EarthSky: Twenty years of discoveries changing story of human evolution

EarthSky has an interesting article that summarizes twenty years of human evolution discoveries.  They write:

Perspectives on our own species have also changed. Archaeologists previously thought Homo sapiens evolved in Africa around 200,000 years ago, but the story has become more complicated. Fossils discovered in Morocco have pushed that date back to 300,000 years ago, consistent with ancient DNA evidence. This raises doubts that our species emerged in any single place.

This century has also brought unexpected discoveries from Europe and Asia. From enigmatic “hobbits” on the Indonesian island of Flores to the Denisovans in Siberia, our ancestors may have encountered a variety of other hominins when they spread out of Africa. Just this year, researchers reported a new species from the Philippines.

All of these discoveries point to the idea that there was considerable population mixing throughout the Middle to Late Pleistocene not just in Africa but throughout the Old World. We know that it took place in China around 120,000 years ago by the evidence from Linjing.  These particular hominins have characteristics found in modern humans, Neandertals and Homo erectus.

Interestingly, the idea that our species did not originate in any single place was an idea pursued by Rachel Caspari almost two decades ago, at a paper given at one of the American Association of Physical Anthropologists conventions.  At the time, it was still thought that the “Out of Africa” replacement model was still the best explanation for modern human origins.We now know that it is not.

Chris Stringer: Meet the Relatives

Chris Stringer has a post in the Financial Times titled Meet the Relatives.  It is sort of a whirlwind tour through the evolution of the genus Homo.    He writes:

The discoveries of Homo floresiensis, Homo luzonensis, Denisovans and Homo naledi in the past 15 years remind us that the fossil record of humans is still very patchy — stone tools are scattered across much of Africa as a witness to widespread human occupation, yet fossil evidence has been recovered from less than 10 per cent of that continent’s area.

The percentage coverage for Asia is hardly any better: there is, for example, currently only one significant human fossil from the whole of the Indian subcontinent. The discoveries of the past few years underline just how much evolutionary history remains unknown, with other extinct lineages no doubt still to be revealed.

Many of the new finds challenge how we classify fossils in relation to Homo sapiens today. I continue to call the Neanderthals a different species from us, based on their distinctive skeletons and skulls; others feel that the recent evidence of interbreeding and increasing evidence of sophisticated behaviour mean that we should merge them, and the Denisovans, into our species.

I think that the discovery of the Xuchang hominins indicates that there has been considerable population mixing for several hundred thousand years.  As I wrote at the time:

These two Chinese skulls stand at the crossroads of these population movements. While showing clear Neandertal characteristics, they also express modern traits, possibly reflecting mixing with the late, modern human arrivals represented by the recent modern human finds at Daoxian. Yet they also express a clear link to ancient East Asian populations. The implications of these skulls are stark: there has been widespread population mixing and regional continuity in Europe and Asia for at least 400 thousand years. Not only did the Neandertals feel enough cultural kinship to mate and have children with these East Asian people, the early modern humans coming out of Africa did, as well. As Chris Davis of China Daily News put it: “One big happy family.”

Whether this represents such behavior at the peripheries of different species or that of one polytypic species is, as yet, unclear.  It is very clear that our understanding of how these populations interacted is rudimentary, at best. 

On the Heels of the Discovery of Homo luzonensis...

A new study from the journal Cell suggests that there were multiple migrations of individuals into Southeast Asia with the Denisovan genome.  Here is the summary from Cell.

Genome sequences are known for two archaic hominins—Neanderthals and Denisovans—which interbred with anatomically modern humans as they dispersed out of Africa. We identified high-confidence archaic haplotypes in 161 new genomes spanning 14 island groups in Island Southeast Asia and New Guinea and found large stretches of DNA that are inconsistent with a single introgressing Denisovan origin. Instead, modern Papuans carry hundreds of gene variants from two deeply divergent Denisovan lineages that separated over 350 thousand years ago. Spatial and temporal structure among these lineages suggest that introgression from one of these Denisovan groups predominantly took place east of the Wallace line and continued until near the end of the Pleistocene. A third Denisovan lineage occurs in modern East Asians. This regional mosaic suggests considerable complexity in archaic contact, with modern humans interbreeding with multiple Denisovan groups that were geographically isolated from each other over deep evolutionary time.

These data, in combination with the new Luzon material further suggest that the population interrelationships in Southeast Asia were complex, with considerable mixing between Denisovans, archaic Homo sapiens, eastward-migrating Neandertals and who knows how many other groups. We know from the material in China that the fossil material of even 100 thousand years back exhibits multiple origins.  As I noted about the Xuchang hominins at the time:

These two Chinese skulls stand at the crossroads of these population movements. While showing clear Neandertal characteristics, they also express modern traits, possibly reflecting mixing with the late, modern human arrivals represented by the recent modern human finds at Daoxian. Yet they also express a clear link to ancient East Asian populations. The implications of these skulls are stark: there has been widespread population mixing and regional continuity in Europe and Asia for at least 400 thousand years. Not only did the Neandertals feel enough cultural kinship to mate and have children with these East Asian people, the early modern humans coming out of Africa did, as well. As Chris Davis of China Daily News put it: “One big happy family.”

The presence of the hominins in Luzon, as well as the new research reported here suggests that this complexity comprised all of East Asia.

Modern Humans Came Out of Africa Earlier Than Thought and Did So Multiple Times

If you have been studying human evolution, this story is not new.We have suspected for some time that humans migrated out of Africa in waves, beginning with Homo heidelbergensis, up through archaic Homo sapiens and, now, modern Homo sapiens.  As the story notes, there have likely been multiple migrations of Homo sapiens, as well. 

From the story in Science Daily:

A review of recent research on dispersals by early modern humans from Africa to Asia by researchers from the Max Planck Institute for the Science of Human History and the University of Hawai'i at Manoa confirms that the traditional view of a single dispersal of anatomically modern humans out of Africa around 60,000 years ago can no longer be seen as the full story. The analysis, published in the journal Science, reviews the plethora of new discoveries being reported from Asia over the past decade, which were made possible by technological advances and interdisciplinary collaborations, and shows that Homo sapiens reached distant parts of the Asian continent, as well as Near Oceania, much earlier than previously thought. Additionally, evidence that modern humans interbred with other hominins already present in Asia, such as Neanderthals and Denisovans, complicates the evolutionary history of our species.

Here is the graphic from the story:

 
What is only hinted at in the article is that when these waves of moderns came out of Africa, they interbred with the archaic hominins that they encountered. Further, this genome was remarkably stable, since there is genetic evidence that there was at least 500 thousand years of separation between them.Also missing from the story is the range of variation that these hybridizations display.  For example, as noted by the triangle on the map, the Xuchang fossils from China seem to show a mix of modern, Neandertal and late Homo erectus morphologies.  As I noted at the time:

The implications of these skulls are stark: there has been widespread population mixing and regional continuity in Europe and Asia for at least 400 thousand years. Not only did the Neandertals feel enough cultural kinship to mate and have children with these East Asian people, the early modern humans coming out of Africa did, as well.

The other peculiar thing about the graphic is that there seems to be no migration pattern through the strait of Gibraltar. This seems odd since this is likely at least one of the routes that were taken by H. ergaster between 1.5 and 2.0 mya, and since it is thought that the Levallois tool technology comes from the Middle Stone Age of Africa, this would have been a likely route of introduction.  There may have been a barrier of sorts, suggested by the fact that the latest surviving Neandertals are from Spain. 

New Jebel Irhoud Date Causes Rethink of Chinese Dali Skull

Newsweek (Yes, that Newsweek) has a story on the reexamination of the Chinese Dali cranium, in light of the revised dates for the Jebel Irhoud skulls.  Kastalia Medrano writes:

Known as the Dali skull, it was discovered nearly 40 years ago in China’s Shaanxi province. It belonged to a member of the early hominin species Homo erectus. Its facial structure and brain case are intact, despite being dated to around 260,000 years ago. The Dali skull is so old that archaeologists initially didn’t believe it could share features with the modern Homo sapiens.

But Xinzhi Wu of the Chinese Academy of Sciences in Beijing believed that due to the overwhelming physical similarities, Homo erectus must have shared DNA with Homo sapiens. After decades of this idea being dismissed by mainstream academia, Wu and a colleague, Sheela Athreya of Texas A&M University, recently reanalyzed the Dali skull and found it may force us to rewrite our evolutionary history after all. It’s incredibly similar to two separate Homo sapiens skulls previously found in Morocco. “I really wasn’t expecting that,” Athreya told New Scientist.

If we’d found only the Moroccan skulls, and not the Dali skull, it would make sense to keep believing all modern humans evolved in Africa. But the similarities show that early modern humans may not have been genetically isolated from other parts of the world, like what we know today as China.

“I think gene flow could have been multidirectional, so some of the traits seen in Europe or Africa could have originated in Asia,” Athreya told New Scientist.

Okay, now the nuts and bolts.Dali has always been thought to occupy that rarified space between Homo erectus  and early modern Homo sapiens.  It was always thought to be between 200 and 250 kya, and optically-stimulated luminescence dating (a variant of thermoluminescence) places it between 258 and 267 kya.  My analyses of the skull suggested that it was, in no way, shape or form, a modern human but it did not tend to cluster with the Neandertals in terms of head shape.  Athreya and Wu have done an extensive multivariate analysis and conclude (from the article, likely behind a paywall:

When just the facial skeleton is considered, Dali aligns with Middle Paleolithic H. sapiens and is clearly more derived than African or Eurasian Middle Pleistocene Homo. When just the neurocranium is considered, Dali is most similar to African and Eastern Eurasian but not Western European Middle Pleistocene Homo. When both sets of variables are considered together, Dali exhibits a unique morphology that is most closely aligned with the earliest H. sapiens from North Africa and the Levant.

These results add perspective to our previous view of as Dali a “transitional” form between Chinese H. erectus and H. sapiens. Athough no taxonomic allocation is appropriate at this time for Dali, it appears to represent a population that played a more central role in the origin of Chinese H. sapiens. Dali's affinities can be understood in the context of Wu's Continuity with Hybridization scenario and a braided-stream network model of gene flow. Specifically, we propose that Pleistocene populations in China were shaped by periods of isolated evolutionary change within local lineages at certain times, and gene flow between local lineages or between Eastern and Western Eurasia, and Africa at other times, resulting in contributions being made in different capacities to different regions at different times.

In combination with the Xuchang remains, the reanalysis of this skull suggests that, indeed, there is a long and complex interrelationship between different hominin groups that dates back some 400 thousand years. From the paper:

In the braided-stream network model, evolutionary change in China was the result of a shifting network of gene flow among distinct regional Chinese populations, as well as between Chinese and Western Eurasian populations. These were not isolated evolutionary lineages; gene flow both within China and between Eastern Eurasia, Western Eurasia and Africa was intermittent and could explain the similarities in aspects of the neurocranium found here between Dali and Western Eurasian Middle Pleistocene humans. Gene flow would best be described as a braided stream network with periods of isolated evolutionary change within a local lineage at times, and periods of gene flow between local lineages or paleo-demes at other times, resulting in contributions being made in different capacities to different regions at different times.

It is pretty clear that our simplistic models of Recent African Replacement and Multiregional Evolution need to be reworked. It is becoming more apparent that what we are looking at either a polytypic species with a huge geographical range or a syngamion of related species that intermixed regularly. The Xuchang skulls were, apparently, not unique.

New BioLogos Post on Xuchang Hominins is Up

My BioLogos post on the Xuchang hominins is up.  Comments welcome there and here.

Another Human Species in China?

The Christian Science Monitor and other outlets are reporting on a new find from Xuchang, China, that seems to possess intermediate traits between archaic and modern Homo sapiens:

In an article published Friday in the journal Science, the researchers note that the skull fragments date to the Late Pleistocene epoch, a time marked by the expansion of H. sapiens and the extinction of other species in the genus Homo. During the early part of that epoch, Neanderthals roamed Europe and western Asia while humans began to journey out of Africa. But fossil records of human species in Eastern Asia from that time period are thin, muddying the picture of that era for a substantial region of the planet.

The skulls found in China were found to bear very close resemblances to those of Neanderthals, including a very similar inner ear bone and a prominent brow ridge. But the brow ridge was much less pronounced than one would expect from Neanderthals, with a considerably less dense cranium, as one might expect in an early H. sapiens. Researchers also found that the skulls were large by both modern and Neanderthal standards, with a whopping 1800 cubic centimeters of brain capacity.

So where do they fit in the grand scheme of things?

"The overall cranial shape, especially the wide cranial base, and low neurocranial vault, indicate a pattern of continuity with the earlier, Middle Pleistocene eastern Eurasian humans. Yet the presence of two distinctive Neanderthal features ... argue for populational interactions across Eurasia during the late Middle and early Late Pleistocene," said Dr. Trinkaus in a statement.

This kind of population mixing makes sense. We already know that modern humans and Neandertals interbred in Europe and that the geographic range of Neandertals stretched from Portugal to Teshik Tash, in Russia and Shanidar Cave, in Iraq.

The remains are dated to Marine Oxygen Isotope Stage 5d or 5e, making them between 105 and 125 ky in age.  Here is a description of the neurocranium from the paper:

The large Xuchang 1 neurocranium closely approximates the shapes of those of Middle Pleistocene humans, especially eastern Eurasians (Fig. 2 and fig. S17). The vault height is low, similar to those of the Neandertals and the higher Middle Pleistocene vaults, and the low vault height is reflected in a low temporal squamous portion (figs. S27 and S28). It is also produced by the very flat midsagittal parietal arc. In contrast, the maximum cranial breadth is the largest known in the later Pleistocene (fig. S15), and it is securely based on an undistorted posterior cranium. Moreover, the widest point is low, on the temporal bones (fig. S17), as in most earlier crania, rather than on the parietal bones, as among Neandertals and most modern humans. In addition, the one complete mastoid process is short and slopes inward (fig. S17), rather than being longer and more vertical, as in modern humans and some Neandertals. These features combine to provide the cranium with an occipital profile similar to those of earlier human crania, contrasting with the rounded profiles of Neandertals and the laterally vertical ones of modern humans.

There are a few things that are immediately interesting about this. First, this skull is YUGE.  1800 cc is monstrous.  The average cranial capacity of modern humans is around 1450 cc and that of Neandertals, around 1550 cc.  Second, the low, flat cranium with the widest point on the temporal bones (just above your ears) are traits of Homo erectus, not modern humans or Neandertals, suggesting strongly that there was some sort of continuity from this group through to modern humans in this region.  Neandertals simply don't have those traits.  Nonetheless, the cranium clearly shows some Neandertal traits in the ear and rear of the vault.  This continuity is characterized by the authors thus: "This morphological combination, and particularly the presence of a mosaic not known among early Late Pleistocene humans in the western Old World, suggests a complex interaction of directional paleobiological changes and intra- and interregional population dynamics."  As more information becomes available, we will have a better idea of how this find fits in the east Asian evolutionary picture.  This is exciting.  Up until this point, we have had very few finds in China that fall within this general time frame, most notably the Dali and Mapa remains.  I will have to rework my section on human origins for the BioLogos site for this region.