Ethiopian Fossils Bring New Intrigue to Evolutionary Debates : Anthropology: Bones of human ancestors found at a 3-million-year-old site rekindle questions of whether early species coexisted and how they lived.

BERKELEY —  Researchers Friday announced dramatic new finds from a 3-million-year-old boneyard in Ethiopia that promise to kindle fresh debate about the earliest known human ancestors, a species best-known for a dwarfish skeleton called “Lucy.”

After a 14-year period during which they were unable to visit Ethiopia, researchers from the Institute of Human Origins here returned to the Lucy site in the remote and desolate Afar region last October and discovered 18 major fossils from 15 individuals.

In 1974, paleoanthropologist Donald C. Johanson and his colleagues from the institute stunned the anthropological community when they discovered the 3.5-foot-tall Lucy at Hadar. She was the oldest known hominid, or member of the human family, and was of an entirely new species.

The new discoveries, the researchers said Friday, confirm the first hominids’ similarity to the apes that preceded them, providing new insight into their evolutionary biology.

These fossils, said institute paleoanthropologist William Kimbel, “throw fresh wood on the fire” of two ongoing debates: whether another, more recent human ancestor coexisted with Lucy and her kin, and whether humankind during the period were still partially tree-dwellers or spent most of their time walking the vast savannas upright.

Among their most important finds are a jawbone that looks very much like those of 12- to 8-million-year-old apes that were long extinct when Lucy inhabited the then-lush shores of a huge lake that preceded the Awash River; an upper jaw that is contemporaneous with Lucy but that bears characteristics of a later species; and an arm bone that was heavily muscled, suggesting that it was used for climbing trees.

The Ethiopian site, called Hadar, lies at the northern end of the 4,000-mile-long geological structure called the East African Rift, which continues southward through Kenya and Mozambique. Sediments laid down in the rift over millions of years have been the source of many of the most famous fossil finds that have illuminated the history of humankind.

In 1975, a year after the Lucy find, Hadar also yielded the so-called “First Family,” a collection of hominid fossils consisting of the partial remains of at least 13 individuals found on a single hillside. Researchers named Lucy and the First Family Australopithecus afarensis, “the southern ape from Afar.”

Researchers have been champing at the bit since then because this source of ancient and unusual fossils has been closed to study, thereby delaying the discovery of potentially even more significant ancestors.

The 10-member team representing Ethiopia, the United States and Israel collected fossils at the site from mid-October to mid-December last year.

Most intriguing among the new discoveries was an upper jaw and partial face taken from the same geological stratum that produced Lucy. The jaw has prominent bony ridges beside the nasal cavity, which are much more characteristic of a later species called Australopithecus africanus.

This “southern ape of Africa” branched off from the human ancestral line shortly after A. afarensis and was the precursor of a strong, muscular lineage that eventually became extinct. The bony ridges are thought to reflect a strengthening of its jaws that arose as the hominids’ diet became tougher to chew, reflecting a cooling of the African climate that began about 3 million years ago.

But surprisingly, the palate of the upper jaw indicated that this specimen is still A. afarensis, Kimbel said. This suggests that certain traits carried over from the earlier species to the later one, further confirming that Lucy and her species were in the process of evolving into a more human-like creature.

Also important was the discovery of the upper arm, or humerus, believed to be from A. afarensis. Both ends of the humerus are missing, presumably gnawed off by hyenas, but what was left behind is most important--deeply excavated attachments for the muscles known to bodybuilders as “pecs” and “lats.”

Those muscles are the primary ones that hominids would use to pull themselves up into trees. The large size of the muscles signaled by the bones, Kimbel said, “is consistent with the theory” that Lucy and her brethren spent a significant amount of time in trees, foraging for food and escaping from danger rather than walking upright.

Also significant is the lower jaw from a hominid, containing the right tooth row. This jaw “is striking in its overall primitiveness,” said Kimbel.