Fossil finds give clues to ancestors

New discoveries of ancient bones throw light on the beginnings of our species

THE two-million-year-old skeletal remains of a juvenile male and an adult female, discovered in a South African cave, may be those of the direct ancestors of the first humans to walk the earth.

The claim -- from an international team led by James Cook University geologist Paul Dirks and Lee Berger, a paleoanthropologist at Johannesburg's Witwatersrand University -- is a big one.

The newfound species has been named Australopithecus sediba, a blend of the established Latin term for "southern ape" and the word for "wellspring" in the SeSotho language.

What sets Au sediba apart from previous early ancestral humans is the fact that the bipedal hominin shares a mix of primitive features common to the ape-like Australopithecines and to more advanced proto-humans such as "handy man" Homo habalis , and "upright man" Homo erectus.

"These fossils give us an extraordinarily detailed look into a new chapter of human evolution and provide a window into a critical period when (hominins) made the committed change from dependency on life in the trees to life on the ground," Professor Berger said.

In two reports published overnight in the journal Science, Professor Dirks, Professor Berger and their colleagues report that the fossils were found close together in cave deposits at Malapa, South Africa, along with the remains of up to 25 animals, among them a sabre-toothed cat, a horse, a hyena, an ancient pig and rabbits.

The two partial skeletons include most of a skull, teeth, pelvis and ankle of the new species.

The researchers found, but have not yet analysed, the bones of at least two other individuals, including an infant and another adult female. Those fossils were also embedded in cave sediment.

William Jungers, head of anatomical sciences at New York State's Stony Brook University Medical Centre, said: "Great new fossils, fabulous preservation, and with associated craniodental and postcranial elements -- it doesn't get much better than that."

But along with all the experts The Australian contacted, Professor Jungers -- an expert in locomotion skills of hominins like "Lucy", a 3.5 million-year-old Australopithecus afarensis, and the hobbit Homo floresiensis, the puzzling hominin found by Australian and Indonesian experts in a cave on the Indonesian island of Flores in 2003 -- is sceptical of the claim that Au sediba is the direct ancestor of all members of the human family's genus, or group, Homo.

By the time the new hominin lived, 1.95 million to 1.78 million years ago, members of the Homo clan had been around for at least half a million years in east Africa.

And ancestral humans such as H. erectus and Homo ergaster had left Africa, arriving in eastern Europe and western Asia at least 1.8 million years ago.

"Fundamentally, this is just another wonderful australopith," Professor Jungers said.

Peter Brown, the University of New England paleoanthropologist who first analysed the hobbit remains, said working out where this australopithecine fits in the family tree would not be easy.

"Between five million and two million years ago, our ancestor evolved into something we would recognise as increasingly human, both in appearance and behaviour," he said. "Unfortunately, while there are hundreds of hominin fossils, particularly from south and east Africa, from this time period, preservation is poor."

That's frustrating for experts such as Professor Brown, since it makes it difficult to work out the processes driving the emergence of the behavioural and biological traits that distinguish humans from other primates such as chimpanzees and gorillas -- in particular, modern humanity's gait, reproductive strategy, growth rate and big brain.

Professor Brown is optimistic the newfound fossils will help put together some of the pieces of the evolutionary puzzle. "But the fragmentary and geographically dispersed nature of the hominin fossil record makes picking the winners and losers in the evolutionary lottery difficult," he said.

Au sediba could easily be an evolutionary dead end.

Professors Dirks and Berger and colleagues acknowledged this in their report.

"The possibility that A sediba split from A africanus before the earliest appearance of Homo cannot be discounted," they wrote.

There's another difficulty with the team's earliest-ancestor claim, one that will only be resolved with the discovery and analysis of more Au sediba remains.

As University of California, Berkeley, paleoanthropologist Tim White told Science, the characteristics shared by Au sediba and Homo could be due to normal variation among australopithecines or because of the young male's juvenile status.

Professor White said of Au sediba: "Given its late age and Australopithecus-grade anatomy, it contributes little to the understanding of the origins of the genus Homo."

Paleoanthropologist Matthew Tocheri of the Human Origins Program at the Smithsonian's National Museum of Natural History in Washington agreed.

"It remains to be seen which presently known -- that is Au afarensis or Au africanus -- or perhaps yet-to-be-discovered hominin is the last common ancestor of the genus Homo," he said.

Sorting that out won't be a straightforward process of finding lots of new fossils and deciding which ancient species looked more like us than another.

For instance, like Professor Jungers, Dr Tocheri has studied the hobbit. He noted there were resemblances between it and Au sediba, among them a small brain size and similar limbs.

"But since these shared primitive characteristics have been in the hominin lineage for a long time, more than three million years, they do not provide the necessary information about which later hominins are more closely related to each other," Dr Tocheri said.

So as with every discovery in human evolution, they not only add new clues, they toss up new questions.

Understanding of the human journey was complicated last March when a group led by evolutionary geneticist Savante Paabo of Germany's Max Planck Institute in Leipzig announced it had genetic evidence that a previously unknown species -- nicknamed X woman -- lived at the same time as modern people and neanderthals, sharing a common ancestor with both species.

Mike Morwood, a University of Wollongong archeologist and co-leader of the hobbit discovery team, said that along with the hobbit, X woman was further evidence that humanity's family tree was really a family bush.

Speaking of the age when australopithecines and early Homo species lived, he said: "This was obviously a period of evolutionary ferment for hominins, and the time when the ancestors of H. floresiense probably dispersed out of Africa."

That fits neatly with what evolutionary biologists know of the emergence of species generally. Nature tosses up a variety of experiments. Some win, some lose. The winners evolve into another species, the losers become extinct.

The question is, what was Au sediba: late surviving australopithecine, late surviving member of a species that earlier gave rise to Homo, or oddball Homo?

Professor Brown's answer: "As in most matters to do with interpretation of the hominin fossil record, history will be the judge."