Ancestral Bushwhack: Hominid tree gets trimmed twice
By Bruce Bower
Many anthropologists suspect that hominids, ancient members of the human evolutionary family, branched into as many as 20 different now-extinct species over roughly the past 6 million years. At scientific meetings in Phoenix last week, two skeptical researchers took different approaches to pruning this species-laden scenario.
Their handiwork expands debate over how to identify hominid species in the fossil record. Still, proponents of what might be dubbed humanity’s family bush remain steadfast.
In a controversial presentation, Tim D. White of the University of California, Berkeley accused many of his colleagues of ignoring geological forces that have substantially distorted the shapes of key hominid fossils and given them a false appearance of anatomical uniqueness.
“The metaphor of a bush seems to be seriously misplaced with regards to the evolution of hominids,” White said at the annual meeting of the Paleoanthropology Society. His talk elaborated on his commentary published in the March 28 Science.
As a case in point, the Berkeley anthropologist cited a 3.5-million-year-old hominid skull that has been assigned to its own genus and species, Kenyanthropus platyops (SN: 3/24/01, p. 180). Many of the specimen’s apparently unique traits actually result from distortion caused by compacted sediment inside the skull that has expanded and pushed against surrounding bone, White asserts.
On close inspection, he says, the Kenyanthropus face contains about 1,100 small pieces of bone separated by a latticework of mortarlike sediment.
White and his coworkers identified varying levels of this geological deformation in 60 fossilized oreodonts, North American pig relatives that lived from 34 million to 24 million years ago. Kenyanthropus displays bone cracking and twisting comparable to that in the most distorted oreodont fossils, White contends.
In his view, Kenyanthropus may actually have been a form of Australopithecus afarensis, the 3-million-to-4-million-year-old species that includes the famous partial skeleton called Lucy, which White codiscovered.
Advocates of a hominid family bush reject White’s charges. Researchers have long noted the deforming effects of sediment on hominid fossils, says Ian Tattersall of the American Museum of Natural History in New York. The Kenyanthropus skull exhibits extensive cracking but remains symmetrical and thus provides a reliable view of the ancient hominid, Tattersall says.
Ironically, a second researcher aiming to stem the tide of hominid species agrees with Tattersall that geological distortion of fossils has been adequately accounted for. However, data from living mammals indicate that early members of the Homo lineage—often sorted into six or more species—probably didn’t make up more than two or three species, says Glenn C. Conroy of Washington University Medical School in St. Louis.
Conroy, who spoke at the annual meeting of the American Association of Physical Anthropologists, used a worldwide data set on mammalian anatomy to identify the number of current species in genera falling within the estimated weight range of early Homo—66 to 143 pounds.
To Conroy’s surprise, only 38 of 1,116 mammalian genera include any members tipping the scales in that range. “I have no idea why,” he says.
These groups usually consist of one to three species. Primate genera, even many of those below the Homo weight range, contained only one to three species.
These data indicate that the hominid evolutionary tree has never been bushy, Conroy holds.
Conroy’s focus on mammals of appropriate body weight is “a reasonable criterion” for estimating numbers of hominid species, remarks Bernard Wood of George Washington University in Washington, D.C.
Letter to the editor: I would like to correct two errors in your article. The scientific meeting of the Paleoanthropology Society was held in Tempe, Ariz., not Phoenix. Furthermore, Tim D. White was not a codiscoverer of Lucy. After my discovery, I invited Dr. White to help analyze and describe Lucy and other fossil hominid finds from Hadar, Ethiopia.
Don Johanson
Arizona State University
Tempe, Ariz.
