Summarise those features shared by humans and other primates, describe their general adaptive significance, and evaluate their contribution to human adaptation.

Felix Bottomley Human Origins and Diversity

Humans are classified by biologists as belonging to the order Primates, a group that includes lemurs, lorises, tarsiers, monkeys and apes. There are certain distinguishing features that set primates aside from other animals, and more specifically mammals. These characteristics are, large brains relative to body size; nails instead of claws; grasping hands, and in most cases grasping feet (due to opposable thumbs); and frontally positioned eyes. These can also be considered as characteristics that are shared by humans and primates. They are also classified by the characteristics they share, these being anatomy, physiology, protein structure, and even genetic material itself. The similarities on which taxonomy today is founded are symptomatic of evolutionary relationships. As a result of this, by looking into the anatomy and physiology of other primates we can better understand what human characteristics we owe to our general primate heritage and what traits are uniquely human. Therefore, we can glean an idea of the features that are shared by both humans and primates. Yet, studies show that many of the differences between primates, especially apes, and humans are differences of degree rather than kind. By this we mean that there are very few characteristics that are not shared by primates and humans, nonetheless these characteristics have been taken much further in humans.

Fundamentally humans are exaggerated primates (Introduction to Physical Anthropology, 8th ed., 2000, p.133). There are certain features that set them above other primates, yet they are not solely a human attribute. Humans have a world wide distribution, linked heavily to the reliance on culture; the sole method of locomotion is bipedalism; and humans have larger brains relative to body size, which in turn leads to the ability to think and write about ideas, along with the ability to speak. However, when one looks at these features one can not help but notice the glaring fact that many other primates share some or parts of these features. For instance gibbons spend ten per cent of their locomotive time on their hind legs, primates also have large brains relative to other mammals, with allows methods of communication but they are not as advanced as to be able to write or think consciously about complex ideas. The distribution of other primates is interesting as they are predominantly a tropical group. Yet, there is the snow monkey of Japan that is able to live in cold mountainous conditions. One must also remember that humans evolved from the tropics, and they are only able to live in such wide ranging locations because of their ability to culturally adapt to extreme situations. This is further evidence that the differences between humans and primates are more a question of degree rather than kind.

There are also other features that are shared by both humans and other primates which have great importance. Both have a k-mode reproductive strategy, which means that the family size is usually very limited to either one or maybe two offspring. This is because of the far greater level of parental care and child dependence in comparison to other animals. There is also the shared characteristic of muscles in the face, that relate to expression, therefore it is obvious when a primate, especially a human, is happy, which is not obvious from the facial expression in other animals.

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There are also three other features that are shared between humans and other primates, firstly a non specialisation in diet and therefore very similar teeth structures; secondly touch sensitive pads in the end of digits, which is related to the fact that both have nails instead of claws. Finally there is a high level of eye, hand coordination with the brain that does not occur in animals such as dogs which are nose-mouth sensitive animals. It was probably as arboreal animals relying on visual predation of insects that primates developed this coordination, in order to grasp the branches, watch their prey closely and then catch it.

There is much general adaptive significance to the shared characteristics of primates and humans. However, there are a few that are of greater significance, the opposable thumb and toe; the eyes and brain; family size; dentition; and nails in place of claws. The opposability of the thumb and toe has, for primates, been vital when grasping either objects or branches. Some chimpanzees are known to be able to use tools, and the evolution of the opposable thumb has made using tools for primates far easier. Many primates are also known to be arboreal, such as many of the strepsirhines (e.g. Lemurs), the ability to grasp branches again makes living in the trees much easier, and it facilitates the movement on or between trees. Linked closely to this is the adaptation of nails. This is particularly important for the smaller primates such as Lorises. Much of the food that primates require tends to be located at the further reaches of a branch, which tends to be much thinner than closer to the trunk. Therefore, if a primate had claws it would very likely injure itself, as the hand would wrap around the whole branch and imbed the claws in the palm of the hand. As a result primates have adapted to this situation by replacing the claws with nails. The adaptation of the eyes and brain of a primate are essential in its survival, as they allow it to hunt and also to leap or jump, and are the main mechanism for allowing the primate to sense the world around it. This is because it relies far less on the sense of smell; as a result primates have stereoscopic vision, which is facilitated by the fact that the eyes of a primate are positioned at the front of the head, leading to binocular vision. Stereoscopic is also aided by the visual information from each eye being sent to both hemispheres of the brain (ibid p.108). The significance of the adaptation of k-mode reproductive strategy in primates has allowed for far greater care of the offspring by the parents. This has thus allowed a much greater probability of that offspring surviving. Essentially what the primate is doing is dedicating more time to one offspring in the hope that it will rear it long, and well, enough to guarantee that the offspring can leave and reproduce itself. This is not the case in many other animals, such as turtles, which produce many offspring at the same time, maybe due to the presence of predators, hoping that maybe one or two will survive. Finally there is the significance of the teeth in primates, which tend to be very non specific. This means that there are a mixture of incisors, canines, pre-molars and molars. This is because primates, and humans, are omnivorous and therefore require the molars to eat leaves and fruit, whereas other food sources, such as meat require the use of the incisors.

The contribution that shared characteristics and their adaptive significance have had on human adaptation is very important in making humans the successful primates that they have become today. There can be no argument that humans are one of the most dominant species on the planet and it is essentially due to the evolution and development of many primate characteristics, as well as our ability to adapt to various environments. It has been more of a developmental adaptation rather than an acclimatisational, behavioural or cultural adaptation. The adaptation of humans mainly focuses on the functional modification of a structure to promote survival in an ecological niche; this could include the ability to live in cold or hot climates, or even at high altitudes. The dentition of humans is an important adaptation; because we are omnivorous we are able to live in a far wider range of ecological niches, surviving on fruit, meat, or even certain types of leaves. This could also be, in some ways, linked to the evolution and adaptation of taste perception, which has come from our primate ancestors, who required taste perception to avoid eating potentially toxic leaves or other food.

Also the development of the opposable thumb has had a large adaptive significance for humans. As we know it allows for the grasping of branches and, more importantly for humans, objects. This has allowed humans to make use of tools and weapons, which in turn has allowed us to hunt much more efficiently, adding to our chances of survival and creating time to develop in other ways, such as technologically. This distinct benefit is coupled with the fact that humans have developed much larger brains. Therefore, there is an obvious advantage over other animals as we can coordinate hunting more efficiently. In conclusion to this, though, there is not that great a link between the characteristics that humans and primates share and human adaptation. Many of the traits that primates have merely primitive forms of those that humans have, and therefore humans are better adapted to living all over the globe, whereas primates are still predominantly a topical animal.