Fossils have long provided snapshots of the human family tree, but a new find in Africa gives scientists a kind of mini home movie showing man's primal development.
Because the 4.2-million-year-old fossil is from the same human ancestral hot spot in Ethiopia as are remains from seven other human-like species, scientists can now fill in the gaps for the most complete evolutionary chain so far.
"We just found the chain of evolution, the continuity through time," said Ethiopian anthropologist Berhane Asfaw, co-author of the study being reported Thursday in the journal Nature. "One form evolved to another. This is evidence of evolution in one place through time."
The species, Australopithecus anamensis, is not new, but its location is what helps explain the giant leap from one early phase of human-like development to the next, scientists say. All eight species were found in a region called the Middle Awash.
"It's like 12 frames of a home movie, but a home movie covering 6 million years," said study lead author Tim White, co-director of Human Evolution Research Center at the University of California at Berkeley. Fossils in the region cover three major phases of human development.
"The key here is the sequences," White said. "It's about a mile thickness of rocks in the Middle Awash and in it we can see all three phases of human evolution."
Modern man belongs to the genus Homo, which is a subgroup in the family of hominids. What evolved into Homo was likely the genus Australopithecus (once called "man-ape"), which includes the famed 3.2 million-year-old "Lucy" fossil found three decades ago.
A key candidate for the genus that evolved into Australopithecus is called Ardipithecus. And Thursday's finding is important in bridging — but not completely — the gap between Australopithecus and Ardipithecus.
In 1994, a 4.4 million-year-old partial skeleton of the species Ardipithecus ramidus — the most recent Ardipithecus species — was found about six miles from the latest discovery.
"This appears to be the link between Australopithecus and Ardipithecus as two different species," White said.
The major noticeable difference between the phases of man can be seen in Australopithecus' bigger chewing teeth to eat harder food, he said.
While it's looking more likely, it is not a sure thing that Ardipithecus evolved into Australopithecus, he said. The finding does not completely rule out Ardipithecus dying off as a genus and Australopithecus developing independently.
The connections between Ardipithecus and Australopithecus have been theorized since an anamensis fossil was first found in Kenya 11 years ago. This draws the lines better, said Alan Walker of Penn State University, who found the first anamensis and is not part of White's team.
Rick Potts, director of the Smithsonian Institution's Human Origins Program, agreed: "For those people who are tied up in doing the whole human family tree, being able to connect the branches is a very important thing to do."