Remarkably well preserved for a two-million-year-old fossil, this child's skull belongs to Australopithecus sediba, a previously unknown species of ape-like creature that may have been a direct ancestor of modern humans, according to a new study in Science.
Scientists think this particular Australopithecus sediba fossil is from a male between 8 and 13 years old. The child's fossils were found in the remnants of a subterranean South African cave system alongside the fossil remains of an adult female in her 30s.
"It's the opinion of my colleagues and I that [A. sediba] may very well be the Rosetta stone that unlocks our understanding of the genus Homo," the biological group that includes humans, study leader Lee Berger, of the University of Witwatersrand in Johannesburg, said in a statement, referring to the artifact that helped decipher ancient Egyptian hieroglyphics.
A. sediba could help anthropologists understand, to a greater degree than any human-ancestor species discovered so far, the transition from late australopithecines—the apelike group of species that came before the first Homo species—to the first direct ancestors of humans, Berger added.
A. sediba lived in a patchwork of grasslands and woods in what's now South Africa. The generally flat landscape was broken up by small hills and cliffs, some of which contained caves like the one pictured above, which is close to the fossil-discovery site.
The remains of other animals that lived at the same time as A. sediba were also found in the deathtrap, including saber-toothed cats, hyenas, antelope, and rabbits.
Scientists speculate that a harsh drought may have driven a desperately thirsty A. sediba adult female and young male to enter the caves in an attempt to find an underground source of water.
Study co-author Paul Dirks of James Cook University in Australia speculates that the animals had also been drawn to the cave during a drought by the smell of underground water.
"Perhaps ... they found themselves in an awkward situation and ultimately fell down and died," the geologist said at a press conference Wednesday.
Photograph courtesy Paul Dirk
Species From WhichA. Sediba Sprang?
Study author Berger thinks A. sediba may be a direct descendant of Australopithecus africanus—a skull of which is pictured above—and may also be the immediate ancestor of early Homo species. He points to the mosaic of australopithecine and human features of the A. sediba skull as evidence.
For example, like A. africanus,A. sediba had a relatively small brain compared to modern humans. But the shape of its brain—which can be determined by the shape of the skull's interior—is very much like that of early Homo, Berger said.
Not everyone agrees A. sediba deserves to occupy such a critical position in the human family tree.
"I don't think there's a lot of compelling evidence to suggests that [A. sediba] lies between Australopithecus and Homo," said anthropologist Bernard Wood of George Washington University. "It doesn't fit what our preconceptions would be about the ancestor of Homo."
For example, A. sediba's arms are too long, and the new species isn't as well adapted for bipedalism as some scientists expect the ancestor to the first Homo to be, said Wood, who was not involved in the study.
Photograph by Shaen Adey, Gallo Images, Corbis
Human-Like Face for A. Sediba?
A frontal view of the A. sediba child skull suggests the new human-ancestor species had some surprisingly human-like features. It's face, for example, is flatter than other known australopithecines. Also, A. sediba had small teeth and a surprisingly human-like, as opposed to chimpanzee-like, nose.
Study co-author Berger said a facial reconstruction of A. sediba is in the works.
"What you'll see, I suspect, is something surprisingly more modern than we would expect in ... other things that have been called Australopithecus," which translates to "southern ape."
Photograph courtesy Brett Eloff and Lee Berger
Human Species Sprang From A. Sediba?
These skull fragments belong to Homo habilis, a species that many scientists think is one of the first member of the genus Homo—the human species group. Known only as Skull 1470, it was discovered in 1972 in Kenya and is thought to be about two million years old.
Berger thinks the newfound fossil species A. sediba may be a direct ancestor of H. habilis or perhaps a later human species, Homo erectus.
But other scientists think the timing is wrong. Skull 1470, for example, is slightly older than the new A. sediba specimens, and there are other human fossils that are even older.
"It's hard to argue this is the ancestor of Homo when it's occurring much later than the earliest members of the genus Homo by half a million years," said anthropologist Brian Richmond of George Washington University in Washington, D.C. (Explore a prehistoric time line.)