A classic conundrum in science concerns the likelihood that something as biologically unusual as humankind would evolve. The question is often posed in astronomical or cosmological terms: how frequent is ‘intelligent life’ in the universe? But as we don't even know yet how frequent is any kind of life (although I'm sure many now suspect it is very common), this line of approach hasn't yet provided particularly satisfactory answers.
A more practical question concerns the local circumstances and boundary conditions on this particular planet, exemplified by the question ‘why did humans evolve in Africa, and not on some other continent?’ A few years ago I was in fact asked this very question by a press officer of my university. I said at the time that there just isn't an answer simple enough for the Aha-Erlebnis she was looking for, and there the matter rested.
But in a recent paper in the Journal of Biogeography, Norman Owen-Smith, a world leader in understanding megaherbivore behaviour and ecology, proposes an answer that is well worth serious consideration (Owen-Smith, 2013). It isn't an answer likely to satisfy a cosmologist and it doesn't necessarily have implications for evolutionary theory in general, but it is still an interesting piece of the unique history of our planet. What Owen-Smith proposes is essentially this: that humans evolved in Africa because, of all the southern continents, only Africa offered an abundance of meat, packaged in medium-sized hoofed mammals (100–1000 kg), mainly ruminants of the bovid family. Other continents didn't offer as favourable a mix. South America and the northern continents had a prevalence of megaherbivores (over 1000 kg), while Australia had a prevalence of smaller species (10–100 kg).
The underlying idea is of course that the ability to supplement the diet with meat from animals killed by carnivores was a critical prerequisite of ‘hominization’ and that the availability of portable-size carcasses would therefore facilitate the process. According to Owen-Smith, megaherbivores, despite their high biomass density, aren't really useful because of their low mortality rates and unmanageably large carcasses. And the unstated opposite is, presumably, that small herbivores don't really provide scavengable carcasses at all. And so humankind evolved in Africa because it was, if not the best of all possible continents, at least the best one available. Fair enough, and far be it from me to dispute the scavenging hypothesis itself or quibble about details such as availability of evolutionary raw material. Next time I'm asked, this will be my story.
But to me the most stimulating part of the study is the one that explains why the African hoofed mammals (ungulates) had this particular body size distribution in the first place. This is where the plot thickens and testable hypotheses begin to stir.
Owen-Smith bases his analysis on genus counts of the late Pleistocene ungulate faunas of each continent, with estimates of body size and diet. Body size is the bit that impacts on hopeful meat-scavengers, but diet is the one that leads to the causal explanation. Forcing all known genera into the simplest possible dietary classification of grass eaters (grazers) versus eaters of non-grassy vegetation (browsers), Owen-Smith shows that what was so special about Africa was really the great diversity and high abundance of medium-sized grazers, not just medium-sized ungulates in general. He also offers a straightforward explanation: the African savanna, unlike grasslands anywhere else, is both nutrient-rich and arid, and the African grasses are more nutritious and better able to withstand grazing than grasses indigenous to other (southern) continents. Mix high elevation (to provide aridity) and volcanic activity (to provide nutritious soils) on a continent spanning the equator and bingo! A sustainable grassland ecosystem full of medium-sized, grazing antelopes appears.
A very nice story and a treasure of compiled information to back it up. This is exactly what our evolutionary scenarios need: concrete and quantifiable ties to the physical and biotic processes and environments of our ever-changing planet. And yet I wonder about the details. The African elephant is listed as a browser although the stable isotope record suggests that African elephantids were grazers throughout their fossil history (Cerling et al., 2010). And how on earth did the fallow deer end up as a grazer? There are a few more such surprises but I don't really believe that they would significantly bias the result. I'm also surprised by the choice to reconstruct the ungulate fauna about the late Pleistocene, rather than the early Pleistocene or perhaps an even earlier interval, more relevant to human emergence. But I suspect that this choice isn't in itself critical either.
My real concern is what the result would have been using the more conventional, three-part classification into browsers, grazers and mixed feeders, or indeed any protocol using available palaeodiet data. Fundamentally there can't really be anything seriously wrong with forcing the diet classification into a browser–grazer dichotomy, just as there isn't really anything wrong with classifying vegetation types as either forest or grassland. Such dichotomies can greatly help our understanding by emphasizing the coarsest level of spatial distribution patterns, and they can be extremely useful for modelling approaches. But they can also hide underlying variability that may turn out to be critical.
The reasons for my worry are only beginning to emerge as the study of herbivore palaeodiets gets into its swing. In particular: what was the real diet of all those medium-sized bovids at the time of human emergence? Evidence, much of it still unpublished, is accumulating both from stable isotopes (e.g. Schubert et al., 2006; Cerling et al., 2010; Kaiser, 2011; Uno et al., 2011) and from tooth wear (e.g. Kaiser & Franz-Odendaal, 2004; Schubert et al., 2006; Codron et al., 2008; Stynder, 2009; Kaiser, 2011) that the browser–grazer polarization seen today in the African ungulate faunas was weaker in the geological past. Admittedly, C3 grasses might masquerade as ‘browse’ in isotope analyses, and microwear analysis might show an anomalous ‘last supper syndrome’, but mesowear analysis doesn't suffer from those biases and thus provides independent verification. There is now increasing multiproxy evidence of genera known as grazers today being mixed feeders in their earlier history, including the interval critical for hominization. There is also evidence, from both living species and fossil ones, that ungulates may oscillate between diets in response to environmental changes at different scales (e.g. Kahlke & Kaiser, 2010; Yamada, 2012). One can still enforce a browser–grazer dichotomy, of course, but if most species were actually using substantial amounts of both grass and non-grass, and perhaps changing their diet in complicated ways, the dichotomy might just not represent reality very well.
Is the relative shortage of committed grazers in the past a problem for Owen-Smith's hypothesis? Not necessarily. There is no need to assume that the critical factor was grazing in the sense of eating mainly grass on a day-to-day basis. A more generalized hypothesis could be based simply on the availability, in one way or another, of abundant grass biomass for a range of species with diverse dietary preferences, possibly reaching back as much as 12 million years (Feakins et al., 2013). The fact that at least two Pleistocene hominin species apparently had a diet mainly based, somehow, on grasses (or sedges) with C4 photosynthesis (Cerling et al., 2011; Lee-Thorp et al., 2012) also seems to be part of this story. Indeed, an extreme reformulation of Owen-Smith's hypothesis might state that while many herbivores, including some of our more remote fossil relatives went for the grass (or sedge) directly, our direct ancestors opted to harvest the same resource at a higher trophic level.
A fateful choice, as it turned out. ‘The ecological features that promoted the Pliocene radiation of grazing herbivores in Africa … provided the necessary conditions to enable the transition of a hominin lineage from ape to ape-man. This led ultimately to modern humans with the hunting skills to eliminate much of the large herbivore faunas they depended on when they later spread across the globe' (Owen-Smith, 2013). And, to crown the irony, the rapidly growing human population of the post-extinction world soon found itself increasingly dependent on direct utilization of cultivated plant biomass, especially grasses.