The hominin braid

Much has been said ’round the HBD-osphere, lately, on the age of the Pygmy (and Bushmen?)/everyone else split. Greg Cochran of West Hunter, for example, supports a split around 300,000 years ago–100,000 years before the supposed emergence of “anatomically modern humans” aka AMH aka Homo sapiens sapiens:

A number of varieties of Homo are grouped into the broad category of archaic humans in the period beginning 500,000 years ago (or 500ka). It typically includes Homo neanderthalensis (40ka-300ka), Homo rhodesiensis (125ka-300ka), Homo heidelbergensis (200ka-600ka), and may also include Homo antecessor (800ka-1200ka).[1] This category is contrasted with anatomically modern humans, which include Homo sapiens sapiens and Homo sapiens idaltu. (source)

According to genetic and fossil evidence, archaic Homo sapiens evolved to anatomically modern humans solely in Africa, between 200,000 and 100,000 years ago, with members of one branch leaving Africa by 60,000 years ago and over time replacing earlier human populations such as Neanderthals and Homo erectus. (source)

The last steps taken by the anatomically modern humans before becoming the current Homo sapiens, known as “behaviourally modern humans“, were taken either abruptly circa 40-50,000 years ago,[11] or gradually, and led to the achievement of a suite of behavioral and cognitive traits that distinguishes us from merely anatomically modern humans, hominins, and other primates. (source)

Cochran argues:

They’ve managed to sequence a bit of autosomal DNA from the Atapuerca skeletons, about 430,000 years old, confirming that they are on the Neanderthal branch.

Among other things, this supports the slow mutation rate, one compatible with what we see in modern family trios, but also with the fossil record.

This means that the Pygmies, and probably the Bushmen also, split off from the rest of the human race about 300,000 years ago. Call them Paleoafricans.

Personally, I don’t think the Pygmies are that old. Why? Call it intuition; it just seems more likely that they aren’t. Of course, there are a lot of guys out there whose intuition told them those rocks couldn’t possibly be more than 6,000 years old; I recognize that intuition isn’t always a great guide. It’s just the one I’ve got.

Picture 1( <– Actually, my intuition is based partially on my potentially flawed understanding of Haak’s graph, which I read as indicating that Pygmies split off quite recently.)

The thing about speciation (especially of extinct species we know only from their bones) is that it is not really as exact as we’d like it to be. A lot of people think the standard is “can these animals interbreed?” but dogs, coyotes, and wolves can all interbreed. Humans and Neanderthals interbred; the African forest elephant and African bush elephant were long thought to be the same species because they interbreed in zoos, but have been re-categorized into separate species because in the wild, their ranges don’t overlap and so they wouldn’t interbreed without humans moving them around. And now they’re telling us that the Brontosaurus was a dinosaur after all, but Pluto still isn’t a planet.

This is a tree
This is a tree

The distinction between archaic homo sapiens and homo sapiens sapiens is based partly on morphology (look at those brow ridges!) and partly on the urge to draw a line somewhere. If HSS could interbreed with Neanderthals, from whom they were separated by a good 500,000 years, there’s no doubt we moderns could interbreed with AHS from 200,000 years ago. (There’d be a fertility hit, just as pairings between disparate groups of modern HSS take fertility hits, but probably nothing too major–probably not as bad as an Rh- woman x Rh+ man, which we consider normal.)

bones sported by time
bones sported by time

So I don’t think Cochran is being unreasonable. It’s just not what my gut instinct tells me. I’ll be happy to admit I was wrong if I am.

The dominant model of human (and other) evolution has long been the tree (just as we model our own families.) Trees are easy to draw and easy to understand. The only drawback is that it’s not always clear exactly clear where a particular skull should be placed on our trees (or if the skull we have is even representative of their species–the first Neanderthal bones we uncovered actually hailed from an individual who had suffered from arthritis, resulting in decades of misunderstanding of Neanderthal morphology. (Consider, for sympathy, the difficulties of an alien anthropologist if they were handed a modern pygmy skeleton, 4’11”, and a Dinka skeleton, 5’11”, and asked to sort them by species.)

blob chart
blob chart

What we really have are a bunch of bones, and we try to sort them out by time and place, and see if we can figure out which ones belong to separate species. We do our best given what we have, but it’d be easier if we had a few thousand more ancient hominin bones.

The fact that different “species” can interbreed complicates the tree model, because branches do not normally split off and then fuse with other branches, at least not on real trees. These days, it’s looking more like a lattice model–but this probably overstates the amount of crossing. Aboriginal Australians, for example, were almost completely isolated for about 40,000 years, with (IIRC) only one known instance of genetic introgression that happened about 11,000 years ago when some folks from India washed up on the northern shore. The Native Americans haven’t been as isolated, because there appear to have been multiple waves of people that crossed the Bering Strait or otherwise made it into the Americas, but we are still probably talking about only a handful of groups over the course of 40,000 years.

Trellis model
Trellis model

Still, the mixing is there; as our ability to suss out genetic differences become better, we’re likely to keep turning up new incidences.

So what happens when we get deep into the 200,000 year origins of humanity? I suspect–though I could be completely wrong!–that things near the origins get murkier, not less. The tree model suggests that the original group hominins at the base of the “human” tree would be less genetically diverse than than the scattered spectrum of humanity we have today, but these folks may have had a great deal of genetic diversity among themselves due to having recently mated with other human species (many of which we haven’t even found, yet.) And those species themselves had crossed with other species. For example, we know that Melanesians have a decent chunk of Denisovan DNA (and almost no one outside of Melanesia has this, with a few exceptions,) and the Denisovans show evidence that they had even older DNA introgressed from a previous hominin species they had mated with. So you can imagine the many layers of introgression you could get with a part Melanesian person with some Denisovan with some of this other DNA… As we look back in time toward our own origins, we may see similarly a great variety of very disparate DNA that has, in essence, hitch-hiked down the years from older species, but has nothing to do with the timing of the split of modern groups.

As always, I am speculating.


2 thoughts on “The hominin braid

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