One of the most interesting of the early Primates which probably in some ways resembled man's oldest ancestor, is the tarsier. We have many fossils of this and one living representative, the Tarsius of Borneo (which is nocturnal and has great forward-looking, owl- like eyes). This, compared with the earliest Primates (the shrews), shows definite signs of the march towards the monkey form. The Tarsius has prehensile hands with disc-like expansions on the fingers. Its muzzle is reduced; its eyes are enormous; its brain is well developed, largely because the cerebrum is emancipated from the duties of a smell brain and is occupied in dealing with vision and touch, very delicately controlling the muscular responses involved in leaping from bough to bough. It was undoubtedly the change from plodding along on the ground on all fours to clambering among the branches that swung the evolution of the little insectivorous mammals towards the anthropoid direction.

There are some authorities who would trace man's origin directly back to a tarsioid ancestor, regarding the monkeys as a development in a different and more specialized direction. But most authorities would include in man's family tree a monkey-like (pithecoid) stage; though it must be understood that modern monkeys have changed considerably from the type that was the ancestral form of both man and monkeys as we know them.

It was in the Oligocene, 35 million years ago, that a creature lived that was more developed than a tarsier but in some ways less developed than any monkey or ape now alive—and very much less than a man. It was fully arboreal and better perfected for that kind of life than any of its own ancestors. It was in fact a monkey. Larger than the Tarsius, its increased size and weight put greater demands upon the climbing mechanism, i.e. strong, grasping hands and feet. A heavy survival value was placed on balance and distance judging, the senses and brain being developed accordingly. There is also a trend in the direction of the upright body, for in a tree it is easier and safer to sit or stand than it is to lie down.