OUR LARGE BRAIN: DOES SIZE MATTER?
Clearly, the answer to this question must somehow lie in the evolution of our brain, for it is the brain that determines how we will behave. Established wisdom tells us that hominid brain size increased gradually over time; from this point, we leap easily to the conclusion that our remarkable cranial organ was gradually burnished by natural selection over hundreds of thousands of generations. This perception directly reflects the power of a movement known as the Evolutionary Synthesis. This grand paradigm of evolutionary theory swept through paleoanthropology (the study of human origins) around the middle of the twentieth century, and has ruled there ever since. To cut a long story short, the Synthesis reduced virtually all evolutionary phenomena to the action of natural selection, acting slowly and consistently on the gene pools of lineages of organisms over vast spans of time. In essence, the focus of the Synthesis was on the accumulation of tiny changes within a continuous reproductive chain extending over the eons. Which is a great pity, because it turns out that the evolutionary process is a great deal more complex than this, with many more levels of action. Under pure natural selection, it is the reproductive success or failure of individuals that is the key element in evolutionary change; and while natural selection is doubtless an important factor in influencing evolutionary histories, it is far from the whole story. Populations, species, and environmental changes are also critical elements in the evolutionary drama.
One of the factors that facilitated the acceptance of the simple linear picture of evolution was the undeniable fact that, the farther back in time one goes, the smaller hominid brains tend to become. In fact, this evident pattern is the strongest evidence that anyone can actually quote for a pattern of linearity in human evolution. Brain sizes (when this information is preserved) are by their very nature easily quantifiable; and it turns out to be quite simple to join these fairly steadily enlarging (though spottily distributed) numbers into a sequence, implying that change is more or less inevitable and that it is only rates of change at different times that may complicate things. But is this ‘evidence’ really so strong? Well, it might be if the notion born of the Synthesis were sustainable: namely, that hominid species were few and time-successive, merging into one another with the passage of time. But the actual pattern that is currently being revealed by the enlarging hominid fossil record is very different. Instead of a gradually changing chain of hominids across time, the signal is one of a diversity of hominid species present on our planet from the very beginning of hominid history.
The paleoanthropologist’s job thus becomes one of recognizing the species within the morphological spectrum our fossil precursors represent, for we can no longer see species simply as arbitrary segments of evolving lineages. As yet, we have hardly started to tackle this task. But it is already quite obvious that the true number of known hominid species is already large, and that those species were morphologically very diverse. Which gives us yet more
reason to believe that the story of human evolution has been one of consistent evolutionary experimentation (with multiple species originations and extinctions), rather than one of within-lineage fine-tuning over the eons. And, if we cannot read hominid fossils simply as links in a chain, it follows that there is a pattern out there – a pattern that we cannot simply discover, but that requires an active effort of analysis.
The central units of such analysis are the species themselves. Numerous hominid species have appeared, have competed in the ecological arena, and have gone extinct (with or without leaving descendant species). If we are properly to discern the pattern of events in the human fossil record, then it is essential that we be able to recognize those species with reasonable accuracy. This is not an easy task. But it is an essential prerequisite to any further studies, including any attempt to determine the pattern of hominid brain size increase over time. And at present we have to admit several things. First, we do not know the true number of hominid species out there in the fossil record (though we can probably make a good stab at determining a minimal number without severely distorting the phylogenetic pattern we perceive). Second, within-species brain size is notoriously variable (the brain sizes of behaviorally normal modern humans, for instance, run from under 1000 to over 2000 ml); and even with a relatively good hominid fossil record, we have no idea of the ranges of brain size variation that characterized even those few extinct hominid species that we can agree on. Third, if we want to calibrate rates of change in brain size over time, we need reliable dating. But even where reasonably accurate dates exist for individual fossils, we have no idea of the overall time ranges (which probably varied widely) of the species they represent. This means that we have no accurate notion of when they might have given rise to descendant species. And, finally, we are very far from reaching anything approaching agreement on the phylogenetic relationships among those species of whose identities we can
be reasonably confident.
So what does the ‘average’ increase in human brain size over the past several million years mean? Yes, go back to over 2 myr ago, and hominid brains were in the ape size range – about a third the size of ours. At 1 myr ago, hominid brains were, in very approximate terms, two-thirds the size of ours. And by about 200000 years (200 kyr ago), before the appearance of Homo sapiens
, some hominid species, at least, had brains as big as our own. There is, then, no question that larger-brained hominids (with many other derived characteristics as well, of course) eventually won out in the evolutionary stakes (though some big-brained species lost out, as well). Overall, then, it seems justifiable to detect a time-related trend. But what’s the pattern?
The traditional tendency has been to join up brain sizes over time in a straight line, with the implicit assumption that slow, steady change linked them all. But as we’ve just seen, that’s hardly a practical option. And if it is correct, as it increasingly appears to be, that human evolution has been, among other things, a story of species competing with their close relatives as well as with other elements in the environment, a different possibility altogether emerges. For it is at least as likely that a relatively small number of discrete enlargement events in different species was involved in the overall trend towards larger brain size as that hominid brains (in diverse lineages) inexorably expanded generation by generation, come hell or high water. Big brains use a huge proportion of the available energy, and there must certainly have been a strong countervailing advantage for them to have emerged as the norm. The conclusion is compelling that this advantage must have lain ultimately in increased ‘intelligence’
(whatever that is). But, at a more basic level, it is at least as probable that more-intelligent hominid species outcompeted less-intelligent ones, as that larger-brained individuals simply reproduced more effectively in successive generations. This must have been especially true in the dramatically fluctuating environmental and geographical circumstances of the Pleistocene ‘Ice Ages’, during which most hominid brain size increase took place.
If the pattern of brain size increase over time among hominids is far from clear-cut, what about other brain attributes preserved in the fossil record? Well, it turns out that they are not much more informative. Endocasts, which are natural or artificially made replicas of the space within the skull that houses the brain, show minor differences in the brain’s external contours among hominids of different periods, but there has been considerable argument over just what those differences mean. The more that is learned about the working of the brain in living people, the more evident it becomes that some brain functions, at least, are quite widely diffused within the cortex, and that the relative surface areas of different brain area...