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New Fossils Help Piece Together Human Origins

John Roach
National Geographic News
January 21, 2005
 
Fossil fragments of an early species of hominid have been unearthed with rhino, giraffe, monkey, hippo, and antelope remains in Africa. Hominids are upright-walking primates including modern humans and extict and related forms. The new fossils are helping scientists piece together the earliest chapters of human evolution.

The fossils were unearthed from the Gona Study Area at As Duma in Ethiopia's Afar region and are dated to between 4.3 and 4.5 million years ago.

The research team said the fossils were of the Ardipithecus ramidus species. This hominid species lived shortly after hominids split from the common ancestor that gave rise to both chimpanzees and hominids some six to eight million years ago.

Hominids gave rise to a number of human and near human species, including the extinct Neandertals and the hobbit-like Homo floresiensis—and the only surviving species, Homo sapiens.

"Gona is only one of two sites that have produced Ardipithecus ramidus," said Sileshi Semaw, the Indiana University paleoanthropologist who led the research team. "As we go back in time prior to four million years ago, we don't have that many fossils of hominids."

As a result, Semaw added, scientists are forced to piece together this early history from tiny bits and pieces.

A. ramidus fossils were first discovered about 12 years ago about 62 miles (100 kilometers) south of Gona by anthropologist Tim White and colleagues at the University of California, Berkeley.

The newly discovered fossils consist of tooth, jaw, finger, and toe fragments belonging to at least nine individuals. The shape of the toe bones indicates that the hominids walked upright on two feet, similar to humans, Semaw said.

The Gona team published their finding this week in the science journal Nature. The research was supported in part by a grant from the National Geographic Society's Committee for Research and Exploration.

Woodlands and Grasslands

Christophe Soligo studies primate evolution at the Natural History Museum in London, England. He said that the most interesting aspect to the research at Gona is the analysis of all the big mammals found in the vicinity of the hominid fossils.

By looking at the teeth structure of these animals and the evidence of ancient plants preserved in the soils, Semaw and his colleagues determined that the environment was a mosaic of woodlands and open grasslands with lakes, swamps, and springs nearby.

"It's not terribly conclusive, but it's nice they did do these analyses," Soligo said. "That's perhaps the most important aspect—[understanding] what sort of environment these species lived in."

While analysis of A. ramidus toe fossils suggest they could walk on two feet, scientists believe the early Ardipithecus hominids probably looked more like chimpanzees than like humans. These hominids were also probably quite nimble in the trees.

"What we think is they would have spent some time on the ground but would also have been able to climb up in trees," Soligo said.

Fossil evidence from Ethiopia's Middle Awash region—where the first Ardipithicus fossils were found—suggests that these early hominids lived in a closed woodland. "At Gona we can't say conclusively where Ardipithecus lived and rested—we'll continue to do field research to learn more about these least-known ancestral humans," Semaw said.

Blurry History

In March 2004 White and his University of California colleagues were working at the Middle Awash. They announced the discovery of fossils dated to between 5.8 and 5.2 million years ago and assigned them to a new species named Ardipithecus kadabba.

Based on evidence from the Middle Awash, the scientists believe that A. kadabba is slightly older than A. ramidus. But Ardipithecus, they say, is the earliest hominid genus after the split from the common ancestor that gave rise to both chimpanzees and hominids some six to eight million years ago.

The Middle Awash researchers also argued that two other species belonged to the Ardipithicus genus: a recently discovered six- to seven-million-year-old species from Chad named Sahelanthropus and a six-million-year-old species from Kenya named Orrorin tugenensis.

"We seem to agree with that suggestion, but this is a work in progress," Semaw said. "What is very important to understand from this time period is we don't have many fossils, and we still have to fill anatomical gaps in the fossil record."

In addition to the Ardipithecus fossils described in Nature, the Gona team has discovered several new hominid fossils from several key time periods including 5.5-million-year-old teeth and a skull belonging to the earliest representative of the genus Homo.

Previously, Semaw and his colleagues announced the discovery in Gona of the earliest known stone tools used by ancestral humans, which were dated to at least 2.5 million years ago. The site preserves a long sequence of tools from these earliest implements to ones that are dated to half a million years ago.

"We have a well-established study area that is yielding a lot of important information for human evolutionary studies," he said.

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