Monday, October 17, 2011

Robust australopith facial structure

SK 48 - P. robustus
A longstanding question in paleoanthropology concerns whether or not the robust australopithecines are a monophyletic group (a group sharing a common ancestor to the exclusion of all other critters). These hominins have massive jaws and facial structures associated with extremely powerful chewing muscles. Two species of robust australopith have been found in east Africa, and one from southern Africa. Some have argued that the robust australopiths from east and southern Africa are closely related and belong to a single genus, Paranthropus. Others have pointed out that the similarities between the eastern and southern forms largely have to do with heavy chewing, and that the observed similarities might easily have evolved in parallel in two hominins with similar diets. This scenario would imply that the robusts in east and southern Africa were instead most closely related to the non-robust (gracile) hominins living in their respective regions. 

In a recent article in PNAS, Villmoare and Kimbel take a fresh look at this debate by analyzing CT scans of several species of gracile and robust australopithecines. They focused on a feature called the anterior pillar, a ridge of bone running vertically down the side of the nose.  This feature has been argued to be a shared derived trait linking the South African hominins P. robustus (robust) and A. africanus (gracile). The anterior pillars of P. robustus and A. africanus turn out to be quite different when viewed internally. Furthermore, the internal structure of P. robustus is very similar to the structure of P. boisei, an east African robust, even though P. boisei usually lacks the visible external anterior pillar. The authors interpret these findings as additional evidence that the eastern and southern robusts are a natural group, and that the superficial resemblances between P. robustus and A. africanus are the result of parallel evolution.

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