Peter Ungar and Matt Sponheimer earlier this fall
[T]he isotope data also suggest enormous and unanticipated differences between contemporaneous taxa with strong morphological similarities, notably the robust australopiths P. robustus and P. boisei. Despite their attribution to the same genus, there is no overlap in their carbon isotope compositions (41), which is a rarity for congeners among extant mammals.
Maybe this should give pause to those who insist that A. robustus and A. boisei are sister species. Ungar and Sponheimer here reiterate the observation that microwear is very similar between A. boisei and A. afarensis:
The apparent continuity of microwear pattern through the putative lineage Au. anamensisAu. afarensisP. boisei could even suggest that morphological changes reflect increasing efficiency for grinding large quantities of tough food. Although living primates that eat tough items typically have sharp shearing crests, eastern African australopiths and especially P. boisei may have evolved a different solution for processing such foods, given the flattened, thickly enameled teeth of their close ancestors (23). Natural selection must work with the raw materials available to it. Thus, the present-day ecomorphological diversity within the primates may not be sufficient for making some paleoecological inferences, which is not surprising given that the vast majority of all primates, especially apes, that have ever lived are now extinct.
This idea was raised earlier, for example in the context of the stable isotope findings on A. boisei (“‘Nutcracker Man’ debunked”). Until we have more stable isotope results from the known sample of A. afarensis or A. anamensis, we won’t be able to test this “tough C4 food” hypothesis. “Ecomorphological diversity” refers to the match between food types and the topological properties of tooth crowns among living primates. Generally speaking, primates with high crowns and high cusp relief with shearing crests are thereby well-suited for eating tough foods like leaves and stems. That’s the common ground between gorillas and colobines, for example. A. afarensis and especially A. boisei have exactly the opposite morphology from what would seem to be the “tough foods” pattern. So why do these species seem to be acting like grazers? Very peculiar.
My own attitude is that if we can’t clearly make sense of the anatomy of A. boisei, then we won’t be able to untangle the diets of the other species. Early hominins evolved along a distinctive trajectory toward larger molars, smaller canines, and bigger jaw musculature within a common body plan. A. boisei represents the extreme of this trend. So if A. boisei is the logical morphological extreme, why does it seem to have such a different dietary strategy than every other hominin with stable isotope evidence?
Meanwhile, if Ungar and Sponheimer are correct in asserting a common dietary strategy in the East African species, then it seems pretty clear that early Homo shares a dietary commonality with the South African species, not the East African ones. One might argue that Homo differentiated from other hominins within East Africa by adopting a fundamentally South African dietary strategy. But I would be more inclined to suppose a South African-derived hominin made incursions into East Africa, possibly repeated ones, as Homo was emerging. Ungar and Sponheimer are correct that natural selection works with the materials available. Population growth and migration are vastly more rapid than in situ evolution. What if the apparent “early Homo” record actually represents a series of successive dead-end migrations from southern Africa?