Hominid Species Essay Research Paper Hominid SpeciesThe

Hominid Species Essay, Research Paper

Hominid Species

The time of the split between humans and living apes used to be thought to have occurred 15 to 20 million years ago, or even up to 30 or 40 million years ago. Some apes occurring within that time period, such as Ramapithecus, used to be considered as hominids, and possible ancestors of humans. Later fossil finds indicated that Ramapithecus was more closely related to the orang-utan, and new biochemical evidence indicated that the last common ancestor of hominids and apes occurred between 5 and 10 million years ago, and probably in the lower end of that range. Ramapithecus therefore is no longer considered a hominid. The species here are listed roughly in order of appearance in the fossil record (note that this ordering is not meant to represent an evolutionary sequence), except that the robust australopithecines are kept together.

Ardipithecus ramidus

It is the oldest known hominid species, found in the Middle Awash region of Ethiopia in 1994 by Tim White and dated at 4.4 million years. Most remains are skull fragments. Indirect evidence suggests that it was possibly bipedal, and that some individuals were about 122 cm (4′0″) tall. The teeth are intermediate between those of earlier apes and A. afarensis, but one baby tooth is very primitive, resembling a chimpanzee tooth more than any other known hominid tooth. Other fossils found with ramidus indicate that it may have been a forest dweller. This may cause modification of current theories about why hominids became bipedal, which often link bipedalism with a move to a savannah environment.

Australopithecus anamensis

This species was found in 1994 by Maeve Leakey in Kanapoi and Allia Bay situated in North Kenya. The material consists of 9 fossils, mostly found in 1994, from Kanapoi, and 12 fossils, mostly teeth found in 1988, from Allia Bay. Anamensis existed between 4.2 and 3.9 million years ago, and has a mixture of primitive features in the skull, and advanced features in the body. The teeth and jaws are very similar to those of older fossil apes. A partial tibia is strong evidence of bipedality, and a lower humerus is extremely humanlike.

Australopithecus afarensis

A. afarensis existed between 3.9 and 3.0 million years ago. The first of its fossils were found in the mid 1970s along the East African Rift valley. Afarensis had an apelike face with a low forehead, a bony ridge over the eyes, a flat nose, and no chin. They had protruding jaws with large back teeth. Cranial capacity varied from about 375 to 550 cc. The skull is similar to that of a chimpanzee, except for the more humanlike teeth. The canine teeth are much smaller than those of modern apes, but larger and more pointed than those of humans, and shape of the jaw is between the rectangular shape of apes and the parabolic shape of humans. However their pelvis and leg bones far more closely resemble those of modern man, and leave no doubt that they were bipedal. Their bones show that they were physically very strong. Females were substantially smaller than males, a sexual dimorphism. Height varied between about 107 cm (3′6″) and 152 cm (5′0″). The finger and toe bones are curved and proportionally longer than in humans, but the hands are similar to humans in most other details. Most scientists consider this evidence that afarensis was still partially adapted to climbing in trees, others consider it evolutionary baggage.

Australopithecus africanus

A. africanus was found by Raymond Dart in 1925 in a limestone quarry (Tuang) in South Africa. Africanus existed between 3 and 2 million years ago. It is similar to afarensis, and was also bipedal , but body size was slightly greater. Brain size may also have been slightly larger, ranging between 420 and 500 cc. This is a little larger than chimp brains (despite a similar body size), but still not advanced in the areas necessary for speech. The back teeth were a little bigger than in afarensis. Although the teeth and jaws of africanus are much larger than those of humans, they are far more similar to human teeth than to those of apes. The shape of the jaw is now fully parabolic, like that of humans, and the size of the canine teeth is smaller compared to afarensis.

Australopithecus garhi

This species was named in April 1999. It is known from a partial skull. The skull differs from previous australopithecine species in the combination of its features, notably the extremely large size of its teeth, especially the rear ones, and a primitive skull morphology. Some nearby skeletal remains may belong to the same species. They show a humanlike ratio of the humerus and femur, but an apelike ratio of the lower and upper arm. Australopithecus afarensis and africanus, and the other species above, are known as gracile (slender) australopithecines, because of their relatively lighter build, especially in the skull and teeth. Despite this, they were still more robust than modern humans.

Australopithecus aethiopicus

A. aethiopicus existed between 2.6 and 2.3 million years ago. This species is known from one major specimen, the Black Skull discovered by Alan Walker in 1985, and a few other minor specimens which may belong to the same species. It may be an ancestor of robustus and boisei, but it has a confusing mixture of primitive and