Fossil finds in China are challenging ideas about the evolution of modern humans and our closest relatives

July 15, 2016, Chinese Academy of Sciences

On the outskirts of Beijing, a small limestone mountain named Dragon Bone Hill rises above the surrounding sprawl. Along the northern side, a path leads up to some fenced-off caves that draw 150,000 visitors each year, from schoolchildren to grey-haired pensioners. It was here, in 1929, that researchers discovered a nearly complete ancient skull that they determined was roughly half a million years old. Dubbed Peking Man, it was among the earliest human remains ever uncovered, and it helped to convince many researchers that humanity first evolved in Asia.

Since then, the central importance of Peking Man has faded. Although modern dating methods put the fossil even earlier—at up to 780,000 years old—the specimen has been eclipsed by discoveries in Africa that have yielded much older remains of ancient human relatives. Such finds have cemented Africa's status as the cradle of humanity—the place from which modern humans and their predecessors spread around the globe—and relegated Asia to a kind of evolutionary cul-de-sac.

But the tale of Peking Man has haunted generations of Chinese researchers, who have struggled to understand its relationship to modern humans. "It's a story without an ending," says Wu Xinzhi, a palaeontologist at the Chinese Academy of Sciences' Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) in Beijing. They wonder whether the descendants of Peking Man and fellow members of the species Homo erectus died out or evolved into a more modern species, and whether they contributed to the gene pool of China today.

Keen to get to the bottom of its people's ancestry, China has in the past decade stepped up its efforts to uncover evidence of early humans across the country. It is reanalysing old fossil finds and pouring tens of millions of dollars a year into excavations. And the government is setting up a US$1.1-million laboratory at the IVPP to extract and sequence ancient DNA.

The investment comes at a time when palaeoanthropologists across the globe are starting to pay more attention to Asian fossils and how they relate to other early hominins—creatures that are more closely related to humans than to chimps. Finds in China and other parts of Asia have made it clear that a dazzling variety of Homo species once roamed the continent. And they are challenging conventional ideas about the evolutionary history of humanity.

"Many Western scientists tend to see Asian fossils and artefacts through the prism of what was happening in Africa and Europe," says Wu. Those other continents have historically drawn more attention in studies of human evolution because of the antiquity of fossil finds there, and because they are closer to major palaeoanthropology research institutions, he says. "But it's increasingly clear that many Asian materials cannot fit into the traditional narrative of human evolution."

Chris Stringer, a palaeoanthropologist at the Natural History Museum in London, agrees. "Asia has been a forgotten continent," he says. "Its role in human evolution may have been largely under-appreciated."

Evolving story

Em sua forma típica, a história do Homo sapiens começa na África. Os detalhes exatos variam de um para outro, mas os principais personagens e eventos geralmente permanecem os mesmos. E o título é sempre 'Fora da África'.

Nesta visão padrão da evolução humana, o H. erectus evoluiu pela primeira vez há mais de 2 milhões de anos (ver "Duas rotas para a evolução humana"). Então, algum tempo antes de 600.000 anos atrás, deu origem a uma nova espécie: Homo heidelbergensis, os remanescentes mais antigos dos quais foram encontrados na Etiópia. Cerca de 400.000 anos atrás, alguns membros da H. heidelbergensis deixaram a África e se dividiram em dois ramos: um se aventurou no Oriente Médio e na Europa, onde evoluiu para os neandertais; o outro foi para o leste, onde os membros se tornaram Denisovans - um grupo descoberto pela primeira vez na Sibéria em 2010. O restante da população de H. heidelbergensis na África acabou por evoluir para a nossa própria espécie, o H. sapiens, há cerca de 200.000 anos. Então esses primeiros humanos expandiram seu alcance para a Eurásia, 60.000 anos atrás, onde substituíram os hominídeos locais por uma quantidade minúscula de cruzamentos.

A hallmark of H. heidelbergensis—the potential common ancestor of Neanderthals, Denisovans and modern humans—is that individuals have a mixture of primitive and modern features. Like more archaic lineages, H. heidelbergensis has a massive brow ridge and no chin. But it also resembles H. sapiens, with its smaller teeth and bigger braincase. Most researchers have viewed H. heidelbergensis—or something similar—as a transitional form between H. erectus and H. sapiens.

Unfortunately, fossil evidence from this period, the dawn of the human race, is scarce and often ambiguous. It is the least understood episode in human evolution, says Russell Ciochon, a palaeoanthropologist at the University of Iowa in Iowa City. "But it's central to our understanding of humanity's ultimate origin."

The tale is further muddled by Chinese fossils analysed over the past four decades, which cast doubt over the linear progression from African H. erectus to modern humans. They show that, between roughly 900,000 and 125,000 years ago, east Asia was teeming with hominins endowed with features that would place them somewhere between H. erectus and H. sapiens, says Wu (see 'Ancient human sites'). 

"Those fossils are a big mystery," says Ciochon. "They clearly represent more advanced species than H. erectus, but nobody knows what they are because they don't seem to fit into any categories we know."

The fossils' transitional characteristics have prompted researchers such as Stringer to lump them with H. heidelbergensis. Because the oldest of these forms, two skulls uncovered in Yunxian in Hubei province, date back 900,000 years1, 2, Stringer even suggests that H. heidelbergensis might have originated in Asia and then spread to other continents.

But many researchers, including most Chinese palaeontologists, contend that the materials from China are different from European and African H. heidelbergensis fossils, despite some apparent similarities. One nearly complete skull unearthed at Dali in Shaanxi province and dated to 250,000 years ago, has a bigger braincase, a shorter face and a lower cheekbone than most H. heidelbergensis specimens3, suggesting that the species was more advanced.

Such transitional forms persisted for hundreds of thousands of years in China, until species appeared with such modern traits that some researchers have classified them as H. sapiens. One of the most recent of these is represented by two teeth and a lower jawbone, dating to about 100,000 years ago, unearthed in 2007 by IVPP palaeoanthropologist Liu Wu and his colleagues4. Discovered in Zhirendong, a cave in Guangxi province, the jaw has a classic modern-human appearance, but retains some archaic features of Peking Man, such as a more robust build and a less-protruding chin.

Most Chinese palaeontologists—and a few ardent supporters from the West—think that the transitional fossils are evidence that Peking Man was an ancestor of modern Asian people. In this model, known as multiregionalism or continuity with hybridization, hominins descended from H. erectus in Asia interbred with incoming groups from Africa and other parts of Eurasia, and their progeny gave rise to the ancestors of modern east Asians, says Wu.

Support for this idea also comes from artefacts in China. In Europe and Africa, stone tools changed markedly over time, but hominins in China used the same type of simple stone instruments from about 1.7 million years ago to 10,000 years ago. According to Gao Xing, an archaeologist at the IVPP, this suggests that local hominins evolved continuously, with little influence from outside populations.

Politics at play?

Some Western researchers suggest that there is a hint of nationalism in Chinese palaeontologists' support for continuity. "The Chinese—they do not accept the idea that H. sapiens evolved in Africa," says one researcher. "They want everything to come from China."

Chinese researchers reject such allegations. "This has nothing to do with nationalism," says Wu. It's all about the evidence—the transitional fossils and archaeological artefacts, he says. "Everything points to continuous evolution in China from H. erectus to modern human."

Dozens of teeth from a cave in Daoxian, China, have been attributed to modern humans and date to 120,000–80,000 years ago. Credit: S. Xing and X-J. Wu

Mas o modelo de continuidade com hibridização é combatido por esmagadores dados genéticos que apontam para a África como a fonte dos humanos modernos. Estudos de populações chinesas mostram que 97,4% de sua constituição genética é de ancestrais humanos modernos da África, com o restante vindo de formas extintas como os neandertais e os denisovanos5. "Se houvesse contribuições significativas do H. erectus chinês, eles apareceriam nos dados genéticos", diz Li Hui, geneticista populacional da Universidade de Fudan, em Xangai. Wu argumenta que a contribuição genética dos homininos arcaicos na China poderia ter sido perdida porque nenhum DNA ainda foi recuperado deles.

Many researchers say that there are ways to explain the existing Asian fossils without resorting to continuity with hybridization. The Zhirendong hominins, for instance, could represent an exodus of early modern humans from Africa between 120,000 and 80,000 years ago. Instead of remaining in the Levant in the Middle East, as was thought previously, these people could have expanded into east Asia, says Michael Petraglia, an archaeologist at the University of Oxford, UK.

Other evidence backs up this hypothesis: excavations at a cave in Daoxian in China's Hunan province have yielded 47 fossil teeth so modern-looking that they could have come from the mouths of people today. But the fossils are at least 80,000 years old, and perhaps 120,000 years old, Liu and his colleagues reported last year6. "Those early migrants may have interbred with archaic populations along the way or in Asia, which could explain Zhirendong people's primitive traits," says Petraglia.

Another possibility is that some of the Chinese fossils, including the Dali skull, represent the mysterious Denisovans, a species identified from Siberian fossils that are more than 40,000 years old. Palaeontologists don't know what the Denisovans looked like, but studies of DNA recovered from their teeth and bones indicate that this ancient population contributed to the genomes of modern humans, especially Australian Aborigines, Papua New Guineans and Polynesians—suggesting that Denisovans might have roamed Asia.

María Martinón-Torres, a palaeoanthropologist at University College London, is among those who proposed that some of the Chinese hominins were Denisovans. She worked with IVPP researchers on an analysis7, published last year, of a fossil assemblage uncovered at Xujiayao in Hebei province—including partial jaws and nine teeth dated to 125,000–100,000 years ago. The molar teeth are massive, with very robust roots and complex grooves, reminiscent of those from Denisovans, she says.

A third idea is even more radical. It emerged when Martinón-Torres and her colleagues compared more than 5,000 fossil teeth from around the world: the team found that Eurasian specimens are more similar to each other than to African ones8. That work and more recent interpretations of fossil skulls suggest that Eurasian hominins evolved separately from African ones for a long stretch of time. The researchers propose that the first hominins that left Africa 1.8 million years ago were the eventual source of modern humans. Their descendants mostly settled in the Middle East, where the climate was favourable, and then produced waves of transitional hominins that spread elsewhere. One Eurasian group went to Indonesia, another gave rise to Neanderthals and Denisovans, and a third ventured back into Africa and evolved into H. sapiens, which later spread throughout the world. In this model, modern humans evolved in Africa, but their immediate ancestor originated in the Middle East.

Not everybody is convinced. "Fossil interpretations are notoriously problematic," says Svante Pääbo, a palaeogeneticist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. But DNA from Eurasian fossils dating to the start of the human race could help to reveal which story—or combination—is correct. China is now making a push in that direction. Qiaomei Fu, a palaeogeneticist who did her PhD with Pääbo, returned home last year to establish a lab to extract and sequence ancient DNA at the IVPP. One of her immediate goals is to see whether some of the Chinese fossils belong to the mysterious Denisovan group. The prominent molar teeth from Xujiayao will be an early target. "I think we have a prime suspect here," she says.

Fuzzy picture

Apesar das diferentes interpretações do registro fóssil chinês, todos concordam que o conto evolucionário na Ásia é muito mais interessante do que as pessoas apreciaram antes. Mas os detalhes permanecem confusos, porque tão poucos pesquisadores escavaram na Ásia.

Quando eles o fizeram, os resultados foram surpreendentes. Em 2003, uma escavação na ilha das Flores, na Indonésia, resultou em uma pequena hominidade9, que os pesquisadores chamaram de Homo floresiensis e apelidaram o hobbit. Com sua estranha variedade de feições, a criatura ainda provoca debates sobre se é uma forma anã do H. erectus ou alguma linhagem mais primitiva que chegou até a África e sudeste da Ásia e viveu até recentemente, há 60 mil anos. No mês passado, mais surpresas surgiram de Flores, onde pesquisadores encontraram os restos de um hominídeo semelhante ao hobbit em rochas de cerca de 700.000 anos10.

Recuperar mais fósseis de todas as partes da Ásia ajudará claramente a preencher as lacunas. Muitos paleoantropólogos também pedem melhor acesso aos materiais existentes. A maioria dos fósseis chineses - incluindo alguns dos melhores espécimes, como os crânios de Yunxian e Dali - são acessíveis apenas a um punhado de paleontólogos chineses e seus colaboradores. "Disponibilizá-los para estudos gerais, com réplicas ou tomografias, seria fantástico", diz Stringer. Além disso, os sítios fósseis devem ser datados de maneira muito mais rigorosa, preferencialmente por múltiplos métodos, dizem os pesquisadores.

Mas todos concordam que a Ásia - o maior continente da Terra - tem muito mais a oferecer em termos de desvendar a história humana. "O centro de gravidade", diz Petraglia, "está mudando para o leste".

Explore further: Largest group of fossil humans are Neanderthals after all

More information: Jane Qiu. How China is rewriting the book on human origins, Nature (2016). DOI: 10.1038/535218a

Journal reference: Nature

Provided by: Chinese Academy of Sciences