Our story has now gotten to about 5 million years before yesterday afternoon. I shall commit an outrage against paleontology by giving short shrift to all sorts of lovely complications and baldly stating that one group of hominids started, for one reason or another, to walk on two feet instead of four. We refer to these new, bipedal hominids as members of the genus Homo.
I will, however, address one nasty complication. The hominids were never truly quadripedal in the first place. After all, they were descended from primates who lived in trees and used all four limbs for grabbing as well as locomotion. Hominid hands aren’t equipped with hard hooves or big pads for heavy-duty contact with the ground. So the hominids were, in point of fact, neither quadripedal nor bipedal -- they were sort of ‘unpedal”. Or perhaps we could say that they were simultaneously quadripedal and quadrimanual. The Homo species didn’t so much give up using their hands for walking as they specialized their feet to do a better job of walking. So it’s best to think of the Homo genus as hominids with greater mobility.
Many people have made a big deal about how the shift to bipedalism freed the hands to do all sorts of intellectually stimulating things, such as making tools, but our hominid relatives use their hands quite adeptly. It’s true that bipedalism gave the hands even greater freedom, but the basic capability to use hands to manipulate objects was already in place with the hominids. What really changed the hominid condition was the new combination of using hands while walking: carrying things. Chimps and apes can pick things up, and they can run pretty fast, but when they try to do both at once, they’re pretty clumsy. Homo species could carry fairly large items -- like babies, for example -- around with them. Perhaps bipedalism was a response to premature birthing -- it’s difficult to disentangle causes from effects in these matters.
Anyway, this new technology of "carrying things" opened up a new opportunity for the Homo species: the hunter-gatherer lifestyle. They could forage over bigger territories, returning to a home base with the results of their labors. And the home base itself could be mobile. Thus developed the basic pattern of Homo species life: a base camp from which foraging expeditions spread out over the territory. When the territory was exhausted, the base camp moved to a new location. This basic scheme is the foundation of the hunter-gatherer lifestyle.
These hunter-gatherers were omnivorous, and so were interested in everything in their biosphere. They weren’t specialized to any species of prey or any particular kind of fruit, nut, berry, or root, so they couldn’t afford to be ignorant of any living thing. They therefore had to develop an encyclopedic knowledge of their environment: when plants blossomed, when trees set their fruit, where different animals travelled and a thousand other details. All this knowledge had to be integrated into a system that helped the hunter-gatherers obtain the greatest amount of food. This required a significant amount of brain tissue devoted to the problem, which is now sometimes referred to as the natural history mental module.
But there’s one other factor that dictated the nature of the system: those damned infants with the fat heads. Since they were helpless, they had to be closely cared for, and they had be carried everywhere, which greatly hampered mobility. So the base camp was the place for the infants. A small group of caregivers stayed with the infants, while the others spread out in their foraging.
And here began sexual specialization. Clearly, females had to be the caregivers -- they’re the ones with the milk. But the Homo groups didn’t need to dedicate all their females to caregiving, and besides, at any given time only a portion of the females had infants to care for. Thus, some of the females could forage. The younger females were more robust and energetic than the older females, so they made better foragers. Of course, they were still needed to nurse the infants, so they couldn’t go far. The system that developed was fairly complicated. The infants stayed at base camp. The older females spent most of their time caring for the infants or preparing food. The younger females alternated between foraging close to base camp and nursing their infants. The men foraged far from base camp, and the greater distances they travelled could only be justified with higher-value food: meat. So the men were the hunters and the women were the gatherers.
This basic system worked beautifully because it was so adaptable to almost any environment. But it, in turn, generated a great variety of new problems which, as you might imagine, demanded even larger brains. The first factor at work was the difference between the overall caloric productivity of the men and that of the women. The women-gatherers supplied a steady diet of low-quality nuts, berries, and roots. The men-hunters provided occasional grand slams when they brought home kills. This created a situation demanding negotiation. A good kill provided more nutrition than one man could eat, but it was only occasional. Therefore, the men needed to negotiate deals with the women in which they shared their kills in return for a portion of the women’s produce. Both sides ended up with a regular low-quality diet boosted by occasional high-protein meals.
But then a new factor arose that made matters much more complicated: sex.