WARNING: The following post contains adult content!
From the very beginning of formal taxonomy in biology, genus and species names have been assigned partly as descriptions of important aspects of a species. Hence, Homo sapiens refers to our species' purported wisdom. The underlying idea is that one correctly understands important key functions that characterize the species. Linnaeus did not have 'evolution' as a framework with which to make such judgments, but we do. And we should use that framework!
However, sometimes, in the rush to publish in the leading tabloids (in this case, Science magazine), a name is too hastily chosen. That applies clearly to the recently ballyhooed 1.9+ million year old South African human ancestor. Substantial fossil remains of two individuals, a male and a female, were found and seemed clearly to be contemporaneous. Many features suggested that they could be marketed as a Revolutionary! revision to our entire understanding of human origins.
Now, too often in paleoanthropology there is little substantial evidence for such a dramatic consequences, similar to the pinnate carnage known as the War of Jenkins' Ear between Britain and Spain in 1739. But not in this case here! In this case, the specimens were dubbed Australopithecus sediba (See the special issue of Science, Sept 9, 2011 [subscription required]), and there are many traits, some more dramatic than others, by which to argue that it is Really (this time) important.
We haven't the space to outline all these features, but although we are amateurs at paleontology (for expertise, we fortunately have Holly the Amazing Dunsworth as part of our MT team from time to time), we feel qualified to comment. One of the most remarkable features of A. sediba is its long, delicate digits on its obviously dextrous hand, and in particular its long and clearly useful thumb. But useful for what?
One who is rooted, or rutted, in classical human paleontology, and thinks that tool use is what Made us Human, the obvious inference is: a dextrous hand for tool use! There is, however, a tiny problem: no tools were found in the site.
Now this can be hand-waived away in defense of the explanatory hypothesis that is derived from classical adaptive thinking. Wooden tools, like stems chimps strip and use to ferret out termites, wouldn't preserve, and at that early stage of our evolution, stone pebbles might have been used--or even made--but would be perhaps sparse and if not remodeled by the creatures, unrecognizable. Or perhaps these creatures hadn't carried their tools to the site. There may indeed be perfectly reasonable explanations for why no tools were found, but one must at least admit that the absence of tools does not constitute evidence for the tool hypothesis.
Still, if you believe there must be a functionally adaptive explanation for absolutely everything, and you are committed to the current framework of explanations about our ancestry, then tool-use is the obvious one. How occasional use of pebbles led to higher reproductive success--which, remember, is what adaptive explanations must convincingly show--is somewhat less than obvious. Also, it is the female specimen whose hand is best preserved, though the male is inferred (from one finger bone!) to have been similarly handy. But females don't throw pebbles to gather berries!
A satisfying explanation
So how solid or even credible is the tool-use explanation? Could there be better scenarios? We think so, and we believe our idea is as scientifically valid as the tired old one of tool use.
It is obvious upon looking at the fossil hand, that its most likely purpose was, not to mince words about it, masturbation. Just look at the hand itself and its reach position (figure 2). Think about it: deft and masterly self-satisfying would yield heightened sexuality, indeed of keeping one's self aroused at all times, ready for the Real Thing whenever the opportunity might arise. Unlike having to wait for prey to amble by, one could take one's evolutionary future in one's own hands--and use one's tool in a better way, one might say.
Being in a dreamy state is a lot less likely to provoke lethal strife within the population nor "Not tonight, I'm too tired" syndromes, compared to the high-stress life of hunting giraffes (much less rabbits) or trying to bring down berries, by throwing chunky stones at them. Our laid-back scenario does not require fabricating stories of how rock-tossing indirectly got you a mate, because pervasive arousal would be much more closely connected to reproductive coupling, a way of coming rapidly to the important climax: immediate evolutionary success.
Indeed, and here is a key part of our explanation, the same fitness advantage would have applied to both the males and the females. If both parties were at anticipatory states more of the time, fitness-related activity would have occurred even more frequently than it does now, if you can imagine that, and quickly led to our own very existence as a be-thumbed if not bewildered species.
Supporting our hypothesis, vestiges of the original use are still around, as for example the frequency with which football and baseball players grab themselves before each play. Of course, humans seem subsequently and unfairly to have evolved to be less gender-symmetric in this regard. But our explanation is far better than the tired stone-axe story-telling with which we're so familiar. For this reason, we suggest the new nomenclature for our ancient ancestor: Australopithecus erotimanis.
Now, you may think our scenario is simply silly and not at all credible. But is it? By what criterion would you make such a judgment? Indeed, even being silly wouldn't make it false. And, while you may view the standard Man the Hunter explanation as highly plausible, being plausible doesn't make it true. Nor when you get right down to it, is the evidence for the stone-age hypothesis any better than the evidence for our hypothesis.
Indeed, if you think carefully about it, even the presence of some worked pebbles would not count as evidence that hands evolved for tool use. The dextrous hand could have been an exaptation, that is, a trait evolved in some other context, and then later was co-opted for a new function--in this case, the flexible hand, once evolved for one use, could then be used to make and throw tools. What we have explained here is the earlier function that made the hand available in that way.
Don't laugh or sneer, because this is actually a not-so-silly point about the science, or lack of science, involved in so much of human paleontology. It's a field in which committed belief in the need for specific and usually simple adaptive scenarios, using a subjective, culture-specific sense of what is 'plausible', determines what gets into the literature and the text-books (though our idea might have a better chance with National Geographic).
Will anthropology ever become a more seriously rigorous science, with at least an appropriate level of circumspection? It's something to ponder.