Fossil Skull Diversifies Family Tree

Anthropologists have long held that the earliest members of the human evolutionary family consisted of a group of closely related species known as australopithecines. A 3.5-million-year-old skull unearthed in Kenya now suggests that the australopithecines had a set of evolutionary companions.

The nearly complete skull represents a new genus and species of early hominids, according to a report in the March 22 Nature. The team that excavated and analyzed the specimen, led by anthropologist Meave G. Leakey of the National Museums of Kenya in Nairobi, dubs it Kenyanthropus platyops.

Of more than 30 skull and tooth fragments found with the skull, the researchers have assigned two to K. platyops.

If anthropologists accept Kenyanthropus into the evolutionary fold, it will change their thinking about early hominids. Consider the new skull’s unusual anatomy. Like Australopithecus afarensis–a species that existed from 4 million to 3 million years ago and includes the partial skeleton named Lucy–K. platyops has a small brain and thickly enameled cheek teeth. Moreover, its small ear holes resemble those of both chimpanzees and an earlier hominid, Australopithecus anamensis.

In other respects, though, K. platyops looks like a 2-million-year-old skull previously found in Kenya. Many researchers attribute that specimen to Homo rudolfensis, an extinct species in our own genus. Leakey’s group reassigns the skull to Kenyanthropus based on such shared traits as a flat, sloping lower face, raised cheeks, and flattened brow ridges.

If K. platyops had deeper evolutionary roots than A. afarensis, as well as a unique relationship to H. rudolfensis, a species that emerged much later, it raises doubts about Lucy’s legacy, Leakey’s team contends. Contrary to the most influential current view, her kind might not have given rise to all ensuing hominids, the team says.

Whether or not the new skull represents a unique genus, it indicates that a distinct line of hominids existed alongside A. afarensis, remarks William H. Kimbel of the Institute of Human Origins in Tempe, Ariz. Further study of Leakey’s finds may resolve whether A. afarensis truly served as an ancestor to all later hominids, says Kimbel, who directs A. afarensis excavations in Ethiopia.

Leakey and her colleagues unearthed the new skull and associated fossil fragments in August 1999 at a site located just west of Kenya’s Lake Turkana. Dating of volcanic rock from below and above the finds–based on measures of potassium and argon isotopes in the rock–places them at about 3.5 million years old.

K. platyops and A. afarensis may have evolved in substantially different habitats, the investigators theorize. Other ancient animals whose remains have been found at the Kenyan site appear to have been suited to a wetter, more vegetated habitat than that frequented by Lucy’s kind, the researchers say.

The new find’s surprising mix of anatomical features indicates that some parts of the skull can change in striking ways without affecting the shape of nearby areas, Leakey’s group adds. For example, K. platyops combined a forwardly positioned cheekbone with small cheek teeth. However, Paranthropus–a hominid genus that lived from around 2 million to 1 million years ago–blended a comparable cheek bone with large, peglike molars.

“Many of the skeletal features of early hominids may have been acquired piecemeal, not as large anatomical complexes,” Kimbel proposes.

Another approach holds that the evolution of crucial parts of the skeleton in various hominids triggered many other bony alterations (SN: 11/25/00, p. 346).

The position of K. platyops in the human evolutionary tree will remain uncertain for some time, according to Daniel E. Lieberman of George Washington University in Washington, D.C. “There’s no simple way to figure out who’s related to whom,” he says.