Recent Discoveries in Human Evolution: H. Sapiens 300,000 years old?

Welcome back to our discussion of recent exciting advances in our knowledge of human evolution:

  • Ancient hominins in the US?
  • Homo naledi
  • Homo flores
  • Humans evolved in Europe?
  • In two days, first H Sap was pushed back to 260,000 years,
  • then to 300,000 years!
  • Bell beaker paper

As we’ve been discussing for the past couple of weeks, the exact dividing line between “human” and “non-human” isn’t always hard and fast. The very first Homo species, such as Homo habilis, undoubtedly had more in common with its immediate Australopithecine ancestors than with today’s modern humans, 3 million years later, but that doesn’t mean these dividing lines are meaningless. Homo sapiens and Homo neandethalensis, while considered different species, interbred and produced fertile offspring (most non-Africans have 3-5% Neanderthal DNA as a result of these pairings;) by contrast, humans and chimps cannot produce fertile offspring, because humans and chimps have a different number of chromosomes. The genetic distance between the two groups is just too far.

Oldowan tool

The grouping of ancient individuals into Homo or not-Homo, Erectus or Habilis, Sapiens or not, is partly based on physical morphology–what they looked like, how they moved–and partly based on culture, such as the ability to make tools or control fire. While australopithecines made some stone tools (and chimps can make tools out of twigs to retrieve tasty termites from nests,) Homo habilis (“handy man”) was the first to master the art and produce large numbers of more sophisticated tools for different purposes, such as this Oldowan chopper.

But we also group species based on moral or political beliefs–scientists generally believe it would be immoral to say that different modern human groups belong to different species, and so the date when Homo ergaster transforms into Homo sapiens is dependent on the date when the most divergent human groups alive today split apart–no one wants to come up with a finding that will get trumpeted in media as “Scientists Prove Pygmies aren’t Human!” (Pygmies already have enough problems, what with their immediate neighbors actually thinking they aren’t human and using their organs for magic rituals.)

(Of course they would still be Human even if they part of an ancient lineage.)

But if an ecologically-minded space alien arrived on earth back in 1490 and was charged with documenting terrestrial species, it might easily decide–based on morphology, culture, and physical distribution–that there were several different Homo “species” which all deserve to be preserved.

But we are not space aliens, and we have the concerns of our own day.

So when a paper was published last year on archaic admixture in Pygmies and the Pygmy/Bushmen/everyone else split, West Hunter noted the authors used a fast–but discredited–estimate of mutation rate to avoid the claim that Pygmies split off 300,000 years ago, 100,000 years before the emergence of Homo sapiens:

There are a couple of recent papers on introgression from some quite divergent archaic population into Pygmies ( this also looks to be the case with Bushmen). Among other things, one of those papers discussed the time of the split between African farmers (Bantu) and Pygmies, as determined from whole-genome analysis and the mutation rate. They preferred to use the once-fashionable rate of 2.5 x 10-8 per-site per-generation (based on nothing), instead of the new pedigree-based estimate of about 1.2 x 10-8 (based on sequencing parents and child: new stuff in the kid is mutation). The old fast rate indicates that the split between Neanderthals and modern humans is much more recent than the age of early Neanderthal-looking skeletons, while the new slow rate fits the fossil record – so what’s to like about the fast rate? Thing is, using the slow rate, the split time between Pygmies and Bantu is ~300k years ago – long before any archaeological sign of behavioral modernity (however you define it) and well before the first known fossils of AMH (although that shouldn’t bother anyone, considering the raggedness of the fossil record).

This was a good catch. (Here is the relevant Dienekes article, plus Model-based analyses of whole-genome data reveal a complex evolutionary history involving archaic introgression in Central African Pygmies, and Whole-genome sequence analyses of Western Central African Pygmy hunter-gatherers reveal a complex demographic history and identify candidate genes under positive natural selection.) If the slow mutation rate matches the fossil record better than the fast, why use the fast–except if the fast gives you inconvenient results?

But now we have another finding, based on the Bushmen, which also pushes the Bushmen/everyone else split back further than 200,000 years–from BioRxiv, “Ancient genomes from southern Africa pushes modern human divergence beyond 260,000 years ago“:

Southern Africa is consistently placed as one of the potential regions for the evolution of Homo sapiens. To examine the region’s human prehistory prior to the arrival of migrants from East and West Africa or Eurasia in the last 1,700 years, we generated and analyzed genome sequence data from seven ancient individuals from KwaZulu-Natal, South Africa. Three Stone Age hunter-gatherers date to ~2,000 years ago, and we show that they were related to current-day southern San groups such as the Karretjie People. Four Iron Age farmers (300-500 years old) have genetic signatures similar to present day Bantu-speakers. The genome sequence (13x coverage) of a juvenile boy from Ballito Bay, who lived ~2,000 years ago, demonstrates that southern African Stone Age hunter-gatherers were not impacted by recent admixture; however, we estimate that all modern-day Khoekhoe and San groups have been influenced by 9-22% genetic admixture from East African/Eurasian pastoralist groups arriving >1,000 years ago, including the Ju|’hoansi San, previously thought to have very low levels of admixture. Using traditional and new approaches, we estimate the population divergence time between the Ballito Bay boy and other groups to beyond 260,000 years ago.

260,000 years! Looks like West Hunter was correct, and we should be looking at the earlier Pygmy divergence date, too.

Two days later, a paper from the opposite end of Africa appeared in Nature which–potentially–pushes H sapiens’s emergence to 300,000 years ago, “New fossils from Jebel Irhoud, Morocco and the pan-African origin of Homo sapiens“:

Fossil evidence points to an African origin of Homo sapiens from a group called either H. heidelbergensis or H. rhodesiensis. However, the exact place and time of emergence of H. sapiens remain obscure … In particular, it is unclear whether the present day ‘modern’ morphology rapidly emerged approximately 200 thousand years ago (ka) among earlier representatives of H. sapiens1 or evolved gradually over the last 400 thousand years2. Here we report newly discovered human fossils from Jebel Irhoud, Morocco, and interpret the affinities of the hominins from this site with other archaic and recent human groups. We identified a mosaic of features including facial, mandibular and dental morphology that aligns the Jebel Irhoud material with early or recent anatomically modern humans and more primitive neurocranial and endocranial morphology. In combination with an age of 315 ± 34 thousand years (as determined by thermoluminescence dating)3, this evidence makes Jebel Irhoud the oldest and richest African Middle Stone Age hominin site that documents early stages of the H. sapiens clade in which key features of modern morphology were established.

Comparison of the skulls of a Jebel Irhoud human (left) and a modern human (right) (NHM London)

Hublin–one of the study’s coauthors–notes that between 330,000 and 300,000 years ago, the Sahara was green and animals could range freely across it.

While the Moroccan fossils do look like modern H sapiens, they also still look a lot like pre-sapiens, and the matter is still up for debate. Paleoanthropologist Chris Stringer suggests that we should consider all of our ancestors after the Neanderthals split off to be Homo sapiens, which would make our species 500,000 years old. Others would undoubtedly prefer to use a more recent date, arguing that the physical and cultural differences between 500,000 year old humans and today’s people are too large to consider them one species.

According to the Atlantic:

[The Jebel Irhoud] people had very similar faces to today’s humans, albeit with slightly more prominent brows. But the backs of their heads were very different. Our skulls are rounded globes, but theirs were lower on the top and longer at the back. If you saw them face on, they could pass for a modern human. But they turned around, you’d be looking at a skull that’s closer to extinct hominids like Homo erectus. “Today, you wouldn’t be able to find anyone with a braincase that shape,” says Gunz.

Their brains, though already as large as ours, must also have been shaped differently. It seems that the size of the human brain had already been finalized 300,000 years ago, but its structure—and perhaps its abilities—were fine-tuned over the subsequent millennia of evolution.

No matter how we split it, these are exciting days in the field!

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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.