Out of Eden: The Peopling of the World Read Free Page B

aspect of our behaviour – something to do with the way we faced the climatic challenge, perhaps – that gave large, energy-expensive brains survival value. The problem of finding food in an increasingly dry environment must have taxed our ancestors’ resourcefulness. Larger brainsclearly helped them in some way. That behaviour must still be with us today, because over subsequent major glaciations during the past 2.5 million years, new human species with larger brains and more skills appeared in Africa. As the climate briefly warmed after each glacial maximum, the Sahara would become green for a few thousand years and the new human species would venture out to try their luck in Eurasia. By 1 million years ago, brain volumes of various human species living both within and outside Africa had increased from 400 to 1,000 cm 3 , and even into the modern size range. In other words, human brains had grown to three-quarters of their modern size long before we came on the scene. 5
    Why did we grow big brains?
    There have been various suggestions as to what the key behaviour selecting for big brains might have been. The ice ages forced Africa into extreme aridity and would have inspired the dwindling numbers of savannah humans to greater resourcefulness and cooperation. We can see the value of larger brains in such circumstances, but why should
our
brains have grown, and not those of the other mammals living at the edge of the savannah? One behavioural characteristic that is still very much with us today is our fondness for meat with our vegetables. Indeed, London-based anthropologist Leslie Aiello and her colleague Peter Wheeler (the originator of the cool heads theory) suggested that we needed to eat meat in order to facilitate our brain growth. 6 Brains need lots of calories to fuel them, and require high-grade nutrients in order to grow. Yet, as Aiello and Wheeler acknowledged, meat-eating is more a means than a motive for brain enlargement. They also argue that the parallel reduction in size of human intestines, as shown by an alteration of the shape of the ribcage in
Homo ergaster
, is evidence for a change to eating more meat than vegetables. In other words, the oldest true humans had lost the vegetarian pot-belly so characteristic ofaustralopithecines. But this change in the ribcage happened before the dramatic brain growth.
    Early humans such as
Homo habilis
,
Homo rudolphensis
, and
Homo ergaster
may have been more scavengers than hunters. Perhaps they learnt that by arming themselves with sticks and stones they could drive larger predators away from kills, and as their tool-making and cooperative hunting abilities improved they became more confident. But without evidence, all such arguments for the role of climate and meat-eating in the enhanced brain growth of early humans remain largely armchair speculation. We know that, at least in Africa, stone tools were used by
Homo erectus
to butcher meat, 7 but to establish a link between meat-eating, worsening weather, and brain growth we would need a comparison with purely vegetarian primates in the same environment over the same period.
    Recently, Sarah Elton, 8 an anthropologist based at the University of Kent at Canterbury, has provided just that – but her results rather shake our sense of the uniqueness of the human line. She measured brain size in a number of fossil skulls from primate species over the period roughly from 2.5 million years (the start of the cooling) to 1.5 million years ago. She studied the two main branches of hominids that diverged during that period,
Homo
and
Paranthropus
, covering a total of six species. As a comparison primate group she chose several prehistoric species of large, grass-eating, baboon-like
Theropithecus
monkeys that lived in the same environment over the same period. The results were startling. The large vegetarian monkey species showed no trend of increasing brain size over that time period, but hominids from both