Tuesday, April 5, 2016

When Did Modern Humans Arrive In East Asia?

Zhiren Cave in southern China is an important site for the study of the origin and the environmental background of early modern humans. 
The combination of Elephas kiangnanensis, Elephas maximus, and Megatapirus augustus, indicates an early representative of the typical Asian elephant fauna. 
Previous U-series dating of flowstone calcite has pinpointed an upper age limit for the fossils of about 100 ka. In order to achieve a better comprehension of the chronology of the modern human and contemporaneous faunal assemblage, paleomagnetic, stratigraphic, and optically stimulated luminescence (OSL) dating methods have been applied to the cave sediments. Paleomagnetic analyses reveal that there is a reversed polarity excursion below the fossiliferous layer. This excursion can be regarded as the Blake excursion event, given the U-series ages of the overlying flowstone calcite, the OSL measurements, the virtual geomagnetic pole (VGP) path of the excursion, the two reverse polarity zones within this excursion event, and the characteristic of the fauna assemblage. 
The human remains and mammalian fauna assemblage can be bracketed to 116–106 ka. Application of OSL dating leads to erroneous ages, largely due to the uncertainty associated with the estimation on the dose rates.
Yanjun Cai et al., "The age of human remains and associated fauna from Zhiren Cave in Guangxi, southern China" Quaternary International (March 24, 2016) (emphasis and paragraph breaks added).

This is an update of a find previous made in 2007:
The 2007 discovery of fragmentary human remains (two molars and an anterior mandible) at Zhirendong (Zhiren Cave) in South China provides insight in the processes involved in the establishment of modern humans in eastern Eurasia. The human remains are securely dated by U-series on overlying flowstones and a rich associated faunal sample to the initial Late Pleistocene, >100 kya. 
As such, they are the oldest modern human fossils in East Asia and predate by >60,000 y the oldest previously known modern human remains in the region. 
The Zhiren 3 mandible in particular presents derived modern human anterior symphyseal morphology, with a projecting tuber symphyseos, distinct mental fossae, modest lateral tubercles, and a vertical symphysis; it is separate from any known late archaic human mandible. However, it also exhibits a lingual symphyseal morphology and corpus robustness that place it close to later Pleistocene archaic humans. 
The age and morphology of the Zhiren Cave human remains support a modern human emergence scenario for East Asia involving dispersal with assimilation or populational continuity with gene flow. It also places the Late Pleistocene Asian emergence of modern humans in a pre-Upper Paleolithic context and raises issues concerning the long-term Late Pleistocene coexistence of late archaic and early modern humans across Eurasia.
Wu Liu et al., "Human remains from Zhirendong, South China, and modern human emergence in East Asia", PNAS (2007).

The basic issue here is that given everything else we know about modern human evolution and migration, from archaeology of hominin remains and tools, from genetics, from the fates of megafauna in places where humans appear, and so on, modern humans have no business leaving remains in East Asia between 116,000 and 106,000 years ago.

We have a comparatively uninterrupted period in which modern human remains are found in East Asia starting ca. 40,000 years ago.  We have evidence of modern humans in Australia and Papua New Guinea ca. 50,000 years ago.  We have human remains in Southeast Asia ca. 65,000 years ago.  We have tool assemblies in India similar to those associated with modern humans in Arabia that both predate and post-date the Toba volcano explosion, ca. 75,000 years ago.  We have modern humans just barely out of African in the Levant, ca. 100,000 years ago.  We have tool assemblies and structures in the interior of Arabia similar to tool assemblies and structures found in Nubia contemporaneously in connection with modern human remains that are consistent with a first modern human emergence from Africa, ca. 125,000 years ago.  We have modern human remains in Africa dating back to at least 150,000 years ago.

There are a few other claimed East Asian modern human remains of about the same time period where the dating and species identification are not as reliable as Zhiren Cave which is the most recent and methodologically sound of the finds, and the 2016 result tends to argue strongly against the date being inaccurate, which would be one way to solve the riddle, as its its routine for later investigation to result in the redating of remains found in a cave.

The other possibility is that the very fragmentary evidence two molars and part of a jaw bone, look modern because they contain archaic human remains that have independently evolved derived traits similar to those of modern humans in these teeth and jaws, over the roughly 1.7 million years since Homo Erectus was the first human ancestors to leave Africa, even though the species who left these remains is not actual an anatomically modern human. (Alternately, these could be remains of a non-Out of Africa Homo Erectus derived species and instead represented another archaic species, such as Denisovans, who briefly replaced Homo Erectus in East Asia, only to be replaced in turn by modern humans no so long afterwards.)

A third possibility is that a very small number of modern humans migrated very rapidly from Africa to East Asia, but that they lacked critical mass and went extinct soon after, with modern humans achieving a permanent presence in East Asia only 60,000 years later.  Archaic features in the samples could reflect some degree of archaic introgression into modern humans acquire en route to East Asia.

There are several problems with this third possibility.

* Why is there no other evidence of modern humans pre-100kya, between Arabia and South China? Surely, such a long distance migration would have left some settlements in between?

* If these first wave modern humans could make the trip to South China, albeit to fail there, why is there a 60,000 year gap between the next group of modern humans to appear there?

* Modern humans have thrived and seen their populations grow exponentially in all other virgin territory they encountered, and the population density of archaic hominins during the Homo Erectus era must have been quite low (given their inferior technologies and already low hunter-gatherer population density), so why did modern humans who would have made a very early foray into East Asia not have thrived?  The Zhiren Cave evidence seems to indicate that these hominins where quite successful elephant hunters, after all?  Were these purported anatomically modern humans still behaviorally primitive?  And, if so, what behavioral change so profoundly improved their selective fitness?

* Should early modern humans from ca. 116,000 to 106,000 years BP be almost identical to early modern humans from ca. 100,000 in the Levant, such that no other correspondence would be possible?  There wouldn't be much time for evolutionary differentiation of the two populations, although founder effects and introgression could lead to some rather rapid morphological changes.

One can imagine satisfactory answers to each of these questions. But, we have really no solid evidence to support satisfactory answers to any of them.

Therefore, at this point, in the absence of ancient DNA from purportedly modern human remains in East Asia that are so old, the stark absence of corroborating evidence of a modern human presence in East Asia at this early date for the next 60,000 years or so, and the sharp deviation from a paradigm that explains all of the other data that this interpretation would require, is simply not strong enough to convince me that these remains are really modern human in origin.

A story that could fit this outlier to the rest of the data would have to be truly extraordinary, and I'm not convinced that this evidence is sufficiently extraordinary to support such an extraordinary claim.

I'm not dogmatically opposed to being convinced by future corroborating evidence that a modern human classification at the date determined is the correct interpretation.

One also has other weird data points in South China, like a purportedly archaic set of remains dated to just 14,000 years ago. (Also discussed at this blog here).  It is less weird for a small relict archaic hominin population to survive long after all others of their kind go extinct, in a place that we know was inhabited by at least one, and most probably two species of archaic hominins before modern humans appeared on the scene, particularly prior to the Neolithic revolution, when modern human hunter-gatherers may have had less of a decisive advantage over other archaic hominin species.

But, it is still part of an overall picture in East Asia in which the few pieces we have are all outliers that don't make any sense in isolation and don't even form a really coherent story when viewed together.  This is particularly odd because in the New World, in Europe and in Southeast Asia, new evidence seems to be strengthening the very paradigms that the East Asian finds seem to contradict.

But, the evidence so far just isn't enough, particularly given the lack of clarity over what happened to Homo Erectus in the time frame roughly after 200,000 years BP.  Did Homo Erectus go extinct or suffer diminished numbers in the face of competition from another archaic hominin species such as the Denisovans?  Did Homo Erectus thrive until modern humans arrived and then rapidly go extinct, but left few traces due to insufficient archaeological exploration, low population densities and tools that are too crude to distinguish from mere ordinary rocks?

(The latest evidence in Homo Erectus archaeology is the discovery of Homo Erectus remains in Vietnam dated to ca. 800,000 years ago, which is right when and where we would expect to find them under current paradigms.)

I don't think that we have good enough evidence, yet, to resolve these questions, although I'd like to hope that our knowledge base might improve enough to understand this period much more accurately and definitively in the years to come during my lifetime.

19 comments:

Joerg Hensiek said...

Great summary, well-thought-out

Joerg Hensiek said...

just one important discovery with relevance to the Zhiren Cave finds was missing in your excellent assessment:
http://www.scientificamerican.com/article/teeth-from-china-reveal-an-early-human-trek-out-of-africa1/


andrew said...

The October 14, 2015 Scientific American article you link to says that a Nature article reports that there a 47 tooth samples that are unequivocally AMH in the cave together with faunal remains, but finds no stone tools, and puts the date at 80,000 to 120,000 years ago. It argues that human corpses were brought to the cave by animals who were eating those remains. I'll have to find the underlying Nature article to see if there is more detail. The Scientific American paper also doesn't disclose if ancient DNA extraction has been attempted or has been successful with any of those teeth.

The puzzle still doesn't fit, but the 2015 study would seem to remove some of the easier outs. In the absence of stronger evidence of continuous habitation, this would seem to be a Roanoke style failed colony that probably died out before modern humans arrived a second time. There is no indication of the route they took, of their material culture, of their genetic affinities, or of their overall appearance, which is honestly quite surprising given the number of teeth recovered.

terryt said...

It is becoming more and more obvious that the simple OoA proposal is grossly inadequate when it comes to explain modern human origins.

"The human remains are securely dated by U-series on overlying flowstones and a rich associated faunal sample to the initial Late Pleistocene, >100 kya. As such, they are the oldest modern human fossils in East Asia and predate by >60,000 y the oldest previously known modern human remains in the region".

That is so for the oldest 'known human remains'. But we can be reasonably sure humans had reached Australia by 50,000 years ago, and possible crossed Wallace's Line around 60,000 years ago when sea level was at its lowest making the crossing 'easier'. I would not expect that modern humans immediately crossed Wallace's line when the reached it, either. That places modern humans in South China at an early date.

"We have tool assemblies in India similar to those associated with modern humans in Arabia that both predate and post-date the Toba volcano explosion, ca. 75,000 years ago"

And they seem unaltered during before and after the eruption.

"Modern humans have thrived and seen their populations grow exponentially in all other virgin territory they encountered, and the population density of archaic hominins during the Homo Erectus era must have been quite low (given their inferior technologies"

I think you exaggerate the erectus 'inferior technologies'. The technologies of the first modern humans outside Africa are indistinguishable from the pre-modern technologies in each region. Besides which the exponential growth of modern human populations occurred where there were no pre-moderns present, such as Australia and America.

"There wouldn't be much time for evolutionary differentiation of the two populations, although founder effects and introgression could lead to some rather rapid morphological changes".

As would hybridisation with pre-existing human 'species', We know that has happened at least twice and there is no reason to suspect those occasions were unique.

"Did Homo Erectus go extinct or suffer diminished numbers in the face of competition from another archaic hominin species such as the Denisovans? Did Homo Erectus thrive until modern humans arrived and then rapidly go extinct?"

I don't think simple 'replacement' is the only option when dealing with human species' interactions. For example it is most unlikely that a 'pure Denisovan' population crossed Wallace's line. The population had presumably already become admixed in roughly the proportions now exhibited.

Onur said...

I think you exaggerate the erectus 'inferior technologies'. The technologies of the first modern humans outside Africa are indistinguishable from the pre-modern technologies in each region.

That is not the case in Europe, where the technologies of Neanderthals and early modern humans are clearly distinguishable from each other.

terryt said...

"That is not the case in Europe, where the technologies of Neanderthals and early modern humans are clearly distinguishable from each other".

I'm not sure that is altogether correct. There still seems argument over whether some cultures were Neanderthal or modern. I agree there was an 'invasion' of definitely modern people in the Upper Paleolithic but it is questionable whether they were the 'first' modern humans in Europe or not.

andrew said...

""There wouldn't be much time for evolutionary differentiation of the two populations, although founder effects and introgression could lead to some rather rapid morphological changes".

As would hybridisation with pre-existing human 'species', We know that has happened at least twice and there is no reason to suspect those occasions were unique."

Hence my mention of "introgression" which means essentially the same thing as "hybridisation" except with an implied statement about the archaic part being the smaller percentage of the total ancestry when the process is complete for the population.

""Did Homo Erectus go extinct or suffer diminished numbers in the face of competition from another archaic hominin species such as the Denisovans? Did Homo Erectus thrive until modern humans arrived and then rapidly go extinct?"

I don't think simple 'replacement' is the only option when dealing with human species' interactions. For example it is most unlikely that a 'pure Denisovan' population crossed Wallace's line. The population had presumably already become admixed in roughly the proportions now exhibited."

I agree that my questions represent extreme possibilities and are not complete. But, I do think that there is a fairly high probability that an archaic population with no modern human admixture did cross the Wallace line before modern humans did and there is a fairly high probability that it was a source of Denisovan admixture in modern humans. Certainly, however, the scenario you suggest is possible, maybe even equally likely. Indeed, if there was a very early modern human migration to Asia, it could be that that population disappeared because it fully admixed into the pre-existing archaic population rather than going extinct in some more dramatic manner.

terryt said...

Thanks for the clarification.

"I do think that there is a fairly high probability that an archaic population with no modern human admixture did cross the Wallace line before modern humans did and there is a fairly high probability that it was a source of Denisovan admixture in modern humans".

Definitely reached Flores, but that is unlikely to be the source of Denisova in Australians. However I remember a recent paper claimed early humans on Sulawesi. Now that might be worth a further look. But I do thing it reasonably unlikely that a single 'species' of ancient human was spread evenly across the whole of Eurasia from Europe to SE Asia via the Altai. It turns out that the source of Denisovan admixture may indeed have been the Altai region. Have you seen this:

http://biorxiv.org/content/biorxiv/early/2016/04/06/047456.full.pdf

A northern route seems to have become established for both M and N mt-DNAs.

capra internetensis said...

The Denisovan that admixed into living humans was only very distantly related to the Altai Denisovan. The last estimate was that they diverged around 350 000 years ago. Far more likely the Denisovan population that mixed with our ancestors wasn't anywhere near the Altai. Similarly, the Altai Neanderthal was the most divergent from the Neanderthals who interbred with the ancestors of living humans (though the divergence is much less in this case).

The authors of the new paper also bring up the modern human admixture in the Altai Neanderthal to support their route and timing. But the modern human ancestry in the Altai Neanderthal was also highly divergent from living Eurasians, so this does not support their case at all, but rather detracts from it.

terryt said...

Thanks for that information.

"the Altai Neanderthal was the most divergent from the Neanderthals who interbred with the ancestors of living humans"

Presumably a separate admixture event. The surviving Neanderthal genes presumably entered the surviving modern human gene pool somewhere closer to Africa. But hasn't it been shown that the Neanderthal admixture in modern humans comes from several different Neanderthal sources? Although the modern human admixture in the Altai Neanderthal was distant from surviving modern humans the admixture most likely happened somewhere near the Altai.

"The Denisovan that admixed into living humans was only very distantly related to the Altai Denisovan".

And could have introgressed into the surviving human gene pool anywhere between the Altai and SE Asia. The big question now is: is the Denisovan in Tibetans from the same source as that in Oceania? One introgression or more?

terryt said...

"Far more likely the Denisovan population that mixed with our ancestors wasn't anywhere near the Altai".

Possibly. Although the authors make this point:

"This time, we have found additional support from Paleogenetics that has demonstrated introgression in the genome of modern humans of DNA from Neanderthals (Green et al., 2010; Prüfer et al., 2014) and
Denisovans (Reich et al., 2010; M. Meyer et al., 2012) hominins that, most probably, had northern geographic ranges (Sawyer et al., 2015)".

capra internetensis said...

Regarding "northern geographic ranges", the Sawyer references says that Denisovans were *present* in the north for a long time (because one of the Denisovan mtDNAs was much older than the others), not that Denisovans were *restricted to* the north. There's no reason to think that Denisovan populations much more divergent from one another than Pygmies are from Eskimos would have to be adapted to the same environment.

As far as Neanderthals go, of course we know that their range extended south at least to the Levant and Iran. We don't know who was living in Arabia or India or whether the putative southern coastal route even existed, but the Near East is the obvious place for Neanderthal admixture to have happened anyway.

Bringing up the modern human admixture in the Altai Neanderthal torpedos their own argument for an early northern route based on the Chinese fossil record, because it raises the possibility of Eurasian AMHs who were *not* ancestral to living humans.

terryt said...

"not that Denisovans were *restricted to* the north".

At this stage we have no eveidence for their presence other than in the north. We have to confine our conclusions to what we know, not to what we wish to believe.

"There's no reason to think that Denisovan populations much more divergent from one another than Pygmies are from Eskimos would have to be adapted to the same environment".

But if some time in the future we are considering a population that had received introgression from some form of 'modern human' it would be relatively easy to determine whether it was from Eskimo or Pygmy. They do differ from easch other genetically, as we could presume all ancient human species did. In fact a fairly convincing argument could be made that ancient diversity would be greater than present diversity because human mobility is likely to have been much less that it has been over the last 50,000 years.

"but the Near East is the obvious place for Neanderthal admixture to have happened anyway".

Which is what many of the same authors argued in the earlier paper:

http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0129839

Quote:

"More recently, it was stated that the introgressed Neanderthal DNA in humans is more closely related to the Mezmaiskaya Neanderthal from the Caucasus than it is to either the Neanderthal from Altai in Siberia or to the Vindija Neanderthals from Croatia"

"Bringing up the modern human admixture in the Altai Neanderthal torpedos their own argument for an early northern route based"

Your imagination is running away a bit here. That is but a very small part of the evidence they offer. The only argument they use is that the presence of Neanderthal genes of any sort indicates a dual presence somewhere north of Arabia. And that Neandethals were one of the two human species who had a northern geographic range.

andrew said...

"At this stage we have no evidence for their presence other than in the north. We have to confine our conclusions to what we know, not to what we wish to believe"

The modern geographic distribution of Denisovan admixture is absolutely evidence for their presence other than in the north. Reasonable people can disagree over how much weight to give the evidence we have on the question of the geographic distribution of Denisovans, but to give it no weight whatsoever is no a very reasonable conclusion.

terryt said...

"The modern geographic distribution of Denisovan admixture is absolutely evidence for their presence other than in the north".

I disagree that it is evidence for the presence of 'pure' Denisovans anywhere other than in the north. The people with Denisovan admixture have just a small proportion in their genome and it could have been carried into SE Asia/Australia from almost anywhere in an already mixed population.

"Reasonable people can disagree over how much weight to give the evidence we have on the question of the geographic distribution of Denisovans, but to give it no weight whatsoever is no a very reasonable conclusion".

Again I disagree. The only evidence for the presence of unadmixed Denisovans is in the north from Europe to the Altai, and we know they have become extinct there. The population that entered SE Asia could easily have become mixed well before they got anywhere near SE Asia. We need evidence of unadmixed Denisova outside the region from the Altai to Europe before we can claim a presence for them anywhere other than that region. As I often say, we have to work with the evidence as we have it not how we wish it to be.

Joerg Hensiek said...

here other evidence from this year, proving that "modern" humans, although not our direct ancestors, did migrate out of Africa at least 100 0000 years ago and mated with archaic humans:

https://www.cshl.edu/news-and-features/neanderthals-mated-with-modern-humans-much-earlier-than-previously-thought-study-finds.html

these folks could be the people of the Zhiren Cave, and this could also explain the archaic features of the findings.

terryt said...

Interesting paper, although the article makes some statements that to me seem strange. Such as:

“the signal we’re seeing in the Altai Neanderthal probably comes from an interbreeding event that occurred after this Neanderthal lineage diverged from its archaic cousins, a little more than 100,000 years ago.”

Why 'diverged'? All it means is that the modern human(s) bred with Neanderthals within a small region. It was the formation of the hybrid that caused the divergence.

"they left their genetic mark in the Altai Neanderthal, about 100,000 years ago, before being lost to extinction themselves".

Maybe not lost to extinction. It is quite possible that the later modern humans who emerged from Africa did not breed with 'Neanderthals' but with a population that was already a hybrid. In which case the genes of the first group to leave Africa would still survive.

It all goes to show that our evolution is far more complicated than is usually believed. And any attempt to define when the 'first modern humans' formed is equivalent to looking for Adam and Eve. It doesn't exist. The process of becoming modern was a long, drawn out affair, not something that happened overnight.

terryt said...

"these folks could be the people of the Zhiren Cave, and this could also explain the archaic features of the findings".

Extremely likely I think. What this tends to show is that 'modern humans' left Africa maybe more than 100,000 years ago. But they did not carry 'modern' haploid DNA. The early modern humans reached south China and the Altai region mixed with Neanderthals. Then they were pushed south by increasing cold and were replaced through the northern half of Eurasia by an expanding Neanderthal population. Then once it warmed somewhat the expansion out of Africa recommenced and carried modern haploid DNA through a population that was basically already 'modern'. As I said, those who look for a single moment in time and space when modern humans first evolved are searching for a rainbow.

terryt said...

Another thing this paper easily explains is the extra Neanderthal component in East Asians. It also offers a possible explanation for the presence of the Denisova component in SE Asia.