The hominids take the cake for one of the most perverse decisions in the history of evolution: climbing down from the trees and resuming ground living. After all that specialization for arboreal living, it seems insane, but it all started so innocently. Imagine you’re a hungry primate reaching out for the last fruit at the extremity of a branch. Your fingers touch it and -- the stem breaks and it falls to the ground! Curses! But then you hear a little voice whispering in your ear, "Why not just run down and grab it?" You scan the forest floor carefully; there’s nobody in sight. No predators to eat you and no fruitivores to steal your fruit -- so far. It’s safe right now, but that could change in a minute. You scamper down the tree, race to the fruit, seize it, and dash madly for the nearest tree, which you climb in a trice. Settling down with your prize, you congratulate yourself on listening to that little voice. It was the voice of Fate speaking to you, for you have just started the slide to hominidhood.
In all likelihood, however, the hominids didn’t come down from the trees for opportunistic reasons -- most probably, they were forced to do so by drying climate. There aren’t any watering holes up in the trees. If you live in a rain forest, you can get plenty of water from the fruit and by licking leaves, but what if the climate changes and your patch of rainforest slowly shifts to a drier milieu? There are no easy evolutionary adjustments you can make to cope with the problem. Koala bears evolved an extreme metabolism to solve the problem, and sloths coped by lowering their metabolic rate to microscopic levels. But those big brains in the primates couldn’t handle such extreme changes. Another solution was necessary, and the one that worked was an alteration of behavior to permit life on the ground.
The new ground-dwelling primates, or hominids, were still strongly inclined to stay in the trees. Living on the surface was still risky, so they always kept close to a tree, and bolted up that tree at the first inkling of trouble. And they learned a trick that the herbivores had long since figured out: safety in numbers. The original primates were loners, but when they came down to the ground, it was always safer to have plenty of eyeballs scanning the neighborhood. So they started living in family groups. Thus began an almost comical sequence of screwy problems and even screwier solutions that propelled those innocent creatures to the Crown of Creation: videogames.
Social living, as any geek will tell you, presents a host of challenges. Many of the simple methods of social coordination developed by other animals just didn’t work for the hominids. The food that hominids eat, for example, is not dispersed and uniform in concentration as is the grass that grazing animals eat. Hominid food comes in small, widely dispersed packets of high value: fruit, roots, small animals. Hominids must forage for food, and this foraging process has all sorts of social implications. Rather than bunching up, hominids must spread out to explore a lot of territory, yet they must stay close enough to provide warning of the approach of a predator. This requires the development of warning calls, as well as a cooperative approach to covering ground. Especially tricky is the problem of sharing. When one hominid finds a particularly desirable meal, he could eat it all himself, or share it with a few others. Will he share it? And with whom? The sharing of food establishes relationships and generates expectations of reciprocity. Remembering who shared food with whom, and who refused to share food with whom, became an important factor in survival. Managing these relationships was even more important. To cope with all these new problems, the hominids developed -- guess what -- more brain tissue dedicated to handling social relationships. This new hunk of brain tissue (which is not actually localized to any particular portion of the brain) is sometimes referred to as the social reasoning mental module.
Now, all this brain tissue was starting to make a pretty big brain, which required an ever-larger skull, and that in turn created a new problem: the infants’ heads were too big to squeeze out the birth canal. Female hominids were dying in childbirth. Something had to be done, and this new evolutionary pressure generated several responses. First, the female pelvis changed in form to permit a larger birth canal. This, unfortunately saddled (ahem) the females with an inferior arrangement for walking, resulting in an ungainly gait.
Another response to all those fat heads was to push the kids out of the womb before they were fully cooked. Dump the kid while his head is still small, then let it grow to full size. But this in turn created yet another problem: how could the hominids take care of babies that were essentially premature? A newborn herbivore is up and running within a few hours of birth, but these hominids infants were helpless and immobile.
Fortunately for us, Darwin had a few more tricks up his sleeve. The hominids figured out that they could care for the infant for several years until he could take care of himself. They also refined a system that had been around for a long time: allomothering. In this scheme, females related to the mother help care for the kids, making it possible to provide lots of care to the infant. Other species such as elephants use this trick too, but the hominids extended and refined it. Of course, in the process, they had to develop even more complicated social schemes which in turn required -- can you guess? -- even more brain tissue. It seemed as if every problem was being solved by throwing neurons at it.
Our next exciting episode: Enter the Hominina!