The Evolutionary Significance of the Daoxian Teeth Discovery
A new discovery in a Chinese cave throws new light on how (and when) modern humans first spread to Asia.
The fuyan cave hominin sample
The world of human evolutionary studies has once again witnessed a discovery that throws new light on a critical time period in human evolution: the origins of modern humans. Recounted in a scientific paper by Wu Liu of the Key Laboratory of Vertebrate Evolution and Human Origins, in Beijing, China, and colleagues,1 47 human teeth have been found at the site of Fuyan Cave, in Daoxian, southern China, which have the characteristics of the teeth of people walking around today but are between 80 and 120 thousand years old!
Before this discovery, the only known modern humans from East Asia were those found in North China, Borneo and the Murray River region of Australia, all of which are between 40 and 50 thousand years old. Before that, there was a gap of some 80 thousand years, until you encounter archaic Homo sapiens, who were still present in the area as late as 120 to 130 thousand years ago (kya).
The presence of the modern humans in East Asia at this time answers a critical question in migration studies: whether or not early modern humans made it from Africa to East Asia before the eruption of the Indonesian Toba super volcano at 74 kya. It has recently become conventional wisdom2 that the reason no modern humans have been found in this region prior to 60 kya is because the eruption, which was 67 times greater than the Krakatoa eruption of 1883 that killed 36 thousand people, created too large a barrier to migration. Because of the Daoxian find, we now know that modern humans did make this trek before the Toba event.
This find also raises intriguing questions about how these early modern humans may have interacted with the pre-modern humans in this area. These pre-moderns, also called archaic Homo sapiens, are fossil representatives of populations that are more advanced than Homo erectus in the direction of modern humans but still retain primitive characteristics, like large brow ridges, slightly smaller crania with sloping foreheads and large faces. If the early date for Daoxian (80 kya) is correct, then there was a time of overlap between them and populations represented by the very archaic looking Mapa cranium, which has very large brow ridges, a large face and a long head. Did they interbreed with them? Did they shun them? Did they meet them at all? The sharp differences between the moderns and their archaic cousins suggest one of the last two possibilities. If, on the other hand, the late date is correct, then there was a time period of about 40 thousand years in which we do not know what kinds of interactions may have taken place. We will have to wait for more information to reveal itself.
The origins and dispersal of modern humans
The Daoxian find also brings into focus two of the most pressing questions involving the origins of modern humans: where and when they came from in the first place. The answer to the first question seems to be Africa. To understand this, we need to turn to the fields of genetics and palaeontology. In the world of genetics, Rebecca Cann, Mark Stoneking and Allan Wilson got the ball rolling in the late 1980s with a paper on modern human mitochondrial (mt)DNA.3 mtDNA, unlike regular DNA, does not recombine, but is passed through the female line, from mother to daughter from generation to generation. They found that, by measuring the amount of variation in mtDNA, genealogies could be constructed and family trees could be traced back to their origins. All roads led to Africa. More research into other aspects of our DNA has only reinforced this conclusion.
The answer to the second question seems to be sometime between 100 and 200 kya. The deepest root of mtDNA lies with the Khoi San people in south Africa at approximately 185 kya. Two other studies place the origin at between 99 kya and 148 kya, although, as David Wilcox points out, the exact date is debated because of how mutation rates are calculated.4
The world of human palaeontology also supports this African origin. At the moment, the earliest evidence we have for modern humans is from the sites of Omo Kibish, in southern Ethiopia, and Herto, in the Awash River Valley, in northern Ethiopia. The Omo remains have been dated at 195 kya and the Herto remains to c. 164 kya, both by the 40Argon/39Argon method. Both finds, while having large brow ridges and bulges on the backs of the heads, are clearly modern in appearance.
Holy land evidence
Aside from the African remains, the earliest evidence we have had of modern humans outside Africa has been the Mugharet-es-Skhul and Jebel Qafzeh fossil collections from Israel, both of which are dated to between 90 and 100 kya. After this, there are no more modern humans in this area until the Manot Cave cranium at 50 kya, which was found with modern stone tools. This gap has led many authors to suggest that modern humans came into this area and were either driven out or simply failed to survive. There is no evidence that they went east, into Asia, from here.
The southern route
But if modern humans did not go through the Holy Land, how did they get to East Asia? New evidence of cultural remains from the Arabian peninsula may have the answer. Two sites, Aybut Al Auwal and Jebel Faya, both at the top of the Horn of Africa, have yielded stone tools dated to between 106 and 125 kya that are similar to those of their African forebears.5 Other researchers, using genetic and cranial shape data, have supported this claim by arguing that there has been more than one migration through this area, beginning around 130 kya.6
Connection with european early modern humans
The authors highlight one important aspect of the Daoxian fossils: while they represent modern humans in East Asia at 100 kya, no such people yet existed in Europe and would not for another 35 to 55 thousand years. The discoverers argue that, while the early moderns found no resistance when settling in China, Neandertals may have kept them out of Europe. I would suggest a far more compelling reason: the cold. The exit from Africa coincided with the beginning of the last glacial period in Europe, approximately 110 kya. The climate record shows that, at its height of the glaciation, the arctic tundra line, which is currently north of the Taiga belt (southern Norway and northern Siberia), was at Vienna. If modern humans had wanted to get into Europe just after they settled in the Near East, they would have had to go through either the Caucasus Mountains or the Bosporus. At the moment, there is no evidence they did. The earliest modern human remains, from the site of Mlade?, in Central Europe, are between 34 and 37 kya, during the last interglacial period, a time when the climate warmed up considerably. These early moderns show up at about the same time that we find our first modern stone tools in the area. I do not think this is coincidental.
Why this new find is not a problem
The fossil teeth from Daoxian cave firmly establish anatomically modern humans in East Asia at least by 80 kya and probably earlier. What they also reveal is that early modern humans can be found in southern China a scant 70 to 80 ky after their origin in Africa.
While these new data from China may seem to completely overturn what we thought we knew about early modern human migration patterns, this is not the case. Using genetics and the human fossil record, we have known for some time that the earliest moderns can be securely placed in East Africa and that they migrated to other parts of the world from there. We have also known that modern humans had become established in East Asia at 60 kya. What this new find does is extend that appearance back in time.
One of the joys of the scientific enterprise is that we are always discovering new things that challenge the way we think and evaluate what we already know. Human evolutionary studies are no different in this way.
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Charles Foster | Inhabit the World