本文作者：小红猪小分队

Source：http://www.newscientist.com/article/mg21829162.200-out-of-asia-our-surprising-origins.html?full=true

With the evolution of the genus Homo, our ancestors became distinctly human. Now we have hints that this great event occurred not in Africa but in Eurasia

IT WAS the evolutionary find of the century: a diminutive human-like fossil that called into question basic assumptions about the origin of our species. No, not Homo floresiensis, aka the hobbit, discovered in Indonesia a decade ago this September. This troublemaker, an equally tiny member of the human lineage – the hominins – was unearthed some 80 years earlier. The discovery in 1924 of the Taung child astonished and unsettled researchers because it didn’t fit their picture of human evolution. It came from a continent few thought was key for human evolution: Africa.

No one now doubts that Africa was the cradle of humanity – the idea goes back to Darwin after all – but early in the 20th century most researchers believed we evolved in Eurasia. The Taung child was the first fossil to challenge that orthodoxy. And Raymond Dart, an anthropologist at the University of Witwatersrand in Johannesburg, South Africa, earned himself an academic mauling for placing his discovery in a brand new hominin category he named Australopithecus (“southern ape”). It would take his critics several decades to admit they were wrong.

But could they have been right about Eurasia after all? Recently, some prominent researchers have come round to the idea that hominins may have left their African cradle much earlier than we thought and undergone critical evolutionary transitions further north. There are even whispers that one of the most important evolutionary events of all – the appearance of our genus,Homo – may have occurred under Eurasian rather than African skies. And the catalyst behind this radical rethink? That pesky little hobbit.

From the beginning, the hobbit, like the Taung child, did not fit the standard picture of human evolution (Nature, vol 431, p 1055). Some of the remains found on the Indonesian island of Flores were just 18,000 years old. So the hobbit was alive at least 10,000 years after every other hominin except our own species had become extinct. The one relatively complete skeleton belonged to an individual who was barely 1 metre tall. Bone fragments suggested other hobbits were even shorter. Then there was the skull. The cranial volume of the single hobbit skull found so far is around 420 cubic centimetres, about one-third the size of a modern human’s. Yet stone tools found with hobbit bones suggested that the hominin was capable of sophisticated behaviour. “It drove me nuts in 2003,” says Peter Brown at the University of New England in Armidale, Australia, who led the team that made the discovery. “What do you compare a unique small-bodied, small-brained hominin dated to only 18,000 years with?”

Some felt the answer was obvious. Some modern humans are born with unusual diseases that arrest growth of the cranium. The hobbit skull could have belonged to one such individual and, in fact, be human (New Scientist, 14 October 2006, p 17).

Robert Martin at the Field Museum in Chicago is a proponent of this idea. He points out that hominin brains have gradually got bigger throughout our lineage’s evolutionary history. If the 18,000-year-old hobbit was really a product of human evolution, it should have had a skull roughly the same size as ours, he says. Instead it is similar to one of Dart’s australopiths, which went extinct about 1.2 million years ago. The body size is also unusual, he adds, but then again some human populations today are small. “The most likely explanation for that small brain is a pathology – especially considering we only have the skull from one individual,” says Martin. Unusual asymmetries in the hobbit skull also suggest that there was something unhealthy about this individual, according to two more supporters of the idea, Robert Eckhardt at Pennsylvania State University in University Park and Maciej Henneberg at the University of Adelaide, Australia.

For Dean Falk at Florida State University in Tallahassee, this kind of scepticism is no more than a repeat of the reception that greeted Dart in the 1920s. Having studied scans of the hobbit skull to work out the shape of its brain, she concludes that H. floresiensis is indeed a new species because of its tiny skull – not in spite of it (Journal of Human Evolution, vol 57, p 597). Any unusual asymmetry is simply down to crushing during burial. But she accepts that this interpretation attracts criticism because it threatens the orthodoxy. “The moment the first Homo floresiensis paper was published there was a vociferous group of scientists screaming ‘no, no, no’,” she says. “It was the same old story as Taung – things haven’t changed.”

Falk’s work suggests H. floresiensis is closely related to a larger extinct hominin called Homo erectus, which we know lived in Indonesia between about 1.7 million and 550,000 years ago. With evidence of 840,000-year-old stone tools on Flores, it is possible that a population of H. erectus arrived on the island around that time and became isolated there. Animals often shrink when isolated on islands, so that may partly explain the hobbit’s small size.

It is another point on which Martin begs to differ. “There is simply no good precedent for island dwarfism affecting brain size in any primate,” he says. The debate continues to rage. “It’s very acrimonious,” says Falk.

Meanwhile, Brown and others have shifted their attention below the hobbit’s neck. There may be only one skull, but there is a veritable treasure trove of bones belonging to at least four hobbits. These show a consistent pattern of features, and together they indicate that hobbits had shoulders, wrists, limb proportions and feet that are, if anything, even more bizarre than the contentious skull (Journal of Human Evolution, vol 57, p 538). “There were just so many primitive features that the hobbit team began to talk seriously about Homo floresiensis being derived from something more primitive thanHomo erectus,” says William Jungers at Stony Brook University in New York, who led some of the analyses. According to Brown, somewhere towards the very top of the list of the hobbit’s likely recent ancestors is one of Dart’s australopiths.

This idea borders on the incendiary. It is one thing to declare the hobbit a valid species and break the cardinal rule that hominin brains get bigger over time. But to go further and suggest the tiny hominin evolved directly from an australopith threatens the decades-old central narrative of the human evolutionary story.

“We’re taught that Australopithecus only ever lived in Africa – that’s textbook,” says Falk. Conventional wisdom suggests the australopiths evolved in Africa about 4 million years ago and died out there 2.8 million years later. Perhaps it was their short legs that discouraged them from making the long trek out of Africa. Certainly it wasn’t until the taller members of our own genus appeared, right towards the end of the australopith age, that hominins began to explore the wider world.

Out of Africa, then back again?

The hobbit remains hint at an alternative. Perhaps an australopith did manage to escape Africa before the Homo genus evolved, and perhaps it survived long enough in Eurasia to evolve into the hobbit.

If so, shouldn’t we have found some fossil evidence for these ancient Eurasian australopiths by now? Not necessarily, says Brown. Environmental conditions in East and South Africa favoured preservation of human fossils in a way that conditions across Asia did not, he says. For Martin, however, the very idea of Eurasian australopiths is untenable. “I can’t understand my colleagues,” he says. “They are incredibly careful to stamp all over suggestions of anything remotely out of the ordinary in human evolutionary thought, and yet some of them swallow this notion completely. There’s not a scrap of fossil evidence to support this idea.”

That’s not entirely true, though. There is one site in Eurasia that could fit with the idea that australopith-like hominins made it out of Africa. What’s more, there are hints that these enigmatic Eurasian australopiths did more than evolve into the hobbits found on Flores: they may have given rise to our own genus.

In 1991, researchers excavating the medieval town of Dmanisi, Georgia, in the Caucasus came across the earliest hominin remains found outside Africa so far. There is still some debate over exactly where the 1.77-million-year-old Dmanisi hominins fit in the human evolutionary tree, but most would classify them as H. erectus. Their age and primitive features suggest they were among the earliest members of this species, implying that H. erectus wasted little time in leaving the East African region in which it first appeared perhaps 1.87 million years ago. Conventional thinking has it that this was the first time a hominin ventured out of Africa, with Dmanisi offering a unique snapshot into the very moment that humans went global.

Then, in 2011, came surprising news from Dmanisi that challenged this picture. Continued excavations had found evidence that the Georgian site was first occupied at least 1.85 million years ago – essentially at the same time that H. erectus appeared in East Africa. This led David Lordkipanidze of the Georgian National Museum in Tbilisi and his colleagues to consider an extraordinary alternative human history. H. erectus could have evolved in Eurasia, they say. If so, the fossils at Dmanisi are not a snapshot of the first hominin migration north out of Africa, but rather catch H. erectus in the act of migrating south into the land of its forefathers (PNAS, vol. 108, p 10432). “It has to be a feasible idea,” says Lordkipanidze.

More broadly, the new dates of occupation at Dmanisi mean that H. erectuscould have evolved from an australopith that left Africa around or before the 2-million-year mark. Falk says her work on the hobbit skull is consistent with this view. The similarities she identified between its brain shape and that ofH. erectus could be explained if both arose from a common ancestor in Eurasia, rather than the hobbit being descended from H. erectus.

While the Dmanisi fossils add extra spice to the hobbit story, when it comes to understanding our own evolution it’s the H. erectus story that really matters. “I think Homo erectus is the first hominin for which you can make a case that it belongs to our genus, Homo, without having to explain away any important exceptions,” says Bernard Wood at George Washington University in Washington DC. A more primitive species, Homo habilis, often placed in our genus, may not belong there or be a direct ancestor, he adds (PNAS, vol 108, p 10375). Crucially, H. erectus is often seen as the direct ancestor of our species. So if it evolved in Eurasia before moving into Africa where our species evolved about 200,000 years ago, modern humanity is arguably the product of both an African and a Eurasian cradle (see diagram).

Wood stresses that the evidence from Flores and Dmanisi is compatible with these radical new ideas rather than strongly supporting them. The fossil evidence from Eurasia is still meagre. That won’t change until researchers accept the possibility that australopiths made it into Eurasia and birthed our genus there, and start looking for evidence. “Unless we open our minds to the possibility that some of that innovation happened outside Africa, we’ll never find it,” he says.

However, the African fossil record is also surprisingly silent on the origin of H. erectus. “There’s a difficult gap between 2 and 3 million years ago in East Africa where the material is incredibly fragmentary,” says Fred Spoor of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. During this time interval, when H. erectus was evolving, “there is nothing that screams erectus” in the African fossil record. It’s an absence that certainly could be explained if this pivotal species evolved outside the continent, he says.

Nevertheless, Spoor is not convinced this is what happened. “It’s an interesting exercise to think of an early pre-Homo erectus migration out of Africa, but there’s really no hard evidence in the form of fossils or stone tools to support it,” he says. He studies hominin skulls, and still thinks the hobbit is best seen as a dwarfed species that evolved from something like H. erectus, not one that descended from a common ancestor of them both. “Others have called this kind of thing ‘X-Files palaeontology’. It does smell a little like that.”

Despite the considerable problems with the African fossil record, Wood thinks that it will be tough to get researchers to take Eurasian alternatives seriously – perhaps, ironically, because Dart and his successors did such a good job of building up the African story. “The struggle to get people to talk about Eurasia wouldn’t necessarily be as hard as Dart found it to get people thinking about Africa in the first place, but it would certainly be a similar kind of fight,” says Wood.

Eckhardt, Henneberg and Martin take a different view. They think that Wood underestimates the power of a seductive new twist in the tale of human evolution, even with a lack of supporting fossil evidence. They suggest that the true modern-day Darts are not those struggling to get people thinking about the Eurasian australopith idea, but those fighting to oppose it.

What both sides can agree on is that there will be important academic clashes in the years ahead as new fossils come to light that either support or refute the ideas that were let loose when the hobbit was unearthed. With those tussles looming on the horizon, it is not surprising that Raymond Dart’s name is mentioned so frequently – after all, he triumphed in the end.

“It took Dart 30 years to be vindicated,” says Eckhardt. “We’re only coming up on year 10 since the finds on Flores.” Keep watching to find out which way the fossil evidence falls.

This article appeared in print under the headline “Our Asian origins”

Colin Barras is a science writer based near Ann Arbor in Michigan