5. The Evolution of Metazoa

5.7 The Primates

The primitive insectivorous eutherian stock possessing as many as 44 teeth gave rise to six types of insectivores. Three lines specialized as gnawing animals and were later eliminated by the prolific rodents (rats, mice) that competed for the same ecological niche. The remaining three types are known as prosimians. The oldest prosimian known – Purgatorius- is 70 million years old and was 40 cm tall. The prosimians still subsist but are not very successful (fig. 5.27). The tree shrews that live in Southern Asia are the most conservative prosimians surviving. Contrary to all other primates, their thumb is not opposable to the other fingers. Their genetic material differs also substantially from that of other prosimians. They are very different from the other prosimians.

Figure 5.27. Prosimians are evolved insectivores, of which the tree shrew is a representative. The primates evolved from the tarsier group.

Lemurs and lorises have a common ancestor. The divergence between lemuriforms and lorisiforms occurred 62 million years ago, in Africa. They began their divergence in Madagascar, during the earliest Eocene, around 54 million years ago. The lemuroids now occupy Madagascar. In this isolated environment, the lemurs produced numerous species with large numbers of animals. Among them there is no trend towards a higher mental development. On the African continent, primates eliminated lemurs. A second group of prosimians is composed of eleven species of lorises. These managed to survive in Africa and Asia. Most of them are timid nocturnal animals feeding on insects and birds. No trend towards a terrestrial life can be noticed among them. They spend their whole lives in trees.

It seems that all monkeys, apes and men evolved from the tarsiers. The origin of the euprimates, i.e. primates of modern aspect, has been debated for 30 years. Euprimates are characterized by grasping hands and feet with opposable thumbs and big toes. They have nails on their fingers instead of claws, forward-facing eyes and large brains. There are four main hypotheses for how these characteristics may have evolved. The first is that they evolved together for visually directed predation of insects at the end of tree branches. A second hypothesis is evolvement for eating fruit on slender branches. A third proposition is for grasping during climbing. The fourth hypothesis is that grasping evolved first, to exploit fruit on terminal branches, and eye-convergence evolved later for visually directed predation of insects.

The first undisputed primates appear in the fossil record about 55 million years ago in the Eocene. The fossils of these archaic primates, the plesiadapiforms, are extremely rare. They were committed arborealists capable of moving on large vertical supports and capable of grasping slender supports with hands and feet. One well-preserved 56-million-year-old skeleton of Carpolestes simpsoni lacks orbital convergence but had a grasping foot with an opposable big toe. It had a nail on the big toe but claws on the other fingers. Other plesiadiforms had claws on all digits and no opposable big toe. Carpolestes provides thus the first evidence for the transition from claws to nails in primates. Binocular vision and leaping evolved in euprimates.

Amphithecus is found in Burma and is 44 million years old. It had only 32 teeth, with thick enamel that shows it to belong to the hominoid family. The animal was about 80 cm tall and weighed 8 kilos. It gave rise in South America to the platyrrhine monkeys. These new-world monkeys have retained many primitive characteristics and showed no evolutive trend towards a terrestrial life during the 30 million years of their existence. They adapted beautifully to an arboreal life and made no attempt to transcend it. Back in the Cretaceous, early placental mammals saw the world in only limited colors, much like some humans who cannot distinguish red from green. One in 12 Caucasians and one in 20 Asian males can’t tell the two hues apart. This is still the case with the vast majority of the monkeys that evolved in the New World. Only the howler monkeys have evolved trichromatic vision.

In the Old World, Amphithecus evolved into catarrhine monkeys, apes and men. Propliothecus that lived 30 million years ago in Egypt initiated this evolutive line. At that moment, the earth’ surface was fragmented, of which large parts were wet (fig 5.28).

Figure 5.28. Vast areas of the world were extremely wet and covered by tropical forests during the mid-Oligocene period, 30 million years ago (small black dots). The moving continental plates however slowly redirected wind and maritime currents in other directions, so that aridity and cold increased in later times. The tropical forests then gave way to open grasslands in many parts of the world.

These Old World monkeys and apes evolved full trichromatic vision for the efficient search of young reddish leaves, which are high in proteins. This gave them a nutritional advantage over other leaf-eaters. A single gene duplication increasing the number of retinal cone pigments was enough to acquire this trait.

An increase in intelligence is noted among these groups. Monkeys are unable to recognize themselves in a mirror. They invariably believe that the image seen is that of another animal of the same species. Self-recognition is noticeable by the chimpanzee. It takes however much exposure to reach this state where the animal does not react socially in the face of its own view. There is a qualitative cognitive difference between great apes in general and other non-human primates. Chimpanzees and bonobos are known to be able to make tools. It has been reported in 1996 that orangutans in the wild manufacture tools, have a tool kit and adjust tool features to current needs14. Thus, the common ancestor of orangutans, chimps and man, a mid-Miocene Ape living about 15 million years ago, was capable of translating cognitive abilities into tool-using skills.

By 25 million years ago, Europe was warm and, in the following 10 million years, the shifting continental Arabian plate pivoted in such a way as to open a land bridge between Europe and Africa, which brought revolution. Across the bridge came early kinds of elephants as well as the anthropoid apes. The European history of this group fills many details left blank by the gap in the African fossil record between 13 and 6 million years ago. One of the most surprising finds is that bipedalism may have arisen directly from apes akin in life-style to the gibbons. If so, then the quadrupedalism of the chimp and gorilla represent reversions.

The great apes, despite a formidable intellectual advantage over other mammals, are on their decline. Orangutans have almost completely gone. Gorillas are scarce; only about 5,000 mountain gorillas are left. Chimpanzees do not fare much better either. These animals never put their intellectual advantages to better use because they were never really challenged to do so.

In contrast to this conservatism, an advanced hominoid species appeared in Kenya (the Kenyapithecus) and India (the Ramapithecus) about 14 million years ago during the late Miocene and early Pliocene epochs. It is now believed that Kenyapithecus is not one genus but two. Kenyapithecus africanus has been rebaptized equatorius, the earliest known ape to leave treetops for the ground, about 15 million years ago. It appears that these apes were part of one great evolving line of dryopithecine hominoids. During that time, tropical forests flourished over vast areas of Africa and Laurasia. Such an environment favored apes but did not require the development of hominid behavior patterns. The dryopithecine hominoids lived in an epoch during which no ecological imperative demanded an evolutionary trend in the hominid direction. Kenyapithecus and Ramapithecus are believed to be the forefathers of the Yeti and the orangutan. One may suppose that some ape populations of that epoch developed behavioral preadaptations to the hominid way of life. These preadaptations assumed no importance as long as the tropical forest existed. As an analogy one may mention the chimpanzees that possess a greater intelligence than is needed to survive in the forest but they were never forced to exploit this superiority. The chimpanzee and the bonobo are considered today as members of the hominid group. The bonobo is closest to Man.

For the dryopithecines, a challenge occurred 7 million years ago during the late Pliocene period. By that time, the moving continental plates had forced maritime currents and winds to take different directions than hitherto. Aridity increased, the forest began to shrink and gave way to open grassland on a large scale. Where the forest remained, in South East Asia and parts of Africa, the apes remained also. Where the forest vanished, the apes disappeared also: they simply perished. However, by making rapid and radical adjustments, at least one preadapted ape population managed to survive. It came face to face with new selective pressures and, through these pressures, undesirable genes were eliminated. This brought about morphological changes, which in their turn exemplify the hominid line.

This reasoning implies that behavioral and ecological changes must have preceded morphological changes. Gigantopithecus, probably at the origin of the Yeti, is the candidate for the initiation of the hominid line. This species lived in India and China, in an environment that appears to have been an open Savannah. The animals may have been genuinely gigantic but this is still questionable: an estimation of their size varies between 5 and 12 feet. In any case, Gigantopithecus was a powerful animal that could safely leave the trees of the forest and live in an open Savannah. This life in open country must have developed its intellectual acuity. It has indeed been found that groups of monkeys living in open country are more resourceful than the groups of the same species that live in a forested environment.

In the open country, Gigantopithecus began to forage for its food. This led to the development of tools in foraging activities. The use of a hand-held object allowed a reduction in the size of the user. Its ability to protect itself and kill was maintained through an increase in speed and agility.

Certain Gigantopithecus specimens, but by no means all of them, show a remarkable resemblance to some of the earliest known Australopithecus fossils. The Australopithecines are definitively hominid in character, appearing later in time (about 5.5 million years ago) and these hominids are also open-Savannah dwellers. It is generally assumed that the emergence of a species of man from this man-ape stock occurred about 500,000 years ago, although "Homo" skeletons have been found presumably much earlier in Kenya, around 3 million years.

An estimation of the temporal separation of the hominids of which man, the gorilla, the bonobo and the chimpanzee are the only remaining species subsisting, from the pongids (orangutans) can be made on the differences in composition of homologous proteins of the serum. Assuming a constant rate of change in the amino-acid composition of these proteins, it is estimated that man diverged from chimpanzee and gorilla about 6 to 4 million years ago at the beginning of the last glaciation while hominids (gorilla, chimp and man), diverged from the orangutan about 15 million years ago. Yet a major trend in Cenozoic primates evolution has been the progressive neo-cortical encephalization (fig. 5.29).

Figure 5.29. The skulls of a lemur, chimp and modern human are juxtaposed. One observes a tendency to a vertical posture (red arrows), a regression of the front (G) and an increase of the volume of the head.

Figure 5.30. Time of separation of the hominids from the pongids and of these hominoids from the cercopithecoids. At the time I drew this chart, gorilla, bonobo and chimp were still assimilated to the pongids, whereas they are now classified among the hominids.

In man, the exigency for a good development of the brain at birth, which is a primate heritage, clashes with the anatomical constitution of erect maternal bodies. This erection reduces the size of the birth canal. Parturition in this case occurs again at an early embryological state. This increase of the brain was accompanied by several other adaptations such as longer gestation periods, increased efficiency of placentation and longer generation times. If one takes generation length into account, then one comes up with the following estimates: the divergence of the hominoids from the cercopithecoids took place about 37 million years ago at the Eocene-Oligocene boundary. The divergence of the lemuriforms and lorisiforms from the hominoids took place about 65 million years ago. Finally the divergence of hominids from pongids dates back 12 to 15 million years (fig. 5.30).

It seems that a bipedal hominid group living 7 million years ago in East Africa separated into two distinct populations through the appearance of the Rift Valley. The population isolated on the dry East Side evolved further into the Homo direction. The population isolated on the western humid side reverted back to a quadruped loco­motion, walking on their knuckles. It evolved into chimps and gorillas. The volume of the brain of a common chimp (formerly called Pan satyrus and now named Pan troglodytes) is 400 cubic centimeters, compared to 1500 in Man. More human than the common chimpanzee are the anatomical features of the bonobo, Pan paniscus. The bonobo separated from the chimp about 3 million years ago and was recognized in 1929 as a species distinct from the chimp. Its social behavior bears striking resemblance with human conduct. As with humans, sex appears to be the key to the social life of the bonobos: when food is given, males get an erection; when females are in heat, they copulate with food-taking males and skip away with their food; when food is in sight, males invite females to copulate and females invite males and other females in sexual encounters. The bonobos copulate one third of the time "more hominum" ("according to the custom of humans", i.e. face to face) (fig 5.31). Gorillas also do.

Figure 5.31. A couple of bonobos copulates face to face.

Contrary to the chimp, the female bonobo is almost perpetually receptive and willing. Oral sex is not ignored. Coitus is only occasional, indicating that sexual plays are essentially social gestures designed as appeasement gestures. The Bonobo society centers on the females and is dominated by them. The sole absolute permanent social association consists in mother and male child. The male never leaves its mother. A second strong social group consists in adult females, which enter frequently in sexual contact, essentially by genital grooming. We have here a model of society centered on love, not war, and the use by females of their sexuality in a manner that resembles strongly a human behavior.

In summary, Homo is a catarrhine anthropoid like all the Old World monkeys. These possess a close spacing of the nostrils in contrast to the New World monkeys, the platyrrhines, which are characterized with a wide spacing of the nostrils. The Old World catarrhine anthropoids are themselves subdivided according to the presence or absence of a tail. These are cercopithecoids and hominoids. Man’ s place is in the hominoid division, sharing a place with the pongids (orangutan) and the less evolved hylobatids (gibbon and siamang). Man himself is classified among the hominids of which he is, with the chimp, the bonobo and the gorilla, the only species subsisting.

References

14. van Schaik et al.: Naturwiss. 83, 186, 1996

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