Date 05-12-2008
GMTime 03:35:31
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Primates
Contents Updated: Tuesday, August 24, 1999
Primates
The very name primates betrays our self-centered view of the world. The order of primates is the first, the prime order, because we are in it. It comprises some 200 species whose characteristic features are binocular vision and grasping hands. Some of the diminutive mammals rather like shrews that inhabited the earth during the time of the dinosaurs took to the shrub and then, at about the time of the dinosaurs’ death, to the trees to become the forerunners of the primates.
These early arboreal mammals were insectivores needing to grasp and move quickly while not falling from the branches. By about 55 million years ago, they had developed the primate grasping hand with its nails instead of claws and a partly opposable thumb. Hands reduced the need for a snout with which to explore, and reduced the need for teeth—jaws became smaller. Their short snout and large, widely spaced eyes meant that vision was more important than smell. To catch insects efficiently and to leap accurately they had evolved binocular vision. The field of view from both eyes overlapped giving them a three dimensional view of the world so that they could accurately judge distance. The movement of the eyes to the front of the skull allowed the braincase and therefore the brain to enlarge.
The ancestor of the primates would have looked rather like the tarsier or, more so, the curious, primitive tupaia of South East Asia. The tarsier has immense eyes for its size (it is nocturnal) but its other features are plainly primate. The tupaia has long separable fingers but no opposable thumb and therefore has no grasp. It has claws instead of nails. It has large eyes but their fields of vision do not fully overlap. It has a snout and a keen sense of smell, and it lives mainly on the ground, though sometimes it runs up trees like a squirrel.
From such a creature evolved the prosimians, the pre-monkeys, of which the lemurs are one group. Through continental drift, Madagascar split from Africa about 55 million years ago and carried with it the primates then living there. Those left behind on the African mainland evolved into monkeys and the rest of the primate line but in Madagascar lesser changes took place. Pressure of competition was less keen and Madagascan lemurs remained essentially unchanged until today, although they did adapt to a variety of ecological niches.
The Madagascan ring tailed lemur has grasping hands with opposable thumbs and nails not claws. Their babies use their hands to cling tenaciously to their mothers. It has binocular vision, it is social and attentive to its offspring, but, on the ground, it walks on all fours. Another Madagascan lemur, the sifaka, can stand on two legs but cannot walk—it leaps along the ground as if it were leaping from branch to branch.
Meanwhile, on the mainland of Africa, about 40 million years ago monkeys evolved, derived it seems from ancestors that split from the lemurs much earlier. Their success drove the African lemurs into the night—they became nocturnal. All African monkeys are active during the day, and to take advantage of the daylight have evolved color vision, a boon for spotting ripe fruit. However they lost much of their sense of smell, unlike the lemurs. Color vision led to them using brightly colored features to signal their species, their sexual receptivity, and to distinguish friends and enemies. They also became raucous chatterboxes with a varied vocabulary of sounds, important for the emergence of speech.
Primates have tended to increase in size as they evolved. 30 million years ago some primates got too large to walk on all fours on the branches like monkeys and instead took to swinging beneath them—these became the apes. Today’s orangutans are the largest tree dwellers. At over 400 pounds they have reached their limit. Gorillas are bigger but they do not swing in trees as adults. A third of adult orangutans have broken bones. As they grow up they must often experience branches breaking under their weight. Adult orangs swing about in the branches of the tree they are already in, because they can feel the strength of the branches as they grip them. But, rather than making a bold swing to the next tree and risking a branch they have not tested, they descend to the ground and walk.
