Branchless Evolution: Fossils point to single hominid root

Scientists working in Ethiopia’s Middle Awash valley have uncovered fossils of a 4.1-million-year-old human ancestor that bolster the controversial proposition that early members of our evolutionary family evolved one at a time on a single lineage rather than branching out into numerous species.

A team led by anthropologist Tim D. White of the University of California, Berkeley unearthed 31 fossils of Australopithecus anamensis, the earliest known species of this ancient hominid genus. The finds, from at least eight individuals, consist primarily of teeth and jaws, but include foot and hand bones and much of an upper right-leg bone.

Anatomical similarities indicate that Au. anamensis evolved directly from an earlier hominid, Ardipithecus ramidus (SN: 1/22/05, p. 51: Available to subscribers at Pieces of an Ancestor: African site yields new look at ancient species), between 4.4 million and 4.1 million years ago, the researchers assert in the April 13 Nature. By 3.6 million years ago, they add, Au. anamensis had evolved into Australopithecus afarensis, the species that includes the partial skeleton known as Lucy.

“There may have been times when one early hominid species evolved into another one without branching off into multiple species,” White says. His view contrasts with that of researchers who suspect that hominids branched into many species over the past 6 million to 7 million years (SN: 5/3/03, p. 275: Available to subscribers at Ancestral Bushwhack: Hominid tree gets trimmed twice).

To prove that Au. anamensis branched from an earlier, as-yet-unknown population would require evidence that the Australopithecus species lived at the same time as Ar. ramidus, the Berkeley scientist notes. No such evidence exists.

The new finds come from two Middle Awash sites, Aramis and Asa Issie. The fossil discoveries occurred between November 1994 and December 2005. The finds extend the known range of Au. anamensis by about 600 miles to the northeast of two Kenyan sites where another team reported finding remains of the species in 1995.

White’s team dated the Middle Awash material by measuring both the rate of decay of argon isotopes in volcanic ash just below fossil-bearing deposits and the magnetic properties of the sediment.

Early hominids in eastern Africa apparently lived in forested areas, the researchers say. Both Asa Issie and Aramis, also the home of Ar. ramidus, contain abundant fossils of monkeys, antelopes, and other woodland dwellers.

Since Ar. ramidus and Au. anamensis lived in the same place and negotiated comparable habitats, it’s plausible that the earlier hominid evolved directly into the later one, remarks anthropologist Alan C. Walker of Pennsylvania State University in University Park, a member of the team that found Au. anamensis fossils in Kenya.

From 4.2 million to 1.2 million years ago, Australopithecus evolved increasingly larger jaws and teeth from one species to the next with minimal or no evolutionary branching, Walker proposes.

Anatomical comparisons of earlier Au. anamensis and Au. afarensis finds, conducted by anthropologist Donald C. Johanson of Arizona State University in Tempe and his colleagues, also indicate that the older species evolved directly into Lucy’s kind. Their study will appear in the Journal of Human Evolution.

“We need more-detailed knowledge about [Ar.] ramidus to test the veracity of the proposed ancestor-descendant relationship between it and [Au.] anamensis,” Johanson says.