Thursday, April 08, 2010

New addition to the human family tree

Two-million-year-old “mother and child” fossils found in South Africa

Fossilized bones representing a new species of likely human ancestors were described today in the journal Science.

The remarkably well preserved fossils were of an adult female and a pre-adolescent male who shared a unique checkerboard mixture of apelike and human features. They were discovered in August, 2008 in a remote site in South Africa by a team led by Lee Berger at the University of Witwatersrand.

Remarkably, the fossils were first spotted by Berger’s son Matthew. “It was a child, found by a child,” says Berger. The Government of South Africa and the University of Witwatersrand are sponsoring a contest among South African children to name the child.

Berger, the study’s lead author, stopped short of saying that the pair were in fact a mother and her child, but said that his team will be using a variety of techniques to try to answer that question. “They were almost certainly part of the same troop,” he says. “So there’s a very high probability that they are related to each other.”

Geologist Paul Dirks, part of the group that studied the fossils and the setting in which they were found, thinks that the two died together, probably in a major flood or mudslide that washed their bodies down into the depths of a cave, where their remains were safe from scavengers and rapidly turned into fossils.

Berger and his colleagues spent the past year and a half studying the fossils of these two individuals, along with the bones of hundreds of animals found near them, and the geological layers above and below them.

This allowed them to pin down the time when these creatures lived extremely accurately. “Our ability to date these sites in southern and eastern Africa has become more and more precise,” says Berger. They almost certainly lived 1.95 million years ago, give or take a few thousand years.

These are not the oldest probable human ancestors. For example, two members of the species Australopithecus afarensis left a trail of surprisingly human-like footprints in a bed of volcanic ash in Laetoli, Kenya 3.6 million years ago, and the tool-using Homo habilis lived some 2.5 million years ago.

However, the new species, which Berger and his colleagues have named Australopithecus sediba, presents a surprising mix of ancient and more modern traits, as if a snapshot had caught the species in the awkward process of morphing from ancient apelike predecessors to more recognizably human ancestors.

Australopithecus sediba is undoubtedly a highly transitional species, with a mosaic of characteristics shared with later hominids but with other features typical of the australopithecines,” says Berger.  Hominids include humans and their direct ancestors or very close relatives, while the australopithecines were earlier small-brained but bipedal species, one of which is thought to have evolved into the first bigger brained, walking and tool using human species.

Berger believes that Australopithecus sediba is either a direct ancestor of one of the earliest members of our genus, Homo, or a very closely related side branch.

The new species studied by Berger and his colleagues had long legs and human-like hips, so they could walk easily. At the same time, they had very long arms and short, but very strong hands and fingers that meant that they were still at home in the trees. “They were very competent walking bipeds,” says Berger, “but with these backup, parachute arms that allowed them to climb trees.”

Even their skulls show a peculiar mixture of features. Their brains were small, around 420 cubic centimeters—less than one third of the volume of modern human brains. However, their faces had many human-like features, including a well-developed nose, well defined cheeks and a sloping but somewhat bulging forehead.

“They would look dramatically different [than other ancient human ancestors]” says Berger. “They would have long, apelike arms but with short, powerful, human-like hands. They would have human shaped hips and long legs, and a modern-like face, but with a very small head.”

A. sediba  skull
photo by Brett Eloff, courtesy Lee Berger, Univ. of Witwatersrand

Berger is also intrigued by the fact that the adult female was nearly as tall as the predicted full height of the male. This similarity in size between males and females might be associated with human-like families and groups, in contrast to the extreme male-female size differences found in primates such as gorillas with a single dominant male guarding and dominating a harem of females.

Sediba has taken a leap toward a social structure where you don’t have a dominant alpha male and you are lowering competition between males, who live together with females and their offspring in a social group,” Berger says.

He compares this new species with its odd mix of ancient and modern features to the Rosetta Stone that first let scholars make sense of Egyptian hieroglyphics. “These are a remarkable sample of fossils,” says Berger. “They’re going to answer a great many questions about human evolution during the period from 2 million to 1.7 million years ago, a period that is very poorly represented in the fossil record.”

Even if long-legged but small-brained Australopithecus sediba didn’t quite make it into the genus Homo, I for one am happy to welcome them to our ancestral family tree.

Robert Adler
for the institute