Last Ape Standing: The Seven-Million-Year Story of How and Why We Survived Read Online Free PDF Page A

sorts of experiments on humans, but all indications are that the same holds true for us.) When food is scarce, fertility also drops and animals mate less frequently, an additional way of slowing down the cycle of life. While the deprivation makes life horrible for the creatures enduring it, from an evolutionary point of view it carries with it the quality of pure genius. Nutritional penury not only extends the life of an animal, but fewer offspring improves the chances of the whole species remaining in the evolutionary sweepstakes. Fewer offspring also places less stress on already overburdened food resources. The whole process of living decides, it seems, to bide its time until the storm passes. Cell growth on every level slows except for one key and remarkable exception: brain–cell growth increases.
    There the cells last longer,
and
they begin to make new versions of themselves faster, or at least the neurotrophins generated by the hypothalamus, which are the precursors of new brain cells, do. Not only that, other experiments show that food deprivation increases an appetite–stimulating peptide called ghrelin, which enables synapses to transform themselves by some molecular magic into cortical neurons. You could say the body and the brain strike a bargain. To compensate for the aggressive growth of new neurons, the rest of the anatomy fasts, stretching scarce nutritional resources that it then redirects to the brain. Or put another way, the body slows down aging and accelerates intelligence. This means that 3.5 million years ago, by the time Lucy and her contemporaries were desperately scavenging at the margins of an unpredictable land, the chronic deprivation they were facing was accelerating the growth of their brains. 4
    So our ancestors had two assets going for them. Upright walking made them more mobile and efficient, able to cover more ground and better equipped to evade the predators that were evolving along with them. Their larger brains meanwhile made them more capable of adapting to dangerous situations on the fly, more adept scavengers, and better at collaborating successfully with one another. All good in these strange and dangerous environments. That they survived despite their desperate circumstances proves that the combination of the two adaptations was succeeding. But there was now a new challenge: the two trends were on a collision course and bound, in time, to make it impossible to survive. Something had to give.

Chapter Two

The Invention of
Childhood (Or Why It Hurts to Have a Baby)
    My mother groaned, my father wept,
into the dangerous world I leapt
.
    —William Blake
    The human birth canal inlet is larger transversely than it is anteroposteriorly
(front to back) because bipedal efficiency favors a shorter anteroposterior
distance between a line that passes through both hip joints and the
sacrum … This size relationship, along with a twisted birth canal shape,
makes human parturition mechanically difficult
.
    —Robert G. Franciscus
    “When Did the Modern Pattern of Childbirth Arise?,”
Proceedings of the National Academy of Sciences
    Two and a half million years ago, around the end of August in the Human Evolutionary Calendar, primates like
Kenyanthropus platyops, Australopithecus afarensis
, and
Australopithecus africanus
begin to disappear from the fossil record. They may not actually have disappeared, but the evidence of them does. Either way, their evolutionary run was apparently nearing an end. However, with their disappearance a new, rich wave of human species began to crest and break on Africa’s broad and windy plains. In the space of one million years, nine new varieties of humans emerged. Stepping back and looking at the aggregated remains scientists have labored to pick out of the hills,valleys, and ancient lake beds of Africa like so many needles from an incomprehensibly huge haystack, these species give you the impression that the savanna apes that had been struggling so long to