New Fossils Help Piece Together Human Origins

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"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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