The Evolution Of Primate Locomotion And Body Configuration - Essay Example

The evolution of primate locomotion and body configuration Body size has a leading control on locomotor performance and the morphology of the locomotor apparatus. In locomotion under the influence of gravity, body mass acts as weight force and is a mechanical variable. Accordingly, the application of biomechanical principles and methods allows a functional understanding of scaling effects in locomotion. In 1956, while searching through a collection of fossils at the American Museum of Natural History, I came across a small piece of forehead bone, identified as a "possible primate," that had lain neglected for half a century. It had been recovered from the Fayum badlands, sixty miles southwest of Cairo, Egypt, by an amateur collector named Richard Markgraf. The rock in which it was found was known to belong to the geological epoch we call the Oligocene (now estimated to have lasted from 34 to 23 million years ago). Although only the size of a quarter, the fossil displayed two defining characteristics of the Anthropoidea, or higher primates--the large evolutionary group that includes monkeys, apes, and humans. I could tell that the right and left frontal (forehead) bones in this small animal were fused along the midline suture into a single bone, as is the case in all the higher primates. And on the right side, just enough of the rim of the eye socket was preserved for me to establish that it was fully enclosed in the back by bony plates (the eyeballs of more primitive primates are normally encircled by just a thin bar of bone). Neither feature had been previously documented in so old a fossil. Better late than never, the small piece of bone joined a short list of other fossils discovered in the Fayum between 1906 and 1910 that also appeared to belong to higher primates. The best of the other fossils--both nearly complete mandibles--belonged to two small species named Parapithecus fraasi and Propliopithecus haeckeli. Both have lower molars with anthropoidean features--in particular, they are broad and flat and have five cusps. (Miyamoto 197-220) In addition, Propliopithecus has the same number of the different types of teeth as other Old World anthropoideans, and the two sides of the lower jaw are solidly fused together in the front, another important characteristic of higher primates. My interest sparked by these tantalizing finds, I began doing fieldwork in the Fayum more than thirty years ago. Since then, my teams and I have succeeded in gathering hundreds of additional primate fossils, documenting the presence of eleven primate species in Oligocene deposits that are 30 to 33 million years old. The largest of these species, a close relative of Propliopithecus, is Aegyptopithecus zeuxis, a cat-size creature that appears to stand at or near the base of the family tree of the Old World monkeys, apes, and humans. We have collected several skulls and faces of Aegyptopithecus, as well as many bones from the rest of its skeleton (see "Dawn Ape of the Fayum," Natural History, May 1984). Many of the eleven Oligocene species have anthropoidean features, including the fused frontal bone, enclosed eye socket, lower jaws that are solidly fused together in the front, and the broadened and flattened lower molars with five principal cusps. In certain details, the upper molars also resemble those of more recent higher primates. Another anthropoidean characteristic is the manner in which the bony ring encircling the eardrum lines the auditory opening at the side of the skull. The eleven species are diverse in many respects, however. By 1985, I had accumulated enough evidence to say that they fell into several different taxonomic families or subfamilies. Given that so much diversity had evolved, I had to conclude that the common ancestor of all the higher primates must go back a long way in Africa. This was only the beginning, however, for in 1983 a Fayum site called Locality 41 had been discovered. Its exposed deposits came from a much deeper layer than those of the quarries we had been mining for fossils. Anthropologist John Kappelman collected samples of Fayum rocks to measure their age. The dating method he used, called paleomagnetic reversal stratigraphy, is based on how the earth reverses its magnetic field at irregular intervals. (Ramaswami 257-273) When a rock forms, it is imprinted with the magnetic alignment of its time. By checking the changing alignments in rocks at numerous successive levels in the Fayum, Kappelman was able to show that the rocks at Locality 41 most likely date to 36 million years ago--in other words, to the late Eocene epoch. The site thus promised to open up a whole earlier chapter in higher primate evolution. After a primate canine was discovered at Locality 41 in 1984, we shifted our collecting efforts to those deposits and soon were rewarded by the discovery of more primate fossils. From these, in 1989 I was able to describe two new kinds of higher primates. The larger of the two, named Catopithecus browni, was documented by several partial lower jaws and one nearly complete skull. As in Aegyptopithecus, the eye socket is enclosed, the frontal bones are fused, and the bony ring supporting the eardrum encircles the margin of the ear opening. (Oxnard 1127-1158) For Catopithecus, the sequence of teeth in the upper and lower jaws is the same as in the Old World monkeys, apes, and humans. And as in all higher primates, the four upper and four lower incisors are spoon- or chisel-shaped and relatively vertically implanted, with the upper central incisors and lower outer ones being larger than their neighbors. On each side the incisors are followed by a single pair of upper and lower canines, then two pairs of premolars (bicuspids in the parlance of dentists), and lastly three pairs of molars. The premolars and molars resemble those of other early higher primates. We have recently discovered limb bone fragments from Locality 41 that on the grounds of size also appear to belong to Catopithecus. The animal would have been about the size of a modern South American squirrel monkey, with a face of corresponding size, but because its brain was not as big, the head was smaller overall. The second species I named from Locality 41, Proteopithecus sylviae, is distinctly smaller and has slightly broader, flatter molars. Tentatively, I have grouped both of these new species in the same family as the later Aegyptopithecus and Propliopithecus. However, several different families may be involved. Recent (and as yet undescribed) finds of Proteopithecus show that it had three pairs of premolars on each side, like New World monkeys and unlike Aegyptopithecus. Last fall I published descriptions of three more animals from Locality 41, each of which appears to represent a different family of higher primates. The first is Serapia eocaena, a small creature that resembles species belonging to the later Parapithecus and similar genera and should be ranked in the same family. The second, which is still smaller--about the size of a pygmy marmoset--is Arsinoea kallimos. The single known jaw has teeth that, in a very general way, look like those of a higher primate. A frontal bone, found about three-quarters of an inch from where the jaw was discovered, exhibits the complete midline fusion found in other higher primates. The third new species, which is the largest of the three, is called Plesiopithecus taras, a name meaning "the amazing half-monkey." Although its molars vaguely resemble those of the other Fayum higher primates, it has a greatly enlarged canine or incisor in the lower jaw that is quite unlike the front tooth of any other Fayum primate. (Pawlowski 257-276) What all of these discoveries add up to is this: as early as 36 million years ago, four or more taxonomic families of higher primates were living in Africa. To derive such diversity from a single ancestral species would have taken a long time--presumably between 5 and 10 million years. Thus higher primates have a considerable, unknown earlier history in Africa. For nearly a century, paleontologists have been seeking to understand the Eocene roots of the Anthropoidea. Numerous Eocene age primates, belonging to some forty or fifty genera, are known in North America and the combined continent of Europe and Asia. These fall into two broad groups, one resembling the modern tarsiers of Southeast Asia and the other the lorises and Madagascar lemurs. Students of primate history have wanted to derive the earliest African anthropoideans from one or the other--the "tarsioids" or the "lemuroids"--or at least to demonstrate that one has a closer relationship. What now seems most probable is that the higher primates are not derived from either of these northern groups, but arose from a third Eocene ancestral line, very likely in Africa. Works Cited Miyamoto, Michael M., DNA Systematics And Evolution Of Primates. Goodman, Morris. Annual Review of Ecology & Systematics, 1990, Vol. 21, p197-220 Oxnard, Charles E., Brain Evolution: Mammals, Primates, Chimpanzees, and Humans. International Journal of Primatology, Oct2004, Vol. 25 Issue 5, p1127-1158 Pawlowski, Boguslaw., Loss of oestrus and concealed ovulation in human evolution. Current Anthropology, Jun99, Vol. 40 Issue 3, p257-276 Ramaswami, L. S., Some Aspects of Reproduction of the Female Slender Loris, Loris tardigradus lydekkerianus Cabr. Kumar, T. C. Anand. Acta Zoologica, 1965, Vol. 46, p257-273 Read More