The Elements of Evolution (61-3) Not Homo?

 Not Homo?

A genus is typically defined as both a clade and a grade. A clade is of lineage, while a grade is of similarity. Snakes are in the same clade as lizards, as they share a common ancestor, but, lacking legs to walk on, snakes are a different genus than lizards by grade.

It is arguable that H. habilis belongs in the Homo genus. H. habilis had a less protruding face than australopiths, as well as other physical distinctions, so does not belong within Australopithecus. But, with its short body, long arms, and other dissimilarities, H. habilis was also physically divergent from H. sapiens, and so is not the same grade as modern humans. The diversity of fossils attributed to H. habilis, and divergences from later Homo, illustrate the difficulties of tracing human descent.