THE year is 1933, and China is partly under the control of invading Japanese forces. During construction of a bridge near Harbin in the north-east, a local man working for the Japanese makes a stunning discovery: an ancient human skull. He recognises the skull鈥檚 value right away. Determined not to hand the fossil over to the occupiers, he buries it in an abandoned well.
There the skull remained until 2018, when, shortly before his death, the man told his family of its existence. In due course, the Harbin skull came into scientific hands. It was worth the wait, not least for the clues it offers those searching for the enigmatic Ancestor X聽鈥� the species that gave rise to humanity.
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For as long as researchers have been finding ancient human-like fossils, one question has聽been nagging away in the back of their minds: how are we related to other prehistoric groups and species? In particular, how do we fit聽in with the Neanderthals and Denisovans who shared Earth with us for most of our existence? It is this question that the discovery of Ancestor X promises to answer. However, the hunt for it聽has been fiendish, with major twists and turns聽along the way. In recent years, it has also聽led to some surprising discoveries, including the revelation that we had misunderstood the way our species, Homo sapiens, evolved. Now, with the re-emergence of the Harbin skull, the search for Ancestor X looks set to get easier in some ways聽鈥� and a lot聽more difficult in others.
When it comes to the big picture of human evolution, universal consensus is hard to find.聽However, by the final decades of the 20th聽century, many researchers accepted a聽few聽key points. The first was that all living people today are so genetically similar to one another that 贬.听蝉补辫颈别苍蝉 must have emerged no聽more than a few hundred thousand years ago. The second was that this happened in a聽corner of Africa, which would explain why most of the genetic diversity that exists in our species today is largely concentrated in Africa. A third widely held idea was that our parent species also gave rise to a second species, Homo neanderthalensis, the Neanderthals. In聽other words, the two species share a common ancestor, dubbed Ancestor X.
Putting all the pieces together, a relatively simple picture emerged. Several hundred thousand years ago, our parent species was present in parts of Africa and Europe. Then, a聽small population of this Ancestor X became isolated in Africa for long enough to evolve new genetic and physical traits and give rise to聽our species. At roughly the same time, another small population of Ancestor X became isolated in Europe and evolved into the聽Neanderthals. Eventually, the isolation ended. 贬.听蝉补辫颈别苍蝉 and Neanderthals both began to spread, consigning all remaining populations of Ancestor X to extinction. Ultimately, 贬.听蝉补辫颈别苍蝉 spread so far and wide聽that it encountered Neanderthals聽鈥� and聽sent them to extinction too.
With this picture in place, the search for Ancestor X was on. By the early 1980s, at the Natural History Museum in London thought he had found it. He had spent years scrutinising the shape of Neanderthal and H. sapiens skulls to work out what they had in common: features they had probably inherited from Ancestor X. Both had large brains, similarly shaped temporal bones 鈥� pieces of the skull on each side of the head that house the ears 鈥� and other features. And, bingo, these were also seen in a third hominin, Homo heidelbergensis. What鈥檚 more, H. heidelbergensis had been present in parts of Africa and Europe between about 300,000 and 500,000 years ago. 鈥淚 thought: well there you are,鈥� says Stringer. 鈥淚t鈥檚 in the right place at the right time to be the common ancestor.鈥�
Common ancestor
But in the 2010s, he and many other researchers had second thoughts. Some of the best evidence about H. heidelbergensis had come from fossils found at Sima de los Huesos in Spain, and by the mid 2010s, it was clear that they actually looked more like proto-Neanderthals. An analysis of ancient DNA published in 2016 lent further weight to the idea that the Sima fossils were early Neanderthals. Furthermore, using this finding together with their age 鈥� around 430,000 years old 鈥� researchers could refine the estimate for when Ancestor X had walked the planet. This pushed it back to between 550,000 and 765,000 years ago. Today, some think even this is an underestimate. For instance, a 2019 analysis of Neanderthal and H. sapiens teeth suggested that Ancestor X was on Earth 800,000 years ago or more. The message was clear: H. heidelbergensis was too young to be Ancestor X.
No matter, though, because a new Ancestor X candidate was already waiting in the wings. Curiously, this suspect 鈥� Homo antecessor 鈥� had been discovered at a site just a few hundred metres to the north of Sima de los Huesos. However, it lived about 800,000 to 850,000 years ago, making it a far more likely Ancestor X, at least on the grounds of age.
If H. antecessor is Ancestor X, it literally puts a different face on the story of recent human evolution. 鈥淗. antecessor exhibits the oldest known modern face so far found,鈥� says at the Spanish National Research Centre for Human Evolution in Burgos. Its delicate cheekbones and flattened features are similar to those seen in living humans and are unlike the heavily built faces of Neanderthals. Because of this, there is now a surprising hypothesis about Ancestor X: perhaps it looked a little like us. 鈥淥ur so-called modern face is an ancient face,鈥� says Stringer. The Neanderthal face might actually be far more evolved.
That being said, Stringer doesn鈥檛 think H. antecessor is Ancestor X itself. In a 2016 analysis in which he considered all we can now infer about our parent species, he concluded that Ancestor X , while the part of the skull encasing the brain looked like that of a primitive H. heidelbergensis cranium , Italy. Such fossils are presumably out there somewhere, but they have yet to be discovered. The hunt goes on.
Meanwhile, there are still a few researchers betting on H. heidelbergensis, including Axel Timmermann at Pusan National University in South Korea and his colleagues, who . They ran a聽climate model on a supercomputer for six聽months to reconstruct how temperature and rainfall might have shaped what resources were available to hominins over the past 2聽million years. They then combined their results with thousands of fossils and other archaeological evidence to work out where and聽when six species of humans聽鈥� including the early 贬.听蝉补辫颈别苍蝉聽鈥� could have lived. Among other things, their results suggested that our聽species evolved from 贬.听丑别颈诲别濒产别谤驳别苍蝉颈蝉 in聽southern Africa as conditions became hotter聽and more arid.
It is a minority view, though. Most researchers now see hints in the fossils that 贬.听丑别颈诲别濒产别谤驳别苍蝉颈蝉 was more closely related to聽the proto-Neanderthals than to our species. That would make it a dead-end side branch of聽our family tree that has no relevance for聽understanding the origin of 贬.听蝉补辫颈别苍蝉. However, a new twist in the story suggests they聽could be wrong.
The problem is that we appear to have fundamentally misunderstood the way human evolution works. 鈥淭he idea humans originated from a small region [of Africa] doesn鈥檛 make much sense,鈥� says at the University of Toulouse, France. 鈥淚t was clear that it was problematic at least 20 years ago.鈥� At that time, he and David Goldstein, now at Columbia University Irving Medical Center in New York, wrote a review paper explaining why.
The science is complicated 鈥� so complicated that it seems many non-geneticists missed the nuance 鈥� but Chikhi says the genetic signals in living humans imply that 贬.听蝉补辫颈别苍蝉 emerged as a 鈥渕etapopulation鈥� spread over a wide geographical area where聽several 鈥渟ubpopulations鈥� . Each of聽these subpopulations was characterised by聽a聽subtly distinct genetic signature聽鈥� and potentially a subtly distinct look.
Significantly, the idea fits well with mounting evidence on the ground. Across Africa, there are fossils between about 200,000 and 300,000 years old that look superficially like H. sapiens, but that each carry a unique mix of physical features. They appear to be representatives of some of the subpopulations that Chikhi says mixed and interbred to establish our species as a metapopulation. This idea, published in 2018, has come to be known as African multiregionalism, but Chikhi and Stringer聽鈥� another advocate of the idea聽鈥� prefer the term pan-African evolution.
This phenomenon may have implications for H. heidelbergensis. A fossil skull from Zambia, labelled as H. heidelbergensis, has recently been dated and . This means 贬.听丑别颈诲别濒产别谤驳别苍蝉颈蝉 was still in Africa at the time聽贬.听蝉补辫颈别苍蝉 was emerging across the continent. And although 贬.听蝉补辫颈别苍蝉 probably traces most of its ancestry to Ancestor X, Stringer says it is possible that one or more 贬.听丑别颈诲别濒产别谤驳别苍蝉颈蝉 groups were among the subpopulations that contributed some DNA to聽our species. To put it another way, even if 贬.听丑别颈诲别濒产别谤驳别苍蝉颈蝉 isn鈥檛 Ancestor X, it might still be among our ancestors.
Dragon Man
Arguably, an even bigger implication of the pan-African evolution model is that it probably applies to the origin of other hominins聽鈥� including Ancestor X. If so, our immediate ancestor also evolved through the mixing of numerous subpopulations of its own parent species spread over a wide geographical area. But where in the world did that happen?
To address this question, international collaborative work is required, particularly between Western and Asian scientists because, in recent decades, it has become increasingly clear that Asia is another hotbed of human evolution. Unfortunately, that hasn鈥檛 always been forthcoming. 鈥淔or a long time, a few researchers had this attitude: we don鈥檛 really trust Asian science, we don鈥檛 even trust the dates on their fossils,鈥� says at the University of Winnipeg, Canada. Whatever the causes (see 鈥淓ast-West relations鈥�), she thinks the situation is finally beginning to change. 鈥淭hankfully, there is now more respect, and more of a push for international collaborations that lead to new ideas,鈥� she says.
Which brings us back to the Harbin skull. It was clear from the moment it came to light that it was a significant find, particularly since, like H. antecessor, it had a face strikingly similar to our own. Last year, Xijun Ni at the Chinese Academy of Sciences and his colleagues, including Stringer, of the fossil, which is at least 146,000 years old. In particular, they wanted to work out where it sits in our evolutionary tree. Surprisingly, the study suggested the Harbin skull 鈥� and a few other fossils, many of them unearthed in China 鈥� belong to a previously unrecognised ancient human population that is evolutionarily even closer to H. sapiens than the Neanderthals. In an accompanying paper, Ni and some of his colleagues. Stringer isn鈥檛 a聽fan of the name because it supersedes one already given to similar fossils, but he doesn鈥檛 mind referring to the new human by its informal nickname: Dragon Man.
This could prove significant for those still seeking Ancestor X. Although analysis of the Harbin skull is ongoing, Stringer speculates that Dragon Man might have emerged from Ancestor X at around the same time H. sapiens and Neanderthals did, implying that , not one of twins. Perhaps, he adds, this third human group will turn out to contain the mysterious Denisovans, discovered in 2010 and considered a sort of Asian sister group to Neanderthals. Discovering the ways in which these sibling species are similar to or different from our own聽may offer key insights into our evolution. Even if we accept our species evolved in Africa, 鈥渆ast Asia is an important region for studying the origin of 贬.听蝉补辫颈别苍蝉鈥�, says Ni.
These developments are a mixed blessing. On the one hand, fossils like Harbin could help further refine our ideas about Ancestor X鈥檚 physical appearance, which might make our elusive parent species easier to recognise when we eventually find it. But on the flip side, Harbin-like fossils imply that the search might be harder than we thought, because Ancestor X could have lived almost anywhere within a truly vast geographical region. 鈥淚t could have been in Africa,鈥� says Stringer. 鈥淏ut it could have been in Europe 鈥� that鈥檚 where H. antecessor is 鈥� or it could have been in west Asia. It could even have been in east Asia. We just don鈥檛 know yet.鈥�
East-West relations
Our species was long thought to have evolved from a small, heterogeneous population living in a tiny corner of Africa. Evidence countering this idea emerged two decades ago, but is only now being widely recognised. at聽Texas A&M University suspects that cultural influences may help explain this. 鈥淎 single 鈥榩ure鈥� form evolving and spreading to replace all others? That鈥檚 colonialism in Europe and that鈥檚 manifest destiny in the United States,鈥� she says.
To be clear, she doesn鈥檛 mean that all researchers of European ancestry are racist, but that racist cultural ideas may influence their work, if unconsciously. What鈥檚 more, a few of these same researchers have been quick to criticise their colleagues in Asia and elsewhere of being under the聽influence of perceived cultural聽biases, she says. This has聽discouraged collaboration and聽made it difficult to work out how the extraordinary fossil finds made in Asia fit into the bigger picture of human evolution.
鈥淭he view is that Asian scientists are biased and ethnocentric, that they like the idea of long-standing regional continuity because the alternative is a replacement model that suggests they haven鈥檛 been in聽the聽area for as long,鈥� says Athreya. She thinks this view is聽incorrect, offensive and聽鈥� above聽all聽鈥� counterproductive because it has encouraged some聽Western researchers to dismiss almost all scientific findings made by human evolution scientists in Asia. Fortunately, attitudes are changing.
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