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Once we recovered from the shock of Darwin's suggestion of a common evolutionary path, parallels have been drawn between humans and apes. For a while, we were compared with such species as gorillas or baboons, where a dominant male controls a harem of females. There was also a forlorn attempt to compare humans with the monogamous gibbons, ignoring the fact that gibbons, though forming permanent pair bonds, are solitary creatures. Most recently, genetic evidence has shown a close relationship with the chimpanzee, and confirmed the existence of some common ancestor of both apes and humans.
We will therefore concentrate on primates, which comprise some 200 species, and are extremely varied in their size and behaviour. In general, primates, including humans, are tree dwelling, tropical mammals, quadrupedal, with all four feet adapted for grasping. They have good eyesight, and generally have acute hearing, with a brain relative to their body size larger than most other species. Their sense of smell however, apart from the prosimians, seems to be less acute. Nearly all are vegetarians, most species mainly relying on fruit, although sometimes supplemented by insects or meat.
The Prosimians are thought to be representative of the earliest primates of some 60 million years ago, very small in size, nocturnal, insect eating creatures. A short while later, the anthropoids appeared - independently, it is thought, in what was to become America and Africa. These were the ancestors of the new world and old world monkeys - an interesting example of parallel evolution.
The first hominoids appeared about 35 million years ago, in Africa, leading to the gibbon and orangutan, plus an ancestor which gave rise about five million years ago to gorillas, chimpanzees and humans.
Primates range in size from the tiny mouse lemur, weighing less than a hundred grammes, to the gorilla, which may weigh over a thousand times as much.
There are relationships between such factors as diet and body size, brain size relative to body size and time taken to reach maturity.
We have suggested that early prosimians were insectivores, a diet that is nutritionally rich. The smaller a creature is, the more energy per gramme of body weight it expends, and needs to obtain through its food. It used to be thought that primate vision evolved to cope with the three dimensional world of an arboreal existence. It is now thought that binocular vision, in particular, along with grasping hands, evolved to capture prey.
Small creatures tend to live very 'fast' lives, usually breeding quickly with a short lifespan. However, the trade off in developing a larger brain to process the information from acute vision, and sometimes hearing, is that gestation times are lengthened. Primates tended, therefore, to have very few young, and live through several breeding cycles, and probably could not have survived in temperate latitudes, where the opportunities are limited by the seasons.
The benefit of a larger body size includes greater reserves to withstand adverse conditions, particularly changes in temperature. It also allows for a large digestive system which can process less nutritious food. Thus monkeys and apes generally eat vegetable matter, especially fruit, while the largest, such as the gorilla, exist mostly on leaves.
A larger body size allows a slower pace of life. It also increases the length of time need to bear young. Most primate young are precocial. While not, initially, able to support themselves fully, they can cling to their mother's fur, and can soon move around on the ground or in the trees. While most group living species have hierarchies of dominance, primates, especially some of the apes, have taken social organisation to new levels of complexity, further extending the period before offspring mature. Thus while lemurs may be adult within a few months, chimpanzees stay with their mothers for up to eight years.
Acknowledgments
Primate Info Net http://www.primate.wisc.edu/pin/
Singapore Zoological Gardens http://www.szgdocent.org/pp/p-main.htm
Citation
Bland, J.,(2002) About Gender: Primates
http://www.gender.org.uk/about/10ethol/a5_prim.htm