Anthropology.

Robust about-face

In the human evolutionary family, the so-called robust australopithecines claim the dubious honor of possessing the weirdest-looking heads. Their massive jaws extend up to about eye level. Hefty, peglike teeth at the back of their mouths give way to much smaller ones in front. A small brain case topped by a bony crest resembles an explorer's helmet perched perilously above a colossal set of choppers.

Researchers usually view the robust australopithecines as a dead-end lineage comprising three species--one in South Africa and two in East Africa--that lived between 2.7 million and 1.3 million years ago. Using a statistical method for organizing species according to recently evolved anatomical features they share, some anthropologists place robust australopithecines into its own genus, Paranthropus.

This evolutionary scenario probably is misleading, contends Melanie A. McCollum of Case Western Reserve University in Cleveland. Most facial traits of robust australopithecines arose as developmental by-products of their unusual set of teeth, she proposes. The anthropologists' statistical method, called cladistics, generates valid evolutionary insights only when it's used to compare anatomical traits that have developed independently of one another, McCollum says.

Her case rests on developmental biology data documenting primate skull changes that occur during growth of the brain, the nasal airway, and the mouth.

Two anatomical traits that robust australopithecines required to accommodate their unusual tooth proportions probably triggered the formation of many skull features used in cladistic studies, McCollum holds. First, the large cheek teeth required support from swaths of lower-jaw bone running up each side of the face, which became longer and thicker in these hominids. This development also instigated a thickening of the mouth's roof and more oral remodeling.

Second, these bone and tooth expansions created an extension of part of the nasal septum, which completely separated the nasal and oral cavities. This resulted in nasal structures pushing up and oral structures spreading down, thus sparking many more anatomical changes, McCollum maintains.

Variations in the shape of their huge cheek teeth also suggest that southern and eastern robust australopithecines evolved separately, a possibility that clashes with the cladistic findings, McCollum argues in the April 9 SCIENCE.

The new report underscores the need to confirm the developmental independence of anatomic traits used in cladistics, comments Tim D. White of the University of California, Berkeley. "Robust australopithecines provide a good example of how cladistics can be misleading," White asserts. --B.B.

Redrawing the human line

Despite criticisms that statistical comparisons of anatomical features, known as cladistic analyses, have a propensity to mislead, Bernard Wood of George Washington University in Washington, D.C., still sees value in them. In the April 2 SCIENCE, he and Mark Collard of University College London reanalyze several cladistic studies and conclude that the two fossil species called Homo habilis and Homo rudolfensis do not in fact belong to the genus Homo. For now, they regard both species--each dated at around 2 million years old--as australopithecines, a genus that includes the 3.2-million-year-old partial skeleton from East Africa known as Lucy.

Wood says that anatomical similarities once taken as support for placing fossil species in the same genus may instead provide insight into independent evolution of those species in a common environment. That still represents a useful application, he argues, although "there's probably more noise than [evolutionary] signal in our cladistic data." --B.B.