Pieces of an Ancestor: African site yields new look at ancient species

New fossil discoveries in eastern Africa offer a rare glimpse of one of the oldest members of humanity’s evolutionary family. More than 4 million years ago, this upright-walking hominid—dubbed Ardipithecus ramidus—lived in an area that contained a patchwork of habitats populated by a wide variety of animals, say anthropologist Sileshi Semaw of Indiana University’s CRAFT Stone Age Institute in Gosport and his colleagues.

In four field seasons from 1999 to 2003, Semaw’s team uncovered 1,500 fossils from 40 sites in Ethiopia’s Gona region. Seven of those sites yielded more than 30 ramidus specimens belonging to at least nine individuals. Those finds consist primarily of jaw fragments, isolated teeth, and toe and finger bones.

The new hominid fossils date to between 4.3 million and 4.5 million years ago, the researchers report in the Jan. 20 Nature. That age estimate rests on measurements of argon gas trapped in volcanic ash and rock situated above and below the finds as well as on evidence from soil corresponding to the known dates of reversals in Earth’s magnetic field.

Fragmentary ramidus fossils from elsewhere in Ethiopia date to more than 5 million years ago (SN: 7/14/01, p. 20: Earliest Ancestor Emerges in Africa).

In Gona, ramidus apparently lived in a forested region with a few open, grassy expanses, as did the 5.6-million- to 5.8-million-year-old Ardipithecus kadabba (SN: 3/6/04, p. 148: Available to subscribers at Early Ancestors Come Together: Humanity’s roots may lie in single, diverse genus). Lakes, swamps, and springs also marked the Gona area.

Semaw and his coworkers reconstructed this ancient environment by considering animal remains unearthed among the hominid fossils. There’s evidence of ancient monkeys, pigs, antelopes, rhinoceroses, elephants, horses, giraffes, and mole rats. Chemical analyses of the creatures’ teeth and of the fossil-bearing soil provided further clues to Gona’s ancient setting.

The shape of a newly discovered ramidus toe bone indicates that this ancient hominid walked upright, the scientists say.

The Gona hominid’s teeth resemble those of two other recently identified hominid genera that lived in eastern Africa between 6 million and 7 million years ago, Sahelanthropus (SN: 7/13/02, p. 19: Evolution’s Surprise: Fossil find uproots our early ancestors) and Orrorin, Semaw says. He suspects that fossils assigned to those evolutionary groups actually belong to Ardipithecus.

Debate over the evolutionary identity of hominids that lived more than 4 million years ago will continue indefinitely, predicts anthropologist Bernard Wood of George Washington University in Washington, D.C. Some researchers emphasize anatomical commonalities in the available fossils and thus lump them into one category or a few common categories; other scientists highlight anatomical differences in the same fossils and thus split specimens into separate species and genera.

“It’s really a matter of philosophy,” Wood says.