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.