SO WHO IS HUMAN #2: Neanderthal

The modifications that led to Neanderthal and sapiens were different to anything that had gone before. Neanderthal developed in cold climates and high latitudes, requiring a number of physical adaptations for survival. Sapiens developed somewhere within the normal geographical span of erectus, and all of its modifications related to brain development. These two super-erectus went down different developmental paths and ultimately sapiens was the victor, though this might have seemed unlikely from an comparison of their physical condition.

Neanderthal 

The neanderthal adaptations were substantial - enough to place them well outside any of the norms for other homo ssp. These included:

- extremely stocky build, with lower leg bones shorter than thigh bones, as is the case with most apes. The broad build lowers the body surface area, preventing heat loss. Some other arctic dwellers, such as some Saami individuals, have the same leg proportions. 

- very strong.   Other apes are much stronger than sapiens, who are relatively light-bodied. Male gorillas are estimated to be 8-15 times as strong as humans, but fortunately they are peaceful vegetarians.  Even chimpanzees can exert about eight times the pull of a human of similar weight. We do not know the strength of a Neanderthal, but they were far more heavily muscled than we are, and probably were at least twice as strong. Scholars are divided on whether they could throw better than humans.Sexual dimorphism in the subspecies was about the same as in sapiens, so the females were also strong.

- large nose. Thought be some to be a cold adaptation; also a feature of Europeans who later occupied the same area. But it is doubtful that this is useful in the cold.

- fair skin Mutations in genes associated with skin colour suggest they had similar fair and red-headed colouring to modern people dwelling around the Baltic. Reduced melanin levels are necessary to allow Vitamin D production under low light conditions, especially when fish are absent from the diet. Some well known recent depictions of Neanderthals have suggested they were red-haired and freckled.

- highly inbred The DNA of Neanderthals in a single location barely varies, and both strands of the X-X chromosome in a recent study were almost identical. It appears that Neanderthals travelled and mated in small  family groups - which would accentuate their differences and accelerate evolution.

- brain and eyes.  Brain size at 1500-1800 cc was significantly larger than sapiens. The occupital bun" or protruding back of the skull implies that it was the occupital lobe that expanded in Neanderthals, rather than the frontal lobe. This expanded occupital lobe is very peculiar and not known in other mammals. The occupital lobe contains the primary visual cortex  and is used for visual perception and colour recognition.

It appears that neanderthal had much better eyesight, adapted to low light conditions in the far north and probably to the arctic winter. The larger eyes also suggest better night vision, so perhaps they were partly nocturnal. It also suggests why they stopped their expansion at about 30-35 degrees of latitude, probably because the light was too bright and the weather too warm for comfort.

Although neanderthal must have been  slow on its feet, it might have compensated by having faster eye-hand co-ordination reflexes, making it into a super-warrior.

The bun appears in some European humans as a sign of admixture:

"There are still many human populations which often exhibit occipital buns. A greater proportion of early modern Europeans had them, but prominent occipital buns even among Europeans are now relatively infrequent."

Ear The skulls of neanderthal are so different from sapiens that even the small bones of the ear can be readily distinguished. This may relate to more detailed pitch modulation by sapiens necessary to distinguish some aspects of speech,

Aggressive behaviour The high proportions of fractures in neanderthal bones, similar to rodeo riders, suggests frequent contact with large combative mammals, suggests they may have leaped upon their prey. It appears they were highly combative; stab and club wounds are also frequent, and there are signs of cannibalism.  Intraspecies competition and their aggressive hunting methods using direct contact probably accounts for adaptive selection in favour of superior strength.

Social behaviour. Like erectus before it, neanderthalus was an apex predator, very capable of hunting the largest and fastest creatures on open plains or in forest. They appear to have had a reasonably complex social organization but probably travelled in smaller bands than sapiens - 5-10 individuals as opposed to 20-30. It is thought they kept to small areas and rarely ventured forth. Injured individuals were nursed back to health - something that erectus had been doing for more than a million years. They maintained the same basic toolmaking practices of flaking sophisticated, very sharp stone implements for a variety of purposes for several hundred thousand years until they ran into sapiens; which suggests they were probably better at observational learning than at creating new methods (as indeed are most modern humans). They conducted burials and included funerary objects in graves, though these are less elaborate than those of sapiens.

Language Their hunting tactics probably required spoken language. Although scholars initially thought that this must be very limited, various genes and facial bones associated with speech seem to be identical in the two subspecies.

Numbers It is thought they were never very numerous, perhaps numbering no more than 10 000 individuals - which might have contributed to their lack of technological progress and to their ultimate defeat.  Given the large range they occupied - all over Europe and into Siberia in the east and Israel in the south - it is hard to know why their populations were not greater, unless they had a low birthrate or the constant losses from feuding kept numbers down.

Location It is rather tempting to think that given the many special adaptations of the Neanderthal, they originally developed in an isolated location such as an island. The one that springs most readily to mind is Fennoscandia

RECENT OUT-OF AFRICA? Not proven Virginia.

Howells (1976) first developed a model of human origins called "Noah's Ark" arguing all living populations have a single recent origin and lineage. He remained however indeterminate on the geographical source. Protsch (1978) and Stringer (1985) developed what became the "Out of Africa" (OOA) model, identifying the single source with Sub-Saharan Africa. A minority of others however proposed the Levant (Vandermeersch, 1981) ... In its original form, the Out of Africa model therefore has been discredited. It has now been revised to include some level of interbreeding. Stringer (2013) has called this model "Mostly Out of Africa" to contrast it to the obsolete "extreme" variant. 

The recent-origins model described above came under immediate attack as it seemed quite unlikely. The first criticism proposed a "Multiregional Theory" in which erectus has been a single species for several million years, but evolved a number of useful characteristics across its spread which leaked into local populations, all of which eventually morphed into sapiens.

Multiregional parallel development

I agree with aspects of this theory. For the reasons given in Homo erectus sapiens, I think homo erectus has been a single species for at least a million years, probably two million. In the post So who is human #1  we showed that homo erectus/ergaster was just like us physically 1.6 million years ago, to most intents and purposes, and they could probably hybridise with modern man.

The problem I have with the rest of the scenario is that it doesn't make a lot of sense from an evolutionary point of view (and neither does Out-of-Africa). This is essentially the parapatric model of evolution - which has not really been observed in mammals.

This rather kindly model of evolution is something like the kindly view of technology - that someone invents a better way of doing something; everyone copies it and we are all better off. Evolution does not work like that - it is in fact random, inefficient and often downright nasty. Species usually have to die in order that others may live, that is natural selection. Very many superior modifications never get off the ground when the carrier dies young. Others are bred rapidly out of existence unless they are dominant and enjoy a very substantial selective advantage.

For this reason, most animal evolution seems to be allopatric/peripatric in nature - subgroups are separated by some event, develop separately and expand; or else they slide into some new niche where they become well adapted, and maybe expand when conditions change in their favour.

It is particularly hard to see how sapiens could have developed from mainstream erectus except through some isolated breeding program - there is nothing special about sapiens that is much use for individuals; all their attributes are only useful in a group of similar individuals. There is no evidence from modern behaviour that  more intelligent, logical, decisive or socially minded individuals are more likely to survive and breed (cheerleaders still prefer jocks, and nerds are social misfits). What is possible however is that groups that contain such individuals and integrate them into decision-making are more likely to succeed.

So what's wrong with out of Africa

During this time period early humans spread around the globe, encountering many new environments on different continents. These challenges, along with an increase in body size, led to an increase in brain size. - Smithsonian Institute.

This quote from the Smithsonian Institute, referring to the early homo period from 2 million years ago to 800 000 years ago and beyond, is absolutely correct as far as I am concerned. The missing part is - how did this large brained homo, fresh from all these challenges across Eurasia, get back into Africa? I think - they didn't!

I am comfortable with the idea of an "original Eden" for both neanderthal and sapiens. The trouble is - Africa is wrong for them both.

Where are these isolated areas in Africa in which they could develop to advanced forms, separated from mainstream erectus? What are the new challenges that Africa presented from two hundred thousand years ago to which they needed to adapt? Erectus had already spent millions of years adapting to Africa, and had reached the top of the food chain to the point it was farming the country just as hunter gatherer tribes do today. Their only real enemies, as today, were other tribes of humans, and it is hard to see evolutionary pressure for their further development. A million and a half years ago they were already at the pinnacle of evolution for Africa and the savanna lands, and it was going to need much tougher challenges to make a super-erectus mutant.

Last glacial vegetation map.

The frequent climatic variations of the last million years have created a laboratory for the development of new species. During the ice ages, sea levels were 8 metres or more lower, From the above map, one can see large differences in the global coastlines during the ice ages (Red Sea and Persian gulf closed, Black Sea, Gibraltar and Sicily closed, large grassland Sahul peninsula in Indonesia, Tasmania and New Guinea joined to Australia in a large continent). The changes at the peak of the interglacials were also pronounced, with higher sea levels and the climate a good deal warmer and wetter - in the eemian the sea level was 8 metres higher than at present there were hippopotamus in the Rhine and Thames. These changes I believe to be the key to the development of the later erectus subspecies.

Neanderthal

It is easy to spot an Eden for neanderthal - Scandinavia. During the last million years, repeated glaciation of the north of Europe occurred. The weight of mile-thick ice pushed down Europe, and when it thawed during ice retreats Scandinavia was made into an island by the ocean. The area was subject to very substantial environmental stress with very different coastlines; the Baltic Sea at one point was a large freshwater lake. Erectus trapped here for twenty thousand years at a time would need to become substantially adapted to the cold and to low light - and this is what neanderthal became. Very substantial inbreeding has been found in the neanderthal genome, consistent with bottleneck populations and travelling in very small family groups.

There have been archaeological finds in caves which strongly suggest human habitation of Scandinavia in excess of 50 000 years ago, but much of the evidence of neanderthal habitation in Scandinavia outside of caves was probably swept away by the expanding glaciers. Or perhaps they did survive, in remnant populations:

In Old Norse sources, trolls are said to dwell in isolated mountains, rocks, and caves, sometimes live together (usually as father-and-daughter or mother-and-son), and are rarely described as helpful or friendly... very strong, but slow and dim-witted, and are at times described as man-eaters and as turning to stone upon contact with sunlight. However, trolls are also attested as looking much the same as human beings, without any particularly hideous appearance about them. 

This sounds extraordinarily like the description of neanderthal that is now emerging - very strong, possibly dim-witted, incestuous, and afraid of bright light.

When the glaciers started to advance again, land bridges reopened and the neanderthal moved south - where just like the Germans and Vikings who followed on the same path, they pushed all before them. In this case they were so much superior to their denisovan adversaries, as we shall see, that they probably wiped them out. But passing into the Middle East, following the cold climate animals that were roaming there around 90 000 years ago, they probably came in contact with their nemesis sapiens for the first time. From here south, the light was probably too bright and the cover too limited for them to safely continue.

Sapiens

There is in fact a similar area that fits the bill for sapiens, at the opposite end of the known world - Indonesia. Here many primates are found, one of the earliest erectus finds - Java man - came to light, and even a small-brained proto-human floriensis lived on Lombok until about 12 000 years ago. The Aboriginal people of Australia are some of the earliest known modern humans, with a record going back 50 000 to 60 000 years. They contain a significant number of heavy-browed individuals similar to erectus, but otherwise have the classic sapiens chin and skull. The Melanesians and the negrito peoples of Africa, generally regarded as the original sapiens inhabitants of the continent, look quite similar to each other.

Apart from Scandinavia, no other area has been subject to the vagaries of climate change as much as the Indonesian archipelago. It has been subject to repeated flooding and drying events, as in Northern Europe changing both the coastline and the environment. As a result, Indonesia has one of the world's highest levels of biodiversity. It has been an absolute hotbed of autochthonous (native) evolution as one species after another attempted to adapt to the changing coastline and habitat.  Nearly half the species in the Wallacea border area between the Wallace and Lydekker lines in the map, for  example, are endemic.

During the ice ages, the giant Sunda peninsula was grassland well suited to early homo and this is presumably how Java man came to be there 1.6-1.8 million years ago. The plain has been repeatedly flooded and has become much wetter during interglacials , and small populations were probably trapped in the jungle-covered islands in the eemian or even before - where they presumably evolved unique bloodlines. Later on when boats were invented (or possibly earlier following tsunamis) they would have made their way to the Sahul continent across the narrow deep water passages near Timor or further north.

The Indonesian archipelago pushes up where the Australian-Indian plate subducts under the Asian plate, and the area is subject to frequent volcanic activity. The massive supervolcanic eruption in Toba Sumatra around 75 000 years ago is said to have triggered a 6-10 year global volcanic winter and possibly triggering the last glacial period. It laid down 15 cm  of ash over the whole of South Asia, killing forests in India. The eruption and subsequent tsunamis must have caused great loss of life, and might have driven any local early-sapiens inhabitants out of the archipelago towards India and Australia.

This type of environment would have been a real challenge even to erectus. In order to maintain viable breeding communities, and to protect against rising sea levels and other dangers associated with rapid global warming, slow cooling and drying events, and tectonic shocks, it might have been necessary to develop    

Hasn't out-of Africa been proved by both paleontology and genetics?

I don't think so. It has not even been proved that erectus had its original base in Africa, though I do think this is likely.

The paleontological evidence that early sapiens was in Africa consists of a piece of skull and some bone fragments from two sites in Ethiopia, dated to 190 000 BP. I am not a paleontologist  but this is very limited information towards establishing what is a fairly fine level of difference erectus-sapiens. Given the substantial differences in skull types between individuals that undoubtedly existed at the time (just as it does now), these probably were not "anatomically modern humans".

The genetic evidence is more complicated and therefore equally complicated to disprove. Human genetics is still in its infancy and it has already made a number of glaring mistakes, absolute statements of fact that have been discredited within a couple of years. No doubt it will make many more before the true results are established.

The principal genetic results state that "genetic Eve" and "genetic Adam" lived in Africa. Relic "long thin lines" are infrequently encountered  in non-recombinant DNA; and after the testing of over 300 000 men a pre-sapiens relic Y-DNA strain was encountered. When more mtDNA lines are tested the same will  probably occur in the female lines.

Even if both Adam and Eve came from Africa, this does not imply that sapiens developed there. The particular mutations that define sapiens probably reside on only a couple of recombinant genes, as yet unknown. It is easy to tack these key genes onto relic DNA from earlier erectus, with the offspring appearing completely human while carrying pre-human non-recombinant lines.

The succeeding article, ????, shows that the Y-chromosome data does not particularly support Africa as the source of sapiens, and is slightly supportive of a south-east Asian origin.

We have not proved here that South east Asia or some other location is the cradle of sapiens, or that Africa is not - we are simply saying that a premature announcement of out-of-Africa was made (with thousands of conjectural articles following the lead)  - and personally I do not think it is even likely, because lightning does not strike twice, especially in evolution.

   
















[1]. Wikipedia. Björn Kurtén wrote "paleofiction"  in the 1980s on this topic. Many bloggers have postulated the troll-neanderthal connection

ORIGINS OF THE HUMAN RACE - START HERE

In this section, I am putting forward an alternative theory to the Recent- out-of-Africa theory of origin of homo sapiens, which is currently dominant among paleoanthropologists, I think the case for African origins is actually quite weak, though possible. Repeating it ad-nauseum in the popular press does not make it any more true. As someone has said, there is so little evidence of our complex origins that one can put forward almost anything and have it fit the facts - and here I will do precisely that.

The Recent Out-of Africa theory claims that homo sapiens is a distinct species, that it developed in Africa around 200-120 Kya (Kya=thousands of years ago), and moved out of Africa, following movements of the game animals they hunted - firstly about 90 kya  (thousands of years ago) in an unsuccessful excursion into the Levant, and then about 60-50 kya when it moved from Africa along the south coast of Asia all the way to Australia, branching further north into East Asia and finally into the forests of Europe about 35 kya.

The alternative theory I am putting forward is:

1. Our forerunners did in fact develop on the savannas of Africa, but 1.5 Mya they had moved throughout all the lightly treed savannas of Eurasia and Africa, where they may have developed in isolated pockets into groups with rather different characteristics. These forerunners looked essentially like us, except they had a different skull shape, and initially, smaller brains. They used tools and fire, and had been doing so for a very long time. They have been called various names, but they were probably the same biological species as us - homo erectus.

2. From about a million years ago, the Earth began to endure hundred-thousand year cycles of freezing and thawing. The freezing dried the temperate latitudes, creating open grasslands, opened up land bridges and much greater areas of the continental shelf.  In the warm cycles, the bridges were inundated, cutting off large groups who were able to develop independently.

3. Probably in the penultimate interglacial, the eemian which occurred about  110 000 years ago (but perhaps earlier in the previous interglacial), two powerful and highly aggressive mutant groups developed in isolated and widely separated areas, both of whom were unlike anything that had gone before, and both of which represented opposite pinnacles of evolutionary development. One subspecies was heavily cold adapted, extremely strong and with unparalleled vision and hand-eye coordination -  perfect warriors. These neanderthals  probably developed in Scandinavia-Britain which was an island at the time, and expanded southwards as the world cooled and the land passages reopened.

The other subspecies remained warm-adapted and fairly gracile like their African forebears. They had a high degree of social coordination, creative intelligence and command, developing novel technologies after two million years of technological stagnation. These sapiens also initially lived in an isolated area where they needed to establish long trade routes in order to survive - possibly the island chain now known as Indonesia, or perhaps even in Papua and Australia.

4. The two enemies met for the first time in the Levant about 90 Kbp with predictable results - the annihilation of sapiens. It was fortunate that the neanderthal were not well equipped to proceed further into the tropics, and not particularly interested in extending their reach into uncomfortable climates, and it was here they missed their chance at global domination.

5. It was to be another 50 000 years before sapiens, armed with improved technology, much greater numbers and disease resistance, were able to try again in the neanderthal heartland. By this time they had established themselves through the entire tropical and temperate belt, eliminating the original Denisovan inhabitants of Eurasia and the relict populations of Africa - with considerable interbreeding where these were most numerous. They had established large populations in the north of India and in the triangle of the Middle East.  They had beaten back the Great Enemy in Siberia, where they learned the techniques for cold survival and outmaneuvering the relatively slow moving neanderthal. In the end, sapiens used exactly the same techniques to wipe out the unbeatable neanderthal that the Europeans were later to use so successfully in eliminating native populations in the New World and Africa - disease, technology and culture, military organisation, and appropriation of resources. They may well have made use of their wealth to invent the divide and conquer tactics that the Romans and Europeans later found so profitable - co-opting one tribal neanderthal group to fight others, and then appropriating the ranges of both parties.

6. It is not entirely true that sapiens wiped out their entire competition through cunning and numbers. They left indications of where these populations once had been numerous by interbreeding - and these show today in the "races" of humanity.

This alternative story, while entertaining, is no more or less fanciful than the current paradigm. In the process of developing it, we will pass through many of the key recent results in paleoanthropology, explaining how they support or fail to support the different scenarios. Even if this story turns out to be largely incorrect, discussing it is instructive and may lead to fresh insights.

The story begins with Homo Erectus Sapiens.

DOES RACE EXIST?

The tendency of our species to find tiny differences in physical appearance between different parts of humanity to be terribly important is most peculiar. We are not alone in this of course, in that various other species make use of physical signalling for courtship which excludes those a little different; while other social animals can treat interlopers with hostility. However the extremes to which we can go are alarming - employing racial vilification, declaring people from other parts of the globe to be inferior or even subhuman, and murdering them, occupying their territory and engaging in ethnic cleansing whenever the opportunity presents.

Interspecies difference is in fact very small - all the "races" perform about equally on most physical criteria, a child from one "race" brought up in a family from another is indistinguishable from any other adopted child. Only two countries in the world, USA and Brazil, still distinguish "race" in their census and one wonders why they bother when there are no functional differences, except perhaps to determine eligibility for affirmative action which has been necessitated by past discrimination.

Genetics and a misguided interpretation of Darwinism has led to some particularly abhorrent doctrines such as Nazism. However, as the methodology of genetics has improved it has become apparent that there are no biological subspecies of sapiens. Analyses such as Fitzpatrick (1998) typically show that 85% of the genetic difference in sapiens is between individuals, and about 15% might be traced to "racial" differences. I can confirm that this is about right - as someone who was lived and worked  in may parts of the globe I have long since ceased to be surprised that I meet the same types of people wherever I go.

However society seems to have gone rather overboard while attempting to put the unpleasant past behind us. Online face tests such as PBS convince us that one cannot identify "race" from faces, at least in America. But might we therefore be missing an important insight?

The fact of the matter is that there are key sites on the globe where "difference" reaches a maximum - for a start. West Africa, Northern Europe and East Asia. Almost anyone can distinguish almost everyone who comes from these places - in facial features and skin colour, while experts on bone structure can easily determine the origins of skeletons from each of these regions. If one looks on the map, there are gradients of difference away from these places which look very much like diagrams for parapatric speciation.

When working from the basic assumptions of this series of articles - namely that sapiens broke out of some ancestral homeland and replaced all other homo while breeding with them to some extent, one might wonder whether these particular places are core sites for other homo ssp - so that Europeans look as they do because they bred with Neanderthals, and Asians and Africans with other groups of early humans.

Looking at the above depiction of a neanderthal woman, one might immediately conclude she is a scruffy but attractive European with big brows and jaw. All the racial determinants are present - reddish hair, white skin, blue-green eyes and the big nose. If she is not a European forerunner, than one would have to presume lightning strikes twice - that there is something about this particular location that has caused these characteristics to develop independently twice, and and not in any other place - a tall order.

Personally I would find it ironic if the Europeans' vaunted racial superiority was actually due to admixture from the "nasty, brutish and short" Neanderthals. However, this opinion is not held by the paleoanthropological community, which holds that these various characteristics are due to local founder effects or parallel mutations.


 

ORIGIN OF SPECIES: Chronospecies

Species are kind of fuzzy. They become real over time, but it takes millions of years - James Mallet

It is surprising how little is formally proved about the origin of species, given that was the title of Darwin’s seminal work and it is the major focus of evolution studies. There is of course a very large literature.  The subject is vitally important to establishing our own origins, and of necessity here only a brief primer of the most important results that might affect homo speciation are given.

Mainly we are concerned with biological speciation - separate groups that cannot effectively crossbreed. I think the other type of species definition, which is based solely on different characteristics, is dangerous in the human context. When Europeans turned up in the New World, they were so extremely different in appearance, habits and motivation they were sometimes regarded as gods. Even as late as the 1930s many people thought the different "races" were virtually different species, with "crossbreds" inherently weak and against the natural order. In fact, because all divisions of sapiens breed so freely and have no discernable functional differences, one would be hard put to even describe them as subspecies (see Does Race Exist).

The theory of speciation is simple enough. Different groups of the same animal develop modifications through genetics that enable them to exploit niches or differences in the environment, and they prosper, Eventually they become so genetically separate from members of their species with other modifications that they are unable to breed with them and they are then forever separate, building their own variations.

 But how and when do they speciate? Many people have the biblical idea that some sudden mutation caused the emergence of Adam, the first of our species, fully formed, at the same time as his partner Eve. In fact this is not possible through any genetic mechanism. Mallet (2006) writes, "The problem was compounded by early systematists' belief that species had an Aristotelian 'essence’ each fundamentally different from similar essences underlying other species." 

We expect that speciation will happen very gradually as different characteristics are incorporated in separate populations. Along the way, we expect that hybridisation is initially possible and may occur repeatedly, but over time fewer and fewer individuals are able to mate successfully as the separate genetic characteristics become fixed. 

Ring species and chronospecies

One of the major criticisms of evolution by creation scientists has been the inability of geneticists to explain or demonstrate speciation. Unfortunately no-one has ever seen a species emerge, it takes too long, and no-one has really been able to examine a breeding population to see what happened genetically as it approached speciation.

Clues to the answer come from so called “ring species”. These are “chains of geographically adjacent populations, in which each population interbreeds freely within itself but somewhat less freely with those adjacent to it, and the populations at the ends of the chain do not interbreed when they come in contact. By the most common definition the populations at the ends of the chain have become separate species."

Only four forms of life have been considered as candidates – the herring or larus gull superspecies complex, the salamanders of the  Central Valley in California, the song sparrow of the Sierra Nevadas in California, and the Greenish Warbler of the Himalayas. [1]

Greenish warbler subspecies around the Himalayas

In a ring species, crossbreeding is substantially reduced between subspecies farther away along the ring. What seems likely is that only a few individuals from distant populations can mate with each other, as an intermediate step on the way to full speciation.

Richard Dawkins (2004) states that ring species "are only showing us in the spatial dimension something that must always happen in the time dimension”. The “ring over time” or chronospecies happens as the breeding group slowly modifies its genetic structure to include various adaptations. We don’t know exactly what mutations prevent viable offspring from forming but eventually some members of the population will have these and they will not be able to breed with the original population or with descendants that have headed down an alternative path. 

I think this probably happened with the Neanderthals when sapiens came in contact with them - some but not all of them could hybridise with their very distant cousins - which explains the relatively small admixture in Eurasian humans.

Spatial mechanisms    

Evolution 101 describes four possible scenarios for the formation  of distinct species, as shown in the diagram. 

The first, "allopatric" scenario involving complete separation is straightforward - there is no mixing of genes and the populations develop independently. What is not quite clear is: how could two populations be so separated, unless they were on separate islands - and why did they not expand their range earlier if that were possible?

Allopatric speciation

This is even more the case in the second "peripatric" scenario, which is the most favoured by evolutionists to be the usual path. The population enters a new niche and then is separated for some reason. It adapts and eventually forms a new species, then expands its territory to cross the original range. While this is a good scenario for the development of specific characteristics, it is hard to see where the separation could be maintained in hominims for the millions of years necessary for speciation. We will return to this in the human context.

The third "paripatric" scenario is essentially the ring species, or what happens with the different human "races" today. A gradient of different characteristics and (perhaps) differential breeding capacity forms across different locations. This definitely happens - but in practice there always seems to be sufficient gene leakage to prevent complete speciation. 

The fourth "sympatric" possibility, in which connected  populations diverge towards their intraspecies extremes to take advantage of local niches, remains speculative since no convincing cases have been observed. It requires both a very strong selection advantage at each extreme, and as well a mating preference among the differentiated individuals. This actually seems to be implied by "out of Africa" theory, since different hominim species would have to have both developed and subsequently lived in contact with each other. 

Genetics and speciation

There are several well documented stages in species separation, defined by the steps in production of viable offspring: 

1. sperm cannot fertilise eggs of other species
2. eggs are fertilised but the embryos (particularly males) die
3. viable offspring are produced but they are infertile

The proven examples of speciation are scanty. Most are the result of hybridisation.

Species with different numbers of chromosomes generally can not pass step 1, because the chromosomes will not line up properly for fertilisation. This is particularly the case when males have more chromosomes than the females. However, in plants, there are documented cases where hybrids of different species (eg radishes and cabbages) can breed with each other but not the parent species. Sometimes this happens through a rare “bad”  hybridisation of species with a  different number of chromosomes that accidentally produces fertile offspring with some combined number of chromosomes. Similar phenomena have been observed in some insects and fish.(Boxhom 1995)

The presence of different strains of parasite in some mosquitoes can prevent mating occurring – typically the egg and  sperm nuclei fail to unite during fertilization. Treating the populations with antibiotics enables hybridization to occur.

 “Behavioural” speciation has been frequently observed and can be easily induced in fruit fly. If separated groups develop different courtship behaviours they will not breed. This behavioural separation is sufficiently common that whether actual biological speciation has occurred can not be established “in the wild”, since the species refuse to breed.

Genetic quasi-speciation has occurred in some cases where the key gene for testicular development has moved on some individuals to another chromosome. Some of the hybrids will then be born without any copy of the gene and cannot produce males. This cannot happen if the gene is located on a non-recombinant Y chromosome, as in almost all mammals.

In mammals, breeding to form sterile mules or viable hybrids is quite common (see McCarthy for a long list). Horses, donkeys and zebras, each of which have different numbers of chromosomes, produce sterile hybrids. These have a common ancestor (MRCA) about ten million years ago. Lions, tigers and other Panthera species do the same. Several attempts to produce a chimpanzee-human hybrid (different number of chromosomes, MRCA about six million years ago) did not result in fertilization (Wikipedia, Mad Science).

Speciation in the hominims

Although as we described in the introductory article, there are a plethora of species names for hominims that differ less from each other than we do among ourselves, there are in fact only two developments that might seriously qualify as speciation. One is the emergence of homo, especially home erectus, from the small-brained australopithecines before 1.6 million years ago. The second is the emergence of sapiens from erectus less than 200 000 years ago. 

The article Homo erectus sapiens stated our opinion that sapiens is not a species - because of fairly obvious hybridisation between sapiens and the other extant ssp of erectus with a common ancestor dating back a million years; and because the specific physical differences of sapiens do not appear to have obvious functional value - sapiens is simply erectus with a deformed head.

The arrival of erectus is a much clearer example of speciation, since they co-existed with australopithecines in Africa and possible elsewhere for such a very long time. Unfortunately we have no genetic evidence, and only a few bones on which to base hypotheses. Exactly how erectus was jump-started from pre-homo ancestors, and where it occurred, is anyone's guess.

These early erectus were functionally physically identical to us, apart from a somewhat smaller brain. They could therefore migrate long distances and, as far as we know, lived in a very wide variety of habitats. It is hard to imagine exactly how they could have engaged in peripatric speciation in an isolated location, unless they were trapped somewhere by rising sea levels and eventually escaped when an ice age opened paths.

This holds even more true for sapiens. It would take a long time to fix the distinctive brain and skull features in the absence of selective advantage. It is hard to think of anywhere in Africa that a race of people might have been separates from their kin for so long. Once again, they might have been isolated somewhere like the Indonesian archipelago and found their way off when boats were invented - or through a tsunami event.

Summary

Mammalian speciation has never been observed, but it seems to take place over an extended period where only some of the population are able to form hybrids with the main population from which they are diverging. Over time the proportion who would be able to mate successfully with the ancestral population goes down. At this stage, there is no evidence that erectus has ever been anything other than a single species; though it may be that different subspecies have hybridised with difficulty.

Because so many species have different numbers of chromosomes and because this is known to limit fertility, chromosome splitting or combination may be a key requirement for speciation.

References

Dawkins, R. (2004). The Ancestor's Tale, 2004:303

Mallet, J (2006). Subspecies, semispecies, superspecies.  Chapter  In Encyclopaedia of Biodiversity. (Elsevier).

[1] Unfortunately the first of these has turned out to be a complex of different species and the second has been shown to occasionally hybridise at the end of the chain,

DEATH OF THE Y: Does the human Y-chromosome have a future?

They are enriched in high-, middle- and low-copy repetitive sequences and contain only a few functional genes. ,,, Y chromosomes: born to be destroyed.

There has been a large accumulation of “junk” in the human genome – about 42% is “retrotransposons” – jumping non-functional genes that easily form multiple copies, amplifying themselves and possibly destroying other genes when they jump. Most of this junk is archaic DNA which through mutation is no longer able to jump or duplicate.

The amount of this retrotansposon junk is very much greater in non-recombinant DNA suh as the human Y-chromosome since it is not removed by evolutionary pressure and, is not cleaned out by the recombination process, which employs a range of protective safeguards against bad copying and intrusive elements).

Hypothetical formation of the Y-chromosome – after Steinemann and Steinemann (2005)

The Y chromosome formed around 160 million years ago when an ancestral mammal developed a sex locus, now called the SRY gene, which causes testicular development. This gene sat on one of two copies of the X chromosome. Subsequently during recombination, a number of other genes useful to males translocated near to SRY in this chromosome or were created there. After that, recombination between X and Y proved harmful, and recombination on this region was suppressed. About 95% of the chromosome – all except for a small area at the tips – was then unable to recombine and was passed on virtually intact to sons.

The Y chromosome has very high rates of mutation because it comes entirely from the sperm, which have to split and rearrange three times during meiosis, with limited chance to check and repair against a duplicate. Natural selection against intrusive elements does not occur very well in non-recombinant DNA. Individual alleles cannot be exposed to natural selection, so that deleterious alleles can hitchhike easily on a good gene. The reverse can also occur – good genes can be lost if they are surrounded by bad ones. Also many perfectly viable lines die out randomly when terninated by girls. This all contributes to the degeneration of Y (in that lines with good Y die out while lines with bad Y live on); and of non-recombinant DNA more generally.

During the lifetime of men, the splitting of sperm cells in mitosis is particularly subject to damage, which is why the fertility of males deteriorates with age and the likelihood of birth defects is greater for older men.

In April 2013 Prof Jenny Graves of Melbourne attracted a good deal of attention when she announced that because of the inherent fragility of the male Y-chromosome and the lossof almost all its functional genes, it would “soon” crumble, making men extinct.

Subsequent investigation has shown that in fact we have not lost any functional genes on the Y chromosome from the time of our branching with chimpanzees 6 million years ago, and only one since diversion with the rhesus monkey 35 million years ago. That is, almost all  of the gene loss from Y took place before 35 million years ago, and quite possibly in the “early” years of the Y. 

In the 58 million base pairs oif the Y-chromoisome, it appears we do not need more than the 86 or so functional genes coding for only 23 distinct proteins to handle our sex differences. It may be that these 86 genes, surviving as they have for 35 million years, have “learned” to defend themselves living in a tangle of useless DNA, which might even act like some sort of styrofoam  packaging protecting the vital materials.

One mechanism by which the Y-chromosome protects itself is by built-in palindromes over critical areas important for male fertility. The Y-chromosome is able to recombine with itself using these palindromes to edit out mistakes and maintain the integrity of the few functional genes. It apparently uses  reversed copies of these genes on itself as a template for verification instead of having a second homologous chromosome.

One wonders from all this if our hard discs could use a clean – if we knew how to do it. The primary driver of our existence is so flawed, random and inefficient it is astonishing it works at all – no machine of our devising could work carrying this kind of useless load. It is not surprising that some are calling the genome  “Unintelligent Design”.

References 

Graves, J. A. M. (2004). "The degenerate Y chromosome—can conversion save it?". Reproduction Fertility and Development 16 (5): 527–534

Graves, J. A. M. (2006). "Sex chromosome specialization and degeneration in mammals". Cell 124 (5)

Steinemann, S&M (2005). Y-chromosomes: born to be destroyed. Bioessays 27.

Was Adam a Lizard? The Long Thin Line

"We should be sampling lines not individuals"

The direct male line is the one that proceeds from Adam to his sons and their sons up to all present men, while the direct female line is the one that proceeds from Eve to her daughters. A number of human geneticists start from the assumption that every direct male and female line must converge back to a recent common ancestor, an Adam or an Eve, while all others “must” die out. Here Adam and Eve are used figuratively, as the common male and female human ancestor of all humanity.  

We agree that there has to be some common male ancestor and some common female ancestor for all of humanity, but we do not necessarily agree that they had to be human.

I actually think that the first humans were a tribe or clan that fixed the flat face mutation which we call "anatomically modern".  They may have lived in one place, but it took them many generations to fix the flat-face mutation between them. If one genetic line can survive from this group for the 200 000 years since the estimated beginning of sapiens, so can two. Or many. No-one is actually killing the alternative lines off. The chimps are our relatives and their line has survived for 5 or 6 million years.

The problem is that the number of descendants of a single individual down direct male and female lines follows the lognormal distribution. This means that a few men (or women) have very many descendants, while a lot of men or women have very few descendants on the direct line. 

We are able to test this using genetics, because the male Y-chromosome is passed down almost unchanged from father to son, while the cell mitochondria are similarly passed down from a woman to her offspring. So you will see many Y-chromosomes that are very similar, because they descend from a man who had very many male-line descendants, and a number that are really different, because they come from men with very few descendants who lived long ago.

The first thing you find in genetic studies of surnames, such as the one I have done in my book "Unravelling the Code" is the preponderance of “long thin lines” – lines with only one or two male survivors in each generation for many centuries. Sometimes families by chance have many adult sons, and then these “long thin lines” can suddenly branch out and become numerous. One man Richard Blood who emigrated to the USA in the 1630s had over 4000 male line descendants there by 1900. Another, John Coode who arrived in Maryland in the 1660s, had only one male line adult descendant in 1900. Quite often a male line that is apparently well-established can suddenly collapse and disappear in a couple of generations because it terminates in girls. 

Because “long thin lines” are the norm rather than the exception, many males find their nearest male line cousins with the same surname, apart from immediate family, are at least sixth cousins. They can be  much further away than this – the Coodes of Nashville’s nearest male line relatives are 12^th cousins, the Coodes of Cornwall. We know that there are unbranching male lines that are much longer than this, going back hundreds of thousands of years. 

  

Without the long thin line we would not be able to distinguish different human genetic clades. In theory the descendants of two brothers ought to have almost exactly the same DNA distribution, because they start with almost the same distribution and they spread out randomly. However, because of the long thin line, and because so many side lines die out, we are typically left with only a few descendants from each brother even five hundred years later – and by that time they can be easily distinguished by Y-DNA because mutations have built up on each line.

Within each clade, a random sample might well yield a very uniform distribution of DNA because their common ancestor is quite recent. Yet the two clades are far apart genetically because they each started with a long thin line.

Small random samples from particular populations tend to be very misleading. We are sampling most of the time from the men who  had many descendants and we are missing most of the men with few descendants, Therefore the distribution can seem to be much tighter than it really is - unless you go specifically searching for certain men of particular lines as we do in my projects. We should be sampling lines, not individuals.

 Because of the long thin line phenomenon, samples are never good enough. You need to test almost the entire population in order to find the rare lines. At the moment only a few hundred thousand men have been Y-DNA tested. In 2013 a really ancient pre sapiens genome, which parted company  from the rest of us maybe 320 000 years ago, turned up in the USA; subsequent investigations in the Cameroons found more of the same. I have no doubt that more will be found as the number of testees expands.

Survival of ancient Y-chromosomes – was Adam a lizard?    

Identify the above: a) Adam b) a close personal friend of Michelangelo c) a lizard

It is possible that the common male ancestor of all humanity could be pushed a long way back, long before anatomically modern humans were around (in fact with the discovery of A00 in the USA, it already has been).  I caused some hostility on a DNA chat group some years ago, and was actually driven off as a “troll”, when I humorously suggested there was no upper limit, and for all we know we are sitting next to Mr Lizard on the bus, or even have Mr Amoeba around for dinner. No-one seemed able to refute this possibility.

It took me a few days to work out why this is biologically impossible. In fact lizards (and monotremes, the most primitive mammals) do not have Y chromosomes; the sex of offspring is determined by the incubation temperature. The male sexual characteristics are also carried on the Y chromosome, meaning that the genitals of really divergent relative might be quite different. So Mr Lizard, apart from causing consternation in the bedroom, would not be able to mate successfully with a mammal female.

It’s possible but very unlikely, depending on exactly where the genetic locus for speciation is located, that someone with a Y-chromosome from an earlier simian species could be alive. It is possible that Adam might not have been erectus. But Y-chromosomes from the Neanderthal, the Denisovans or any other erectus subspecies could be sitting out there right now, since there is no impediment to breeding.

The same applies to mitochondrial DNA, Female speciation is not carried in the mitochondria as far as we know, so unlike Y, in theory it is possible for any pre-human ancient line to survive. We already saw how one Denisovan bone sample carried relic mtDNA from some much more ancient erectus (well, we think it was erectus) and the same could be true in humans.

So WHO IS HUMAN #1? - Erectus

Tracing the origins of the human race has been one of the major areas of scientific endeavour ever since Darwin produced his theory of the origins of species. The Leakey discoveries in East Africa from prompted a vigorous search for the "missing link" - resulting in the discovery of "Turkana Boy" in 1984 - a 10 year old boy who lived 1.6 million years  ago.. His tribe were fire users and had been using stone tools for a very long time. He belonged to a group of  accomplished hunters and his "species" - usually called ergaster - was probably king of the savanna. But at 880 cc his brain size was very much larger relative to his size than any species who had gone before. He was essentially us, with a  smaller brain and a peculiar head.

 We have suggested that in fact modern humans could breed with erectus who lived almost a million years ago, that we are the same species with different heads, and that therefore the focus on sapiens is perhaps not the most important, at least in physical and evolutionary terms. It is erectus ergaster that is the evolutionary miracle.

 The focus of this piece is the physical adaptations that led from other apes to erectus. The most important evolutionary innovations that distinguish us from the other primates, the ones where genetics and selection must be the prime determinants, as I see it, are:

• bipedal gait, and various forms of manual dexterity including "power and precision grip", clubbing and throwing
• bare skin and sweat glands - the "Naked Ape"; permitting distance running
• the large brain and everything that goes with it.

For each of these, we would like to know why, how and where.

Preamble - the world in which erectus developed

The rise of homo has its genesis in a geological event of global scale, the crash of India into Asia and the uplifting of the Tibetan plateau. which reached its peak about six million years ago. The billions of tonnes of newly exposed rock from this collision soaked carbon dioxide out of the air to one of the lowest levels ever,. This cooled the climate gave an advantage to plants using the C4 photosynthesis pathway, mostly grasses. These had been present for over 25 million years, but it was only at this time that the true savanna belts that spanned Africa and Asia developed. These grassy lightly wooded plains became home to millions of hoofed grazing animals, replacing the earlier browsers who fed on forest vegetation, and to carnivores able to take advantage of this huge new source of food.  

At the same time, rapidly changing climate conditions and the emptying and subsequent filling of the Mediterranean caused at first very wet conditions then drying and cooling conditions across Northern Africa. Broadleaf forest replaced rainforest  and was replaced by woodland and savanna, stretching in a changing mosaic of habitats.

Several hundresd species of apes had become widespread in the forests in the preceding Miocene era and now some of them adapted to woodland, savanna and even desert. As well as the baboon who has shown itself able to adapt with different foraging tactics to living in a wide variety of plains environments,  new species of highly mobile, upright bipedal apes appeared that eventually spread worldwide into every type of habitat.

Innovation 0. Bipedalism 

Four legs good, two legs better

There is an argument, based on orangutan behavior (Thorpe et al. 2007) that straight leg walking is actually the ancestral behaviour of primates living in rainforest canopies, and that chimpanzees and gorillas discontinued it when living in more open forest in favour of knuckle walking. At any rate, straight legged bipedal primates with a brain capacity not much different from a chimpanzee entered the open savanna from about four million years ago and continued there until about two million years ago.

The number of finds of these very early small brained upright apes living in open areas has been few, and the finds in different areas have tended to be named not just as separate species but separate genera; we will refer to them generically as australopithecines. The sequence of descent is very far from being established, but there is no doubt the wide range of changing habitats in Africa at the time led outlier groups of forest apes to become specifically adapted for various conditions, and to expand into new areas as climate changed.

Upright walking was virtually a prerequisite for advanced tool use, since it freed the hands for other activities. At some point around 3 million years ago  a small-brained ape learned how to chip stone flakes using other stones and to use these for cutting (see the example of Kanzi below), and homo set upon the long path of growth and technological progress.

Innovation 1. Precision and power grips 

These hand functions are essential to tool making and use.  "Only humans are able to apply the considerable force necessary for holding objects securely and steadily pinched between the thumb and one or more fingers", to apply the "three jawed vice" of the opposable thumb and several fingers. A few monkeys and apes can apply the secure "power grip", where a cylindrical object such as a hammer haft is seated in the closed palm and locked in with the thumb.

It is disputed that the short bipedal homo precursor australopithecines, who lived in Africa between 4 million and 2 million years ago could use tools more successfully that any other ape, or were hunters. However Moya-Sola et al (1999) maintain that the hand of australopithecines had some of the features of "precision pinch" while the bipedal ape Oreopithecus who lived in Sardinia 5 million years earlier had this capability, and they suggest it was in fact co-evolutionary with habitual bipedalism and food gathering.  A contemporaneus hominid ape Orrorin in Kenya had a longer thumb than the australopithecines, making secure grasp possible.

Innovation 2 Throwing and clubbing 

These skills are stated by Young (2003) to promote reproductive success strongly, through establishing dominance, obtaining food, defending against predators and other hominid groups, and because it combines well with bipedalism (also noted by Darwin in 1871). These outcomes certainly hold among humans though they have not been observed among other apes.  Spears and clubs are actually a tremendous advantage when handled well - today a Masai warrior can stop a charging buffalo or lion with a braced spear and can drive off a marauding elephant with a well thrown knobbed club (personal observation). Young suggests this is the earliest hominid specialisation.

While accurate throwing of spears requires the precision grip, forceful throwing requires the application of leverage from the elbow and wrist flick, assisted by rotation of hips and shoulders. A chimpanzee throws stones and other materials, but usually underhand at no more than 20 mph, whereas a twelve year old human child can reach three times that speed, javelins are released at 70 mph, and cricket bowlers and baseball pitchers reach 90 mph or more.

There are almost as many opinions as to when humans gained the accurate throwing ability as paleontologists. Roach (2013) says that erectus had the skill two million years ago, Larsen says it probably evolved  several hundred thousand years ago, while Finlayson (2009) thinks it is peculiar to sapiens and is what helped them out-compete the neanderthals.

Innovation 3. Bare skin and sweat glands - persistence hunting

We think that early homo (who in the absence of information regarding speciation we shall call homo habilis), had learned a hunting trick still performed today by the San (Bushmen) people which requires persistence and endurance, not a great deal of coordination between hunters, little in the way of tools except a water carrier for fluid replenishment, and the specialised skill of  tracking.

The San run down game which would normally be asleep in the shade in the heat of the day. Hunters spend two to five hours harassing an animal over twenty miles until it finally collapses from heat exhaustion. Persistence hunting can be used successfully against the fastest animals - even the cheetah.

We can do this because we are heat-adapted, with tall narrow bodies, profuse sweat glands, "bare" skin with sweat glands for cooling and fine "vellus hair" which acts as a wick for evaporation. So we can keep running for hours in hot conditions, unlike other animals[1] who need to stop and pant to throw off heat. The technique was probably our first hunting method after adopting bipedalism, since our ability to do the quick charges required for ambush hunting was diminished by the upright gait. 

The technique only works in hot open places with daytime temperatures around 40oC and is only recorded in Africa, Australia and Mexico. Although it expends a great deal of energy, oxygen and moisture, (Carrier 1984) it has a very high success rate compared with other hunting methods and in terms of meat yield is only beaten by hunting with dogs  (Liebenberg 2006).

Eating easily digested meat substantially reduces the necessity for digestion and provides energy for other innovations - such as the Big Brain. The loss of body hair and the development of sweat glands in carnivorous bipeds without technologies such as the spear thrower, trapping or dogs must be strongly selected in hot places.

It is suggested by Liebenberg that tracking is such an advanced skill that long-distance running may have evolved while scavenging rather than in hunting, and was developed for beating other animals to the carcass. But in scavenging there is not such an obvious genetic advantage for hair loss and sweat gain.

The loss of body hair, a basic mammal characteristic which assists in the maintenance of a steady body temperature,  leaves humans at a fairly significant disadvantage in terms of adaptability to alternative environments and climate change, and a random observer of the time might well have assumed that these new hairless creatures were painting themselves into an evolutionary dead end, trapped in a single environment. 

From the start there was a need to develop skin pigmentation to protect against deadly and damaging UV rays. Hominims probably discovered very early they could wear skins for protection against the cold, allowing them to head for colder regions. However the need to maintain a correct Vitamin D/folate balance in producing healthy infants has meant that humans have needed to alter their skin colour quite quickly in different latitudes. The Chimpanzee Sequencing and Analysis Consortium has discovered that the genes for skin colouration in humans are among the most significantly modified from those of the chimpanzee, 
  
Because women are not known to hunt, this theory might predict that men were the ones that needed to be hairless, when in fact women have much less body hair than men. Darwin thought that sexual dimorphism was the reason for the male beard and female hairlessness, stating that men prefer women without body hair.  We remain skeptical of sexual selection by the human male, who is demonstrably not particularly fussy when it comes to mating. However - it does remain odd that erectus kept this innovation even when it learned better ways to hunt and when it moved to zones where it was useless - so sexual selection might have payed a role here. It could also have helped with speciation from australopithecus (presuming  homo is in fact a species). Otherwise it is hard to see how it happened, as the two species were neighbours for at least a million years without one wiping out the other or (apparently) forming hybrids..

There is also the possibility that the loss of pelt was even earlier than habilus, stemming back to the time when fully bipedal apes entered the plains. Many apes have large bare areas for sweating and cooling, and an expansion of this capacity in bipedal apes is not a really major innovation. As we have no way to check which hominims had pelts - except to say that tropical hunters such as habilus and ergaster probably did not - this debate will remain unresolved.

Innovation 4. Large brain

Primates already have much larger brains for their size than other animals, developed probably to handle hand-eye co-ordination and to calculate trajectories in a challenging three dimensional arboreal setting, Tricolour vision, which makes it possible to distinguish fruits and red immature leaves, may also require extra brain power.[1] Developing even larger brain sizes probably required nothing more than enhancement/duplication of genes already present in the apes, or even just breeding selection for larger-brained individuals.

Apes show a very large variation in brain capacities. The cranial capacity of gorillas, for instance, varies from 340-750 cc. No doubt they could be bred to favour the larger end of this range without much in the way of genetic modification, but there is no selection pressure for them to do so.

The various australopithecines had average brain sizes fairly similar to a chimpanzee, around 400cc.  We have only a few samples for erectus, but if the average brain size was double this, then even allowing for the increase in stature it represents a very substantial physical modification. Exactly why and where this happened is a matter for conjecture.

The disadvantages of a bigger brain

The big brain is a huge disadvantage for breeding individuals, which is probably why no other animal than homo has developed it. It does not confer enough fitness in itself relative to its high maintenance costs - a quarter to a fifth of the total energy and oxygen consumed. As well it has a very large cost in terms of natal difficulties and nurture. Because human babies have such big heads, humans find birthing far harder than other apes and there are frequent birth casualties for both mother and child. 

Humans have an extremely long period of immaturity compared with other species, and each human reaching adulthood requires a very high investment by parents and the rest of the tribe. It has been suggested that Turkana Boy, with his smaller brain, was considerably nearer maturity than a modern child of his age. 

Overall - the extra food required per individual consumed because of the large brain - directly; through natality loss and damage, and from the extra time spent caring for children and taken away from food gathering, is probably of the order of 40 per cent. In other words, other things being equal, large brained hominids could only sustain about sixty percent of the population of a smaller brained equivalent. Clearly, other things were not equal, but exactly what is speculative. 

The advantages

It is not so easy to find compensating advantages. A decent set of teeth and claws and a fast charge is better value for solo hunting. Higher intelligence is helpful to survival in locating and processing new sources of food, for remembering the location of seasonally available foods, and in hunting and tracking technique, but it is not enough to justify the massive extra energy expenditure - to the point where some scholars have suggested it must be a sexually selected trait with females selecting more intelligent mates (though the lack of dimorphism is not encouraging, and the current behaviour of the human female does not lend much support).

To me, there is only one answer. Many evolutionary biologists focus only on advantages to individuals  - but advantages to whole communities are more beneficial and quicker in assimilating and transferring genes. Humans are a social animal and the positive effects of higher intelligence are greatly magnified in company. The bigger brain permits division of labour - largely unknown in other species  -  and gives time to individuals to develop specialised techniques and make new discoveries which can be shared with the whole group.

In particular any band with several intelligent and empowered members is at a considerable advantage over other bands in terms of access to resources, trade, negotiation and the conduct of warfare, and over predators in terms of maintaining vigilance and organising defence. The whole band does not have to be intelligent, and the mutation can act to the advantage of even the less intelligent or adaptable members of the band well before it is fixed in the whole group. Because the intelligent members will be high-status and perhaps better able to negotiate courtship, they will mate quickly and their genes will spread through the group as it prospers.

Evidence is that early homo was not at the top of the food chain - the hominims from Georgia and habilis in Africa have been found eaten in the dens of extinct cat species. The battle between cats and hominims may have been a factor in the evolution of both.

Today, leopards are expert primate hunters and are quite effective in attacking and disembowelling baboons; they may try to attack a human in the same way if the necessity arises. Baboons are particularly gregarious and unlike most apes live in multi-male groups - typically about 50 which is also believed to be the preferred size of proto-human bands. Like hominims they live on the plains with no cover, and the big-toothed males protect against and drive off big cat predators (Busse 1980).  Australopithecene males were considerably bigger than the females and probably did the same thing. Modern big cats appeared about the same time as early humans, adapted for hunting the game including primates that had appeared on the new savanna grasslands , and one might speculate they evolved together, with cats developing more deadly ambush skills and humans developing bigger brains and better organisation to compensate.

A standard joke in the game lodges of Kenya is a book called "The Most Dangerous Animal in Africa". When you open it you find a mirror. It has always been my contention that because humans arose amidst the fiercest and most dangerous animals on earth, where they survived and eventually prevailed, that when they expanded into other areas they made short work of the local megafauna, which stood no chance.

So when and where did the big brain happen, if indeed it happened only once?

As large as you need and as small as you can

The great variety in sizes of brains (modern humans range from 900 to 1800cc)  within any hominim group makes any comparison of sizes over time unpredictable. Some authors have said that size increased in two bursts - once from habilus to ergaster-erectus about 2 million years ago and again from erectus to early sapiens about 500 000 years ago. Others have said that the expansion in brain size was steady up till 300 000 years ago. Neanderthals definitely had larger brains on average (being the only hominim for which we have a significant sample). 

We know there is a tradeoff between energy use/natal damage and brain size. There may have been overshoot of the optimal size; it is claimed that human brains have shrunk from an average 1500cc to 1350cc over the last 20 000 years. Various theories as to why this has happened are hard to substantiate.

Where it happened is anyone's guess - while most of the finds are in Africa there are a few in Europe and Asia but not sufficient to prove that later big brained hominims are descended directly from earlier small brained ones, which is part of the multiregional theory. Of course the out-of-Africa theory presumes that all significant advances took place in Africa.

 Summary

The very small sample of hominid bones prior to about 30 000 years ago makes teh drawing of conclusions difficult, but the sequence of bodily adaptations that characterise humans are generally agreed to have taken place in the order: 

Bipedalism => Power/precision grip and clubbing => human proportions, bare skin and sweat glands => accurate throwing, probably complete by 1.6 million years ago. 

Average brain size continued to expand during the period, probably accompanied by increasingly complex forms of social interaction. The key innovations of tool chipping and fire setting can actually be carried out by a small-brained modern ape, and they probably took habitual form by the time of ergaster 1.6 million years ago. 

Exactly what happened to the many forms of small proto-human australopithecines that shared Africa with early homo for a million years, disappearing about 2 million years ago, is unknown.  

References

Blainey, G (1983). Triumph of the Nomads. (Pan Australia).

Busse, C (1980).Leopard and lion predation upon Chacma baboons. Botswana Notes and Records 12.

Carrier, D R (1984). The energetic paradox of human running and hominid evolution. Current Anthropology 25: 483.

Chimpanzee Sequencing and Analysis Consortium (2005). Initial sequence of the chimpanzee genome and comparison with the human genome. Nature 437: 69-87. 

Darwin, C (1871). The Descent of Man, and Selection in Relation to Sex. (London: John Murray).

Finlayson 

Larsen, S (2013), opinion in http://www.nytimes.com/2013/06/27/science/evolution-on-the-mound-why-humans-throw-so-well.html

Liebenberg, L (2006). Persistence hunting by modern hunter-gatherers.  Current Anthropology 24http://www.mattmetzgar.com/wp-content/uploads/2007/08/persistence_hunting.pdf

Moya-Sola et al. (1999). Evidence of hominid-like precision grip capability in the hand of
the Miocene ape Oreopithecus. Proc. Natl. Acad. Sci. USA Vol. 96, pp. 313–317

Roach  (2013). Elastic energy storage in the shoulder and the evolution of high-speed throwing in Homo. nature 498: 483-6.

Thorpe, SKS, Holder R and Crompton RH. (2007) Origin of human bipedalism as an adaptation for locomotion on flexible branches Science 316:1328-1331.

Wranger, R (2009). Catching Fire: How Cooking Made us Human. Profile Books,

Young, R W  (2003). Evolution of the human hand: the role of throwing and clubbing. Journal of Anatomy 202: 165-74.

TECHNOLOGICAL ADAPTATIONS

These are one-off technological advances that have been copied and learned, 

Fire 

Fire has been with us so long that it is actually responsible for several biological adaptations.

Cooking breaks down the tough fibre in foods which can then be eaten and easily digested more easily. As a result we need a shorter gut than most mammals (Wranger 2009) - which allowed our long legs to develop, and freed up energy for the larger brain. Our teeth no longer needed to be as robust, and "modern" lighter dentition is probably an adaptation to the cooking of food.

Some foods such as grains and beans can only be eaten cooked as they contain toxins which are mostly destroyed by heating. The acquisition of fire therefore extended the types of foods that could be acquired, and extended the numbers and the range of humans. Collection of wild grass seeds by hunter gatherers on the plains eventually led to agriculture in suitable sites.

Fire has been the ultimate weapon in the hominim arsenal and it seems we probably had it by the time of Turkana Boy. Almost all animals are afraid of fire and a fire will keep large animals at bay - khoisan tribesmen in Botswana today will set a fire as soon as they see an elephant (personal observation).  This may be the way we eventually obtained relief from predators.

The timing of the acquisition of fire is debated. Wildfires became much more common with the advent of open grasslands and savanna, and probably even australopithecus knew of its effects and scavenged charred animals. Perhaps some group of slightly bolder or more intelligent apes discovered how to fan embers to flame and cook freshly caught food, after a fire.

One might wonder whether australopithecus remained afraid of fire and would not approach early homo campsites, aiding in speciation. However some apes are considerably more adept at handling fire and tools than we would generally give them credence for. Kanzi, a bonobo in captivity,  learned to collect firewood, light a fire with matches, set up a griddle and cook. He has learned about 500 words of sign language, and can break flint flakes and create tools for various purposes, using them as scrapers and drills.[2]

Lighting a fire with matches is one thing, but keeping and managing fires seems to be a peculiarly homo trait. Carrying embers around with a tribal group and keeping them alight requires a fair amount of perseverance and social organisation. Lighting a fire with basic materials requires a lot of skill, possessed by relatively few "civilised" people today. 

The short gut and lighter teeth have been with us for several million years, which suggests an early acquisition date. Evidence around ergaster sites in East and South Africa is consistent with cooking and the regular setting of fires. Certainly by the middle paleolithic, all human tribes were practising maintenance of the landscape and hunting using fire, and cooked most of their food, as evidenced by many ancient fireplaces and greatly modified "modern" teeth. 

[1] Horses, camels and kangaroos also sweat, as do most old world primates. Panting is more efficient in cooling the brain and conserving moisture - but it requires the animal to stop.

[2] New York Daily News 30 Dec 2011. Mail Online 23 August  2012.

[3] Birds, insects and tropical fish have even better colour vision, without requiring particularly large brains, so perhaps this is not a factor.