A new continent for human evolution

Abstract

Spectacular fossil finds made in China, along with an ambitious research programme and soon to be complemented by studies of ancient genomes, are raising the profile of Chinese palaeoanthropology. New findings are likely to add complexity to the established ‘out of Africa’ model describing the emergence of our species and its spread around the world. Michael Gross reports. Spectacular fossil finds made in China, along with an ambitious research programme and soon to be complemented by studies of ancient genomes, are raising the profile of Chinese palaeoanthropology. New findings are likely to add complexity to the established ‘out of Africa’ model describing the emergence of our species and its spread around the world. Michael Gross reports. Peking Man, a skull discovered near Beijing in 1929, is one of the widely known fossils of our hominin relatives, but has so far led a lonely life in a cul-de-sac of our evolutionary tree, now dated to around 780,000 years ago. Early movers like Homo heidelbergensis spread into Europe and Asia, but the history of humanity happened elsewhere, as our own lineage only arose much later, in Africa. The current standard model of human evolution posits that the ancestors of today’s non-African populations left the continent some 60,000 years ago and spread into Europe and Asia, with a small amount of interbreeding on the way. This led to the fractions of Neanderthal DNA detectable in all non-Africans and Denisovan DNA found in some parts of Asia and Oceania (Curr. Biol. (2014) 24, R295–R298). However, this view may still be biased by the fact that human and hominin remains are better preserved and more easily accessed in some regions than in others. Underappreciated regions like China may still change the story. Although researchers in China never had much luck with attempts to trace a line from Peking Man to the current Chinese population, recent discoveries may yet add interesting complexity to the evolution and migration of humans in Asia. One of the important recent findings directing attention of palaeoanthropologists towards China was the discovery of 47 well-preserved human teeth in an exceptionally well-defined context in the Fuyan Cave in Daoxian, Southern China, reported by the teams of Wu Liu and Xiu-jie Wu from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences at Beijing, together with Maria Martinón-Torres from University College London, UK (Nature (2015) 526, 696–699). Strikingly, these teeth are anatomically as modern as any described yet, although their minimal age can confidently be set to 80,000 years old. Essentially, the fossil-rich layer in which they were found was entirely covered and sealed by calcite flowstone, upon which a stalagmite grew. Thorium dating of the stalagmite yielded a minimum age of 80,100 years with a confidence interval of only 1,200 years. (This age is out of the range for carbon-dating methods, which can only be used for fossils up to around 50,000 years old.) The upper limit of their age has been estimated to 120,000 years based mainly on the mammalian fossils occurring in the same layer, which are typical of Late Pleistocene fauna. The presence of these teeth in southern China clearly contradicts the widely held belief that the first known migration of Homo sapiens into the Middle East failed to establish a lasting presence, and that the expansion of our species beyond the Arabic peninsula only succeeded around 60,000 years ago. This latter migration event is well established and constrained in time by the arrival of humans in Australia and in northern China around 40,000 to 50,000 years ago. In broad agreement with archaeological finds, recent studies of Australian genomes have dated the genetic split between Australo-Papuans and Asians to 58,000 years (Curr. Biol. (2016) 26, R1119–R1121). The Daoxian teeth, however, provide evidence that our species arrived in southern China at least 40,000 years earlier than in Europe and in northern China. While the route to Europe was shorter, climate considerations, especially during the last glacial period, would suggest that moving within the latitudes to which early humans were accustomed may have been easier for them, as it required fewer adaptation efforts. Similar considerations may apply to the expansion into the north of China, where archaic hominin remains were found to be roughly contemporary to the modern Homo sapiens finds in the south. In addition, the presence of Neanderthals in Europe, who had been away from the tropics for longer and were better adapted to the colder climate, may have slowed any northward expansion, as the authors note at the end of their paper. Whether or not the extinction of Neanderthals is causally linked to the nearly simultaneous arrival of modern humans is an open question. The early, yet modern-looking teeth from Daoxian appear to suggest that the presence of Neanderthals, along with the inclement weather, may have blocked the way for earlier expansions. Conceivably, the weakness and decline of Neanderthals may have opened the door into Europe 40,000 years ago. Alternatively, or additionally, modern humans forcing their way in may have violently displaced the previous residents.New land: Multiple sites in China have recently yielded fossil remains that may change our understanding of human evolution and dispersal. (Image: The Institute of Vertebrate Paleontology and Paleoanthropology, China (IVPP).)View Large Image Figure ViewerDownload Hi-res image Download (PPT) Thus, the earlier than anticipated globalisation of our species can also help us understand the details of what happened closer to the ancestral home of our species in Africa. DNA analyses from the teeth may provide the vital clues. The group of Qiaomei Fu at the IVPP is already working with the teams of Liu and Wu to extract and sequence DNA from the finds. Moreover, caves of the type studied in Daoxian are abundant in that area, so further excavations may yield additional insights into the lives of these early migrants. What palaeoanthropologists are most eagerly hoping for is the discovery of cultural artefacts such as tools associated with the human remains, which so far have not been found at Daoxian. Interest in Chinese excavation sites was further stoked earlier this year when the group of Xiu-jie Wu from the IVPP together with Erik Trinkaus from Washington University at St. Louis, USA, reported the discovery of two partial hominin skulls also dated to the Late Pleistocene, at an age of between 105,000 and 125,000 years (Science (2017) 355, 969–972). The fossils were excavated at the Lingjing site, in the Henan Province, central China. While they only represent the top of the head and are lacking faces, the skulls reassembled from scattered fragments are sufficiently complete to estimate brain sizes and allow detailed anatomical comparisons with contemporary Neanderthals and modern humans. The analyses showed that the skulls harboured surprisingly large brains, with the more complete skull yielding a volume at the upper end of the natural variation for both Neanderthals and modern humans. Moreover, they display a mixture of features known from Neanderthals and therefore considered archaic as well as modern features also found in modern humans. For instance, they have brow ridges like Neanderthals, but a thinner version of them. Other bone structures of the skulls are also less robust than in typical Neanderthals and thus more like modern humans. The researchers interpret their findings as evidence for a specific kind of Asian archaic hominin, which could be an Eastern variant of Neanderthal or something not yet described. “From their fossil record, eastern Asian late archaic humans have been interpreted to resemble their Neanderthal contemporaries to some degree, with considerations of whether the fragmentary remains of the former exhibit features characteristic of the latter. Yet it is only with the discovery of two human crania […] that the nature of these eastern Eurasian early Late Pleistocene archaic humans is becoming clear”, said group leader Xiu-jie Wu from the IVPP in a press statement. Co-author Erik Trinkaus sees the skulls as evidence of migration: “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 — one (iniac and nuchal morphology) unknown among earlier eastern crania, and the other (labyrinthine proportions) evident in only one similarly aged eastern Eurasian fossil — argue for populational interactions across Eurasia during the late Middle and early Late Pleistocene”, he said. “Similar interactions can be inferred from the presence of Neanderthal ancient DNA in western Siberia and in the Tianyuan 1 early modern human from northern China. These data therefore argue both for substantial regional continuity in eastern Eurasia into the early Late Pleistocene and for some level of east–west population interaction across Eurasia.” The authors carefully avoid any mention of a location some 4,000 km northwest of Lingjing, the Denisova cave. A bone fragment discovered in this cave yielded the genome analysis that sensationally established a third hominin species as a Late Pleistocene contemporary of Neanderthal and modern human, namely the Denisovans. Although no morphological features can so far be linked to this genome information, they have become an important part of the jigsaw of human evolution, as traces of their DNA survive in populations in south Asia and Oceania. In a news story accompanying the Lingjing report in Science, Ann Gibbons made the speculative link between the skulls and the genomes, citing several palaeoanthropologists who said that the skulls might very well turn out to belong to the elusive species. Wu and Trinkaus, however, resisted the temptation to speculate on such links, arguing that Denisovans are so far only defined by their DNA. Attempts to retrieve DNA from bone fragments found together with the Lingjing skulls have so far been unsuccessful, although Qiaomei Fu says that there is still the possibility that DNA may be recovered from the site. Thus, a connection cannot be proven yet — but neither can it be ruled out. As DNA evidence from the Chinese fossil finds is still lacking, there is the risk that genome and fossil evidence describe separate versions of the story that are difficult to reconcile. However, the IVPP is taking steps towards bridging that gap. It has set up a laboratory for the sequencing of ancient genomes led by Qiaomei Fu, who previously worked as a PhD student and postdoc with Svante Pääbo at the Max Planck Institute for Evolutionary Anthropology at Leipzig, Germany, where she became involved with the sequencing of the Neanderthal and Denisovan genomes. “Currently, the main purpose of this laboratory is to research topics in evolutionary and population genetics, using molecular genetic approaches to clarify ancient human relationships, archaic hominid introgression into modern humans, migratory routes of early modern humans into Asia and interactions between early modern humans and contemporary local modern humans in Asia,” says Fu, who last year was celebrated as one of the ‘Science stars of China’ (Nature (2016) 534, 456–461). “I certainly think that China is one of the most interesting areas of the world in terms of exploring genetic relationships among ancient populations and fossil groups,” Pääbo comments. “Fortunately, the IVPP has established a state-of-the-art laboratory to study ancient DNA. We are fortunate to work together with them on this.” Chinese researchers are now well positioned to fill in what used to be a large white space in the map of human evolution and dispersal. Further carefully excavated and characterised finds along with the advanced genome sequencing facility could soon establish the missing connections and, for instance, give Denisovans a face, as well as place fossil finds in the genetically established family tree of our species. Pääbo is optimistic about the prospects of China’s quest for human origins: “I think that we can look forward to many new insights from China in the next few years,” he concludes.