Here's what's nagging me:
You'll recall John Hawks opined that the discovery undermined the popular Expensive-Tissue Hypothesis, or ETH. The ETH is often wielded by carnists as proof that humans must eat meat, in order to fuel our large brains; and further, that it proves meat-eating kick-started and drove hominid brain development through the Pleistocene. In short, the ETH is summarized in popular language as "meat made us smart" (though that was never actually what its authors argued).
It occurred to me that some readers of this blog may not quite get what Hawks meant by commenting that,
(a) 2.6-million-year-old butchery tradition should already have refuted the hypothesis that meat-eating caused the expansion of brain size in Homo. ... the observations Braun points out pretty much demolish the 15-year-old story of 'expensive tissue.' Australopithecus seems to have had a small gut, and a bigger brain than chimpanzees. If there was a tradeoff, A. afarensis had already made it.So, here's some background on the ETH, followed by my thoughts on its destiny.
The ETH was first proposed by Leslie C. Aiello and Peter Wheeler in 1995, who argued that the human brain had such a high metabolic cost that it could only be maintained through a trade-off of energy from other metabolically expensive tissues, thus making those other tissues smaller than they'd otherwise be. As they put it in the abstract to their initial article, The Expensive-Tissue Hypothesis: The Brain and the Digestive System in Human and Primate Evolution :
Brain tissue is metabolically expensive, but there is no significant correlation between relative basal metabolic rate and relative brain size in humans and other encephalized mammals. The expensive tissue hypothesis suggests that metabolic requirements of relatively large brains are offset by a corresponding reduction of the gut. The splanchnic organs (liver and gastrointestinal tract) are as metabolically expensive as brains, and the gut is the only one of the metabolically expensive organs in the human body that is markedly small in relation to body size. Gut size is highly correlated with diet, and relatively small guts are compatible only with high-quality, easy-to-digest food. The oft-cited relationship between diet and relative brain size is more properly viewed as a relationship between relative brain size and relative gut size, the latter being determined by dietary quality. No matter what is selecting for relatively large brains in humans and other primates, they cannot be achieved without a shift to a high-quality diet unless there is a rise in the metabolic rate. Therefore the incorporation of increasingly greater amounts of animal products into the diet was essential in the evolution of the large human brain.Many people have read this to mean that "meat made us smarter," but that is not quite what Aiello and Wheeler meant. As Aiello put it in another paper (emphasis added),
Food of low digestibility requires relatively large guts with elaborated fermenting chambers (stomach and/or small intestine) while food of high digestibility (such as sugary fruits, protein and oil rich seeds and animal material) requires relatively smaller guts characterised by simple stomachs and proportionately long small intestines. ...This strongly suggests that the observed association between diet quality and relative brain size (Parker and Gibson, 1979; Clutton-Brock and Harvey, 1980; Milton, 1987, 1988, 1993; Leonard and Robertson, 1992, 1994, 1996) is really a relationship between relative brain size and relative gut size, the latter being determined by dietary quality. The main conclusion is that no matter what is actually selecting for increase in brain size in humans and non-human primates, a high quality diet is necessary for encephalization. It relaxes the metabolic constraints on encephalization by permitting a relatively smaller gut, thereby reducing the considerable metabolic cost of this tissue.So, a "high-quality diet" need not necessarily be one composed mostly or even exclusively of animal foods. Aiello clearly argues that an increase in meat-eating drove encephalization, but the definition she offers is not dependent upon meat-eating. If Richard Wrangham's cooking hypothesis turns out to be right, it could very well be the case that the high-quality diet needed for the ETH was based, at least at first, on cooked fall-back foods like roots, tubers and seeds, along with fresh fruit, supplemented by scavenged meat.
It is possible to read too much into this qualification, however, and allow wishful vegan thinking to shape our analysis. The temptation to paint our ancestors as peaceful frugivores is strong in the vegan community, and we may end up rejecting the ETH for purely emotional or fanciful reasons. This, as I've said before, is a trap. So, we need to be honest with ourselves. Whatever the veracity of the ETH, there is no doubt that hominids ate other animals, at least some of the time. Any vegan who argues otherwise is misinformed, and will get soundly trounced in a discussion with informed carnists.
But that doesn't mean we need to acquiesce to the ETH, either. There are legitimate, non-vegan-related reasons to question it, and that's what Hawks was driving at.
Indeed, one of the ETH's fundamental assumptions might be mistaken.
The ETH depended, in large part, on the observation that hominid brain evolution was punctuated by two sudden growth spurts, one at about 2 million years ago (coinciding with the first appearance of the genus Homo), and another at about 500,000 years ago (the appearance of the species Homo sapiens). Each event, it was held, was powered by adoption of increasingly high-quality diets, usually presumed to be meat-centered.
That was the theoretical landscape 15 years ago. Since then, several new fossil finds, and new interpretations of older fossils, have changed the landscape, and it is no longer clear that hominid brain evolution was punctuated. And the dividing line between Australopithecus and Homo is getting blurrier by the day.
Dean Falk of Florida State University sums it up well (emphasis added):
Although brain size remained conservative during the evolution of Paranthropus, it increased in Australopithecus and between the latter and specimens that lived more recently (~ 1.7-1.9 ma) inThe Figure (12.)3 Falk refers to provides two charts. The one on top plots cranial capacities for select hominins against time, and includes among them the genus Paranthropus (robust australopithecines), who are generally not considered ancestral to Homo. On this chart, the pattern for brain size displays the punctuated pattern mentioned earlier, appearing flat until around 2.0 Ma, then increases suddenly with the emergence of Homo.
Africaand the . The overall morphology of these more recent specimens is transitional enough so that some workers place them in Australopithecus while others include them in early Homo (Balter & Gibbons 2002, Wood and Collard 1999). If, indeed, these specimens are transitional, then the received wisdom that brain size suddenly ‘took off’ in the genus Homo around 2.0 mya needs serious reevaluation (Falk 2004b, Falk et al. 2000). Thus, rather than there being a jump in cranial capacity in early Homo, cranial capacity may have begun increasing in the Australopithecus ancestors of Homo a million years earlier (Falk et al. 2000). With the redating of Java sites (Swisher et al. 1994, Huffman 2001) pushing certain cranial capacities further into the past, there is no longer the discontinuity in the trend for increasing cranial capacity (Falk 1987b, 1998) that once contributed to the suggestion that brain size evolution underwent ‘punctuated’ events (Hofman 1983, Ruff et al. 1997, Leigh S, 1992). Rather, the recent discovery of LB1, the small-brained type specimen for Homo floresiensis (Brown et al. 2004, Morwood et al. 2004), lends an entirely new perspective to the study of hominin brain size evolution (Falk at al. 2005): From australopithecines through extant Homo, upward selection widened the range of brain-size variation, while australopithecine-sized brains may have continued to provide the lower boundary (at least, until very recently). Thus, to some extent, Fig. (12.)3 encapsulates the interplay between selection for brain size (vertical vector) and selection for neurological reorganization (horizontal vector). Republicof Georgia
The bottom chart, however, only plots brain sizes of hominids thought to be ancestral to Homo; that is, it leaves out Paranthropus. And here the pattern looks very different -- a steady, non-punctuated, linear climb beginning ~1.0 million years before the emergence of Homo. No sudden leaps upward.
In short, the top chart's data -- showing punctuated evolution of brain size -- is skewed by the inclusion of hominin genera and species not considered to be our ancestors. When we include only our own ancestral line, there appears to be no sudden leap in brain size, after all.
The recent discovery and description of Australopithecus sediba has only muddied the waters even further. At least one researcher is arguing that the genus Australopithecus should be sunk altogether. He reiterates his case in response to the stone-tool-using find from Ethiopia.
In this context, it looks like the ETH may turn out to be what John Langdon called an umbrella hypothesis: an attempt to explain a multitude of human features by means of a single adaptive breakthrough, with an hypothesis that appears parsimonious, but isn't. The ETH's fundamental assumption that human brain evolution was punctuated looks less likely today than it did 15 years ago.
In short, if the evolutionary pattern of hominid brain size is steady rather than punctuated, and if butchery and meat-eating have been part of hominid behavior since the days of the australopithecines, then the likelihood that meat-eating and hunting sparked sudden increases in cranial capacity goes down considerably.
This is not to dismiss the ETH out-of-hand. It's a serious and legitimate hypothesis, with experimental support in the literature (unlike the Aquatic Ape Theory for which Langdon coined the phrase). It may turn out to be true; many other respected theories began this way. But the appeal of its simplicity is also its danger.
Right now, the ETH does not offer a mechanism for how, precisely, increased meat-eating made hominids smarter. To be fair, Wrangham's elegant and appealing cooking hypothesis doesn't, either. Nor does the new, trendy theory linking brain evolution to climatic cooling. All they do is point to compelling evidence that their pet factor removed an energy constraint on brain evolution. But energy budgets cannot explain everything. Without a mechanism, an hypothesis remains limited in its veracity.
The future fate of the ETH is worth watching. I suspect it will pass on once all the taxonomic confusion about hominids is cleared up, and a clearer picture of our evolution takes shape.
But then, I could be wrong.
 Aiello, L. C., & Wheeler, P. (1995). The expensive-tissue hypothesis: The brain and the digestive system in human and primate evolution. Current Anthropology , 36 (2), 199.