Why the ancient humans of Asia are so mysterious

There are a lot of different kinds of uncertainty in science. Sometimes the uncertainty comes from having very little data. For instance, we know very little about the first couple million years of hominin evolution, because from that time period we have only a few specimens: the 7-million-year-old Sahelanthropus tchadensis, the 6-million-year-old Orrorin tugenensis and Ardipithecus kadabba from around 5.5 million years ago. That’s precious little for a 2-million-year span, so there’s only so much we can say.

A reconstruction of Homo longi, also known as dragon man
Chuang Zhao

However, we can also experience uncertainty when we have a shedload of data but don’t know how to interpret it. That’s the problem we face when we study Asian hominins from the past million years. There is an absolute blizzard of fossils, and no consensus on how to make sense of them. The Journal of Human Evolution has just published a special issue on the topic, so now seems like a good time to take a look.

Asian mysteries

First, some back story. The oldest known hominins are all African. The first species known to have migrated out of Africa was Homo erectus, which was living in what’s now Georgia 1.8 million years ago. H. erectus subsequently spread throughout much of mainland Asia, and also to what’s now Indonesia: they reached Java by perhaps 1.5 million years ago and survived there until 108,000 years ago.

Later on, several other hominins also roamed Asia. While we associate Neanderthals with Europe, they also lived in western Asia and sometimes went a long way east, for example to the Altai mountains of Siberia. Meanwhile, eastern Asia was home to the Denisovans, while the island of Flores had the hobbits (Homo floresiensis) and the Philippines had Homo luzonensis. It’s often hypothesised, or even assumed, that H. erectus was the ancestor of all these populations, but we don’t have DNA from H. erectus so we don’t actually know.

And then along came modern humans (Homo sapiens). Our species seems to have expanded from Africa in multiple waves: H. sapiens remains are known from Israel from 177,000 years ago and 120,000 years ago, but genetic evidence suggests most non-Africans today are descended from a migration out of Africa around 60,000 years ago. Other hominin groups vanish from the fossil record not long after.

In other words, there were two main waves of migration from Africa to Asia. The first was Homo erectus, which then gave rise to a bunch of other groups. The second was Homo sapiens, which replaced all of them.

Seems simple enough, right? The thing is, there’s a bunch of fossils from southern Asia that don’t neatly fit, and nobody is quite sure what to make of them.

The Hualongdong jawbone

One of the studies from the aforementioned special issue describes a lower jawbone from Hualongdong, a cave in eastern China. Researchers co-led by Wu Liu at the Chinese Academy of Sciences in Beijing re-assembled the bone and studied its shape.

They found it has a mix of features: some bits are chunky, some slender. In some ways it looks like a modern human jawbone. However, we have pronounced chins and the Hualongdong jawbone doesn’t, and on other measures it looks like an older species. The overall mix of features is unique.

The jawbone is about 300,000 years old. That’s a curious bit of timing, because it’s about when the oldest known Homo sapiens were living in what’s now Morocco, in northern Africa.

Is the Hualongdong jawbone an early Homo sapiens? It doesn’t have all the features, but then the oldest African specimens don’t either, and there’s a long-running argument over whether they count as “true” Homo sapiens or just a close ancestor. (Here is your regularly scheduled reminder that species demarcations are arbitrary, because evolution is a continuous process.)

On the other hand, if the Hualongdong jawbone really does belong to the same population as the Moroccan Homo sapiens, it would mean hominins made the journey from northern Africa to eastern China within a few tens of thousands of years. That’s surely possible but I would want to see evidence.

An alternative explanation is that the Hualongdong jawbone is a case of convergent evolution: that is, the same features evolved independently in certain east Asian and African populations. If this is so, we could interpret it as a bit of a weird coincidence, or as evidence that similar evolutionary pressures were acting on hominins in both Africa and Asia. Again, your mileage may vary on which interpretation is more plausible, but convergent evolution certainly seems like an option.

And remember, this is just one jawbone. There are lots of other hominin fossils from eastern Asia that are similarly odd. The Dali cranium from China is 260,000 years old and has also been (controversially) interpreted as early Homo sapiens. Other specimens don’t quite fit H. erectus or H. sapiens: some have old-looking shapes but lived recently, others look recent but are actually ancient. And we haven’t even got to the stone tool record, or instances of prehistoric art.

At this point you may be feeling a tad confused. If so, don’t panic. Remember, the whole point of this story is to explain why researchers aren’t sure what to make of the east Asian hominin record. The world experts on this topic are confused, and every new find seems to make it worse.

Say no to new species

Let’s see if we can make some sense out of all this. I’m not going to give you any kind of definitive story, but I’m going to offer the sorts of things that might bring some clarity.

I want to start by expressing deep wariness about claims of new species. The researchers studying the Hualongdong jawbone don’t say it’s a new species, and while I don’t think we know what it is, I think they’re right not to make such a claim.

You might remember that in 2021 researchers described a particularly big hominin cranium from Harbin in China, dated to between 146,000 and 296,000 years ago. Some of the team assigned it to a new species, Homo longi (dragon man). However, most think this is premature based on only one bone.

For one thing, dragon man might actually be a Denisovan, a group that so far is mostly known from their DNA. The same has been suggested of a cranium from Xujiayao in China, dated to around 160,000 years ago, and several other odd bones.

Elsewhere, an archaeological dig in Israel identified bones that researchers claimed were the common ancestor of modern humans and Neanderthals. Or they could be Neanderthals migrating back from Europe into Asia, giving a misleading impression.

Previous claims of new hominin species have sometimes fallen down when new evidence emerged. Longtime readers may remember my August 2022 newsletter, when I told the story of the Red Deer Cave people, also from China. When the bones were first described in 2012, they did not quite fit with Homo sapiens, so researchers speculated about a new species. However, when their DNA was sequenced in 2022, it turned out they were H. sapiens after all.

It might be asking too much to obtain DNA from these specimens, but we might be able to get proteins from their teeth: while not as informative, this would be enough to reveal evolutionary relationships.

Untangling the web

Now let’s try to make sense of the mess.

The first thing to say is that we should abandon the notion of simple stories. Remember, all of recorded history happened within just a few thousand years – whereas we’re talking about a timespan of 2 million years since Homo erectus first wandered outside of Africa. While we might discern some overall trends (like, say, hominins migrating out of Africa), we should expect to see a lot of confusing detail as populations moved back and forth, squabbled, intermarried, got lost in the desert and who knows what else.

In particular, I think we should consider human migrations much more carefully. The genetic evidence is unambiguous that our species hails from Africa: that conclusion was true in the 1990s and it’s only been strengthened by the swathes of genomic data we’ve obtained since. But that doesn’t mean the traffic was one way. Overall, yes, we come from Africa. But I’m now convinced there was a lot of back and forth.

This first became clear to me in 2021 when I wrote about the prehistory of Arabia. When the Arabian climate became more hospitable, people moved from Africa into Arabia – and when the climate turned on them they either moved back or died. There seem to have been at least five waves of migration into Arabia, spanning hundreds of millennia. Similarly, in last month’s newsletter I described what look like multiple migrations onto the Tibetan plateau.

More concretely, maybe some of the features we now associate with Homo sapiens first evolved in southern Asia and were then carried into Africa by a westward migration – before being carried right back out of Africa and all around the world when our species went global. This would explain the Hualongdong and Dali remains, without contradicting the genetics.

If scenarios like this happened, then western Asia – places like Arabia and the Middle East – played a key role in our evolution. Western Asia was the hub through which people and genes passed before heading north, west, south and east.

Perhaps in line with this, one of the other studies in the Journal of Human Evolution compares the teeth of two populations: Chinese Homo erectus, and Homo antecessor from northern Spain. Despite living around the same time, the two have distinct teeth, suggesting that they split from their shared ancestor at least 700,000 years ago. According to the researchers, “Southwest Asia may have played a relevant role in the origin of a common ancestor for these two taxa” and “could be the origin of some migrations toward eastern Asia, such as those included in Chinese H. erectus”.

If there’s one simple thing we can draw out of all this, it may be that prehistoric populations were more connected than we imagine. We tend to think of prehistoric people living in isolated tribes, each with its own patch of land. But that isolation is partly a reflection of how hunter-gatherers live today, when agriculture has forced them to the fringes of society.

In earlier times, the hunter-gatherer network spanned continents. I don’t mean that people living in modern Tanzania were personally acquainted with those living in what’s now Laos – clearly, they weren’t. But there was a long chain of communities linking them, like a giant game of Six Degrees of Kevin Bacon.

If that’s true, then once upon a time a new and advantageous gene variant could spread rapidly across thousands of kilometres, as people moved and intermarried. In other words, prehistory may have been globalised.

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