Big Brains and Fingernails

How did the distinctive traits that make us human evolve?  So what are those traits, anyway?  Here are a few: binocular vision, fingers with nails and soft pads instead of claws, fingers adapted to grasping rather than running, upright posture freeing those hands to grasp and manipulate.

Roland Ennos, in his fascinating new book, The Age of Wood, notes that all those traits were already present in the bush baby, a tiny primate that looks a bit like a furry human.

Though they are similar to us in so many ways, bush babies are only distant relatives. Fossil evidence and DNA analysis show that their lineage split from ours around 50 million years ago. Yet they share with us many key derived characteristics: binocular vision, with the eyes both pointing forward; an upright body posture; differentiation of the limbs between hind legs and feet for locomotion, and arms and hands for gripping; and soft pads and nails on the tips of their digits, instead of claws. We usually think of these characteristics as being human adaptations, but they actually first evolved to help primates live in trees.

Ennos, Roland. The Age of Wood: Our Most Useful Material and the Construction of Civilization (p. 4). Scribner. Kindle Edition.

Bush babies are pretty different from us of course.  Not only are they tiny, but their brains are small, and they live mainly on bugs.  They live in trees and exploit their excellent vision to navigate their arboreal environment and spot the insects they eat.  A diet of insects is nutritious, but insects tend to be small, so a larger animal has a lot of trouble making a living eating them, at least in the trees.

Our closer relatives, the monkeys, are also arboreal, as are essentially nearly all primates, except humans.  They can get larger than bush babies because they eat leaves and fruit.  The fruit eaters have larger brains, because keeping track of what trees have fruit when requires more memory, but these smarts also allow them to develop more socially.

The next big step in our direction is made by the great apes.  What is so great about great apes?  Mainly their size.  The size of their bodies and brains.  Like the monkeys, they live mainly on fruit and are highly arboreal. Like monkeys, they sleep in trees, but unlike monkeys, they construct elaborate nests out of branches.  This, it turns out, requires not only considerable strength but also an understanding of the dynamics of wood.

Remarkably enough, these adaptations for lives in the trees preadapted humans for life on the ground.  Of course we needed a few more evolutionary tricks, like fully upright walking, distance endurance, still larger brains and more fine muscle control, to become who we are today.