Primates in the Trees

The earliest mammals were, well, basically just rats. So if you are a creationist who is insulted by the thought that you are descended from a monkey, it just gets worse and worse the farther back you go. These early mammals scampered around on the forest floor, eating bugs and dead things while trying to avoid getting stepped on. But once the dinosaurs had been pushed out of the picture by the Fickle Finger of Fate, the mammals started colonizing some of the ecological niches opened up by Dino Demise Day. In the process, mammals differentiated in all sorts of directions; some became grazers, some became browsers, some became diggers, some became hunters. But one group, the primates, happened to explore the exciting opportunities of life in the trees.

Arboreal life has two major appeals. First, many trees had come up with the clever idea of placing their fruit up high where the big animals couldn’t get to it. An animal who lives in the trees has a food supply with no competition from animals on the ground. Second, arboreal living is intrinsically safer than ground living. Any predator big enough to eat you will be too heavy to follow you out onto the skinny branch. So the primates happily settled into their new ecological mix.

There were some problems with living in the trees. A lot of early primates fell out of the trees. When you’re lying on the ground stunned or injured, your ecological fitness is not very high. Primates responded to this problem with three adjustments. First, they developed grasping appendages: hands, feet, and sometimes tails. The more hands there are to grab with, the less chance you have to fall.

Second, they substituted muscular coordination for muscle mass. Climbing around in trees doesn’t take a lot of strength, and you don’t have to move particularly fast -- you just need to be able to haul yourself around smoothly and efficiently. Of course, this required bigger brains, which primates promptly grew.

Third, they greatly enhanced their visual capabilities. To understand why visual capabilities were so important, you have to put yourself in the position of a hungry primate: about 50 feet up in the tree. You’ve pretty well cleaned out the fruit in your tree, but the tree next door is laden with plump fruits. Unfortunately, it’s not quite within direct reach; the branches connecting your tree with that tree are too small to support your weight. You have two choices: you can climb down and risk a dash across the open ground, or you can jump. In the great game of life, the ones who chose Door #2 won. However, to jump successfully from tree to tree required two enhancements to the basic primate body. First, even more muscular coordination, so that the jump could be timed and aimed perfectly. Second, greatly enhanced visual skills. To plan a safe jump, our primate ancestors developed a grand suite of new visual skills. Binocular vision was the most obvious of these; it required only a minor repositioning of the eyes on the skull, as well as more neurons for stereo vision processing -- a LOT more neurons. But there were other skills as well. One was texture processing, the ability to read the texture of a surface and determine its shape and slope. That takes a great many neurons. And then there was color processing. A dog, who is color-blind, can’t see the difference between a sour green apple and a ripe red one, but any old primate can do that. Color vision confers all sorts of benefits in terms of reading the foliage, the fruits, and the other components of the arboreal environment. Of course, color vision requires (guess what?) LOTS more neurons in the brain. But, in for a penny, in for a pound, and the primates developed even bigger brains. This new section of the brain dedicated to visual processing was so special that it is sometimes referred to as the visual-spatial reasoning
mental module.

By the time the primates had mastered their arboreal environment, they were highly specialized, and the most distinctive component of primate specialization was a huge brain.

And then some of them turned right around and went back to the ground!
They Called us Hominids

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