Richard Potts in the 16 August 1996 Science, links some major evolutionary changes with "variability selection," that is, selection for traits that "enable resilient and novel responses to new conditions," especially flexibility where environmental conditions fluctuate. Potts posits that the changable conditions in Africa between 5 and 3 million years ago might account for the evolution of bipedality, which could "accomodate [both] terrestrial and arboreal settings." In addition, increase in brain size, technological innovation, and manipulation of symbols may also have been "novel responses" by our lineage to environmental oscillations. Potts notes the extinction of Paranthropus and the Neandertals, both with a more restricted geographic range, "whereas those [species] with greater mobility and behavioral diversity (early Homo erectus and modern H. sapiens) persisted." In other words, rather than accounting for important hominid traits as responses to particular environmental conditions (e.g. the expansion of the savannas), we should look to changable, fluctuating conditions and/or mobility into new circumstances.

The October 7 New York Times (p.F9) reports the discovery of a new specimen of Australopithecus boisei. It is 1.4 million years old and is the largest member of its species found, the only one with an intact mandible, and the first from Ethiopia. Although the discoverers lump the fossil into the existing taxon, it does display some differences that make aspects of it more similar to A. robustus, A. aethiopicus, and even Homo. Such distinctions have apparently led the discoverers (who include Tim White of Berkeley) to expect a renewed dabate between "lumpers" and "splitters" of fossils from this period. They, however, are considering this fossil a regional variant of boisiei. A full report is in the next Nature.

The basis for the turnover pulse hypothesis of Elisabeth Vrba is supported by recent work by geochemist Thure E. Cerling (Monastersky 1998). It is becoming increasingly clear that a major change in worldwide vegetation patterns occurred at about 8 million years ago, with C-3 plants dominating before the change and C-4 plants dominating ever since. Because plants following the C-3 photosynthesis pathway tend to be trees while those following the C-4 pathway tend to be grasses, it would appear that a more heavily forested world was replaced by one dominated by grasslands at about 8 million years ago. The expansion of grasslands at the expense of forests in the Late Miocene may explain the widespread extinction of ape species during this period and may have benefitted those species whose physical adaptations&emdash;including, perhaps, the ability to walk bipedally&emdash;enabled them to survive in the grasslands.

Glen Conroy et al. have recalculated the brain size for the Australopithecus fossil known as Mr. Ples (Gibbons 1998c). Previously thought have a volume of greater than 600 cc, far higher than the other australopithecines that it resembled morphologically, Mr. Ples's brain size seemed to fall into the range of later hominids. The recalculation to about 513 cc places Mr. Ples more firmly among the australopithecines.

New fossils found at Kanapoi and Allia Bay in Kenya seem to confirm the status of Australopithecus anamensis as the oldest hominid species (Leakey et al. 1998). All of the fossils identified as anamensis were found in tuff directly dated to between 4.17 and 4.07 million years ago. New fossils recovered include pieces of maxilla and mandible, capitates, and teeth that are, according to the authors, clearly more primitive than Australopithecus afarensis.

As previously reported in the Update, a new analysis of the cranial capacity of the fossil known as Mr Ples has caused a revision of its estimated cranial capacity (Conroy et al. 1998). Found in 1989 at Sterkfontein, South Africa, Mr. Ples was identified as Australopithecus africanus with an anomalously large braincase exceeding 600 cubic centimeters. This is more than 20% larger than any previously reported cranial capacity for the species.

A new analysis conducted by Conroy et al. using computer imaging shows that Mr. Ples's cranial capacity is actually far smaller, about 515 cubic centimeters. The authors of the study state that computer imaging on a number of other hominid crania indicates that the cranial capacities of these skulls may have to be reassessed downward as well. This study implies that our current models of the tempo and mode of brainsize increase in hominid evolution may need a major reassessment.

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