IT’S instantly recognisable – one of the most iconic scientific illustrations of all time. The original version of The March of Progress, drawn for a popular science book in 1965, lined up all the early relatives of humans known at the time in chronological order. The artist, Rudolph Zallinger, sketched them striding purposefully across the page, seemingly becoming more advanced with each step. It gave the impression – despite the book saying otherwise – that human evolution was a linear progression from small-brained tree climbers to bipedal big-brained modern humans.
This much-copied image has been criticised for its oversimplification, but until recently our evolutionary past was not actually thought to be a great deal more complex, give or take the odd dead-end side shoot. Now, however, a string of new fossils are forcing a rethink.
In particular, it looks if as many species of human-like apes were around during the crucial period from 2.5 to 1.8 million years ago, when the first upright apes with relatively large brains evolved. What’s more, the East African hominin long seen as our direct ancestor may be just a cousin, with our true roots lying further south. Our family tree may have to be completely redrawn.
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The story once looked so straightforward. Around the beginning of the 20th century, anthropologists began unearthing fossils of a big-brained, bipedal species later dubbed Homo erectus. There is now little doubt that H. erectus, which lived from around 1.8 million to 550,000 years ago in both Africa and Asia, was one of our direct ancestors. It could make sophisticated stone tools and probably controlled fire, too.
Soon afterwards, fossils of even earlier human relatives began to emerge in Africa. The australopiths, as they are collectively known, first appeared around 4.5 million years ago, long before H. erectus. They walked on two legs like us but had much smaller brains. So how were they related to H. erectus? A few years before was published, anthropologists thought they had found the “missing link”.
A hominin found in Tanzania’s Olduvai Gorge in 1960 neatly fits the australopith-Homo gap. The fragmentary remains belong to a species with a brain roughly 50 per cent larger than the average australopith, but half the size of our own brains. It first appeared around 2.3 million years ago, just as most australopiths were vanishing but before H. erectus had evolved. And the East African region it lived in had formerly been home to small-brained australopiths, and was later inhabited by H. erectus.
Its discoverers opted to make this hominin the first member of our genus, naming it Homo habilis. For the last 50 years, H. habilis has been central to the story of our origins, says Lee Berger at the University of the Witwatersrand in Johannesburg, South Africa. Put simply, H. habilis was the right hominin in the right place at the right time to be our direct ancestor.
It wasn’t the only hominin around in this place at this time. In the years before H. habilis was discovered, another australopith now known as Paranthropus boisei had been found. P. boisei has some unusual, almost gorilla-like features, though, and is clearly a side branch rather than a direct human ancestor.
The evidence, then, seemed to point to a simple picture not wildly different from The March of Progress. And despite the many discoveries made since the 1960s, a few researchers still think the basic picture is this simple. According to Tim White at the University of California in Berkeley, the hominin evolutionary tree looks rather like a classic cactus, with one central lineage and only the occasional side branch, such as P. boisei (see diagram).FIG-29251901.jpg
But others think recent discoveries point to a much more complicated picture. Hints that our origins were more tangled appeared just a few years after the discovery of H. habilis. In 1972, a research team working in the Koobi Fora region of northern Kenya found a skull from the time that H. habilis was alive with a brain slightly larger, and a face considerably broader and flatter – that is, with prominent cheek bones – than that of any known specimen of H. habilis.
So where does this 2-million-year-old skull fit within our evolutionary tree? Is it related to H. habilis, despite its unusual appearance? Or was it a separate species?
Direct ancestor
Anticipating the second outcome, one researcher called it Homo rudolfensis. But Meave Leakey, based at the Turkana Basin Institute in Nairobi, Kenya, and a member of the team that found the skull, prefers to stick to specimen numbers. The skull’s number is KNM-ER 1470, or 1470 for short.
For almost 40 years, the 1470 skull remained an anomaly. That changed in 2007 when Leakey and her colleagues began finding similar fossils in the same region. “It’s incredibly satisfying to finally have found fossils that match 1470,” she says.
The discoveries, which include a new skull fragment with the flat facial features of the 1470 skull, were revealed last year (). The new find belonged to a juvenile, not an adult. This shows that 1470 was no anomaly, say the team, but instead belonged to a distinct species that was born and grew up with a flat face. Although in some respects that flat face is rather like ours, it is far too broad to be one of our direct ancestors, says Fred Spoor of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, who worked with Leakey on the new Koobi Fora fossils.
With H. habilis and P. boisei, this means at least three species of hominins were living in East Africa around 2 million years ago. Or were there more? One of the enduring mysteries in human evolution is where the first species widely accepted as our direct ancestor – H. erectus– originated. Its roots are a mystery, with some researchers even suggesting it came out of Eurasia about 1.8 million years ago (91av, 11 May, p 40).
However, a scrap of skull from Koobi Fora that dates back 2 million years calls this theory into question. It might be small, but this fragment is strikingly reminiscent of H. erectus, raising the tantalising possibility that it lived in East Africa at the same time as H. habilis and the 1470 lineage. The fact that they coexisted doesn’t rule out H. habilis as the direct ancestor of H. erectus, but it does suggest a more complex relationship than we thought.
Four hominin species living at the same time would be exceptional. And yet Bernard Wood at George Washington University in Washington DC suggests there was another hominin was also present.
This species is the least known of them all. In the 1990s, Wood analysed all the Koobi Fora material discovered up until then. Although the 1470 skull lacked a jawbone, Wood predicted that it would have had a powerful jaw with large teeth. As it happened, a lone jawbone in the Koobi Fora collections matched his expectations. Ever since, this fossil has been thought to belong to the 1470 lineage.
No longer. Leakey and her team have since found two jawbones that are a much better fit for the new flat-faced specimen – and the 1470 skull – than the jawbone Wood identified. “You win some, you lose some,” says Wood.
But that leaves the unusual fossil jaw. In a commentary published alongside last year’s Koobi Fora research, Wood made a bold suggestion: the jaw bone may be evidence of yet another hominin lineage alive 2 million years ago (). Leakey’s team doesn’t necessarily share this interpretation, Spoor stresses.
Potentially, then, East Africa was home to five species of hominin just as our genus was finding its feet. How they are all related is far from clear (see diagram), but H. habilis still looks like the most likely direct human ancestor.
Out of South Africa
Or so it seemed until an unexpected discovery thousands of kilometres to the south of Koobi Fora and Olduvai Gorge. In 2008, Berger’s then 9-year-old son Matthew found the first fossil of yet another hominin, at a site in South Africa called Malapa, north-west of Johannesburg. Berger’s team soon unearthed a pair of 2-million-year-old skeletons so exquisitely preserved there may even be bits of skin still attached to some of the bones. They unveiled this new hominin in 2010, calling it Australopithecus sediba.
Along with the East African hominins, this means that there were as many as six species living in Africa 2 million years ago – a level of diversity unprecedented in 7 million years of hominin evolution. A. sediba is arguably the most surprising of the six.
In some ways, such as brain size, A. sediba resembles other australopiths. But what makes it a strange ape is that in other ways it resembles humans (see below). “Across the body, head to toe, A. sediba shares a remarkable number of characters with Homo,” says Berger.
The more Berger looked at the skeletons, the more convinced he became that A. sediba is a pivotal species in our ancestry. He thinks the characteristics A. sediba possesses, including small Homo-like teeth and a tapered Homo-like waist, put it on the lineage leading to H. erectus ().
But A. sediba, as critics are quick to point out, is everything that H. habilis is not: it’s a small-brained australopith living in southern Africa 2 million years ago – a good 300,000 years after the larger-brained H. habilis first appeared in East Africa. They say A. sediba is the wrong hominin in the wrong place at the wrong time to be our direct ancestor. “It’s just too young to lead to Homo,” says Spoor.
Berger has a simple answer to this criticism: H. habilis, the oldest member of our genus, is not one of our direct ancestors. Its relatively large human-like brain gives the impression that it is, but appearances can be deceptive. “A. sediba is a better candidate for the origin of erectus than habilis ever was,” he says. “Its hand, dentition and what we can see of its skull morphology – other than the cranial capacity – are more like those of H. erectus.”
What this means, he says, is that large brains evolved twice. A small-brained East African australopith evolved into the larger-brained H. habilis around 2.3 million years ago, but this lineage died out. A little later, a southern African australopith closely related to A. sediba evolved into the large-brained H. erectus, and this lineage went on to give rise to the rest of humanity. So Berger is not claiming that A. sediba itself is our direct relative, but that our actual ancestor was a very similar australopith that lived in around the same region at around the same time.
Berger’s ideas have met strong criticism. White, for instance, dismisses Berger’s claims for A. sediba. Where Berger sees a suite of Homo-like characteristics, White sees a peculiar mixed anatomy that couldn’t possibly serve as the blueprint for our genus. But A. sediba is, at least, indisputably a new species, says White. The same can’t be said of all the East African finds.
“Wood and his colleagues are partitioning the record too finely by saying these minor variations are indicative of separate lineages,” White says. There are differences between modern humans, but we all belong to the same species, he says. Likewise, he argues, all the finds from Koobi Fora belong to one species, despite the differences in shape.
Spoor rejects White’s argument for the Koobi Fora fossils. “I’d like to see Tim present some statistics that show there is any primate species diverse enough to combine all of the features we found,” he says.
With the shapes of the bones open to interpretation, is there any other way to address the issue? Remarkably, there is. The carbon in the enamel of teeth holds clues about the kinds of food their owner ate. If the owners of the differently shaped bones had distinctive diets it would boost the argument that they are separate species.
Until recently, researchers thought all hominin diets were relatively similar. Most australopiths probably ate seeds and tubers, while members of the Homo genus may have added a little meat to their vegetables. We now know things were a lot more complicated.
In 2011, Thure Cerling at the University of Utah in Salt Lake City led an analysis of P. boisei teeth. The species had such large, durable teeth that it had always been considered a quintessential seed and nut eater. But Cerling’s analysis suggested, surprisingly, that P. boisei might have spent its days grazing grass – a specialist diet unique for a hominin and virtually unheard of among apes ().
Last year, Berger and his colleagues took a look at the carbon isotopes in A. sediba‘s teeth. They revealed a diet unusually rich in tree leaves, fruits – even bark – despite the fact that grasses dominated its environment. Again, it’s a specialist diet unlike that of any other hominin (Nature, vol 487, p 90).
Cerling’s latest work, published last month, hints at dietary specialisations in some of the other hominins alive 2 million years ago. Most significantly, the unusual flat-faced 1470 lineage may have shared a similar diet to P. boisei – teeth that have been associated with the 1470 lineage carry the same unusual grass-rich isotopic signature. Fossils normally associated with H. habilis, meanwhile, suggest this species was less reliant on grasses for its food.
Dietary work, then, supports the idea that there were indeed several hominin species living cheek by jowl, by showing that they were exploiting different resources. “This work really does open up the issue of exactly how these guys were parsing up the space,” says Cerling.
What it doesn’t do is resolve the issue of whether H. habilis or an A. sediba-like australopith was our direct ancestor. Only the discovery of clear intermediates will help settle this argument. But if the past few years are anything to go by, new finds are likely to raise far more questions than they answer.
Wood, at least, is confident of the way the wind is blowing. He predicts that by 2064 – a century after the first H. habilis finds were described, and 99 years on from The March of Progress – our family tree will be even bushier and more tangled than he currently envisions it. If the discoveries keep coming thick and fast, we might not have to wait that long to find out whether or not he is right.
This article appeared in print under the headline “The unexpected ape”
