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Hominid Timeline

A Hominid is any creature of the family Hominidae (order Primates), of which only one species exists today--Homo Sapiens, or human beings. Extinct species of the family are indicated in fossil remains, some of which are now quite well known: Homo Erectus, Homo Habilis, and, going further back in evolutionary history, various species of Australopithecus.

The family most closely related to the Hominidae today is the Pongidae, the anthropoid apes including the gorilla, the chimpanzee, and the orangutan. These are believed to have diverged from a common ancestral line during the Late Miocene epoch (11.2 million to 5.3 million years ago). The physical characteristics that distinguish hominids from the pongids are erect posture, bipedal locomotion, rounded skulls with larger brains, small teeth (including unspecialized canines), and such behavioral characteristics as communication through language.

In the slow process of evolution, countless generations are needed to acquire these characteristics, so it is extremely difficult to draw a line between pongid types and hominids in the fossil record available. Teeth and skull fragments constitute most of the evidence, and these are often not well enough preserved to show adaptations that might serve to distinguish one species from another.

The oldest definitely known hominid genus is Australopithecus, of which several species have been identified in Africa. This type was bipedal and had manipulative hands and an opposable thumb, and there is some evidence of primitive toolmaking. The origin of Australopithecus dates to some 8 million to 5.6 million years ago, although the fossil record beyond 4 million years ago is inconclusive. There is evidence of a direct relationship between this type and a later species, Homo Habilis, the earliest-known finds of which are dated to about 2 million years ago; Homo Habilis was discovered and named by anthropologist L.S.B. Leakey, who believed it to be the earliest true toolmaker.

Homo Erectus, now believed to date to about 1.6 million years, was apparently the first hominid to forsake the dry, hot climate of tropical African savanna lands and move into more temperate zones. Fossils showing distinct hominid characteristics have been found in Java, China, eastern Africa, northern Africa, and Europe. It was Homo Erectus who mastered fire and also produced a range of well-made tools.

The physiological differences between the hominids and the pongids indicates how these two adapted differently for different environments. The most significant anatomical differences in the hominids are changes in the pelvis, femur, and foot, all clearly related to an erect posture and walking on two legs; these were a distinct advantage in an open savannah environment. The pongids, by contrast, had characteristics that were best adapted for brachiation (swinging by the arms) in a heavily forested environment. They had a smaller thumb than the hominids but a strong and opposable big toe. Young pongids of existing species are active and acrobatic in the trees, but as they grow older they spend more time on the ground, walking on all fours and taking the weight of the upper half of the body on the knuckles of the hand. The pongid pelvis clearly shows evidence of this quadrupedal locomotion. Another adaptation indicating brachiation and walking on all fours is the reduction in size of the hind legs.

The early hominids had larger braincases than pongids and protuberances on the base of the skull that articulated with the spinal column; the position of these occipital condyles farther back on the skull shows that the head was held erect. This also enabled the neck to bear the increased weight of the braincase. In the fossil pongid skull, on the other hand, the position of the occipital condyles, farther forward on the skull, shows that the spine and head were carried horizontally.

The pongid face was prognathous, i.e., it had projecting and massive lower jaws. The teeth show adaptation to a vegetarian diet; the molars are large and worn by grinding. The pongids were adapted for cutting and tearing large stalks, crushing with their large molars and chiseling with their sharp canines. In hominids the arc of the teeth is rounded and higher, and their disposition shows adaptations for cutting and eating seeds, fruit, grasses, and meats--in other words, the hominids were also hunters (or at least scavengers) as well as gatherers of food.

Copyright (c) 1997 Encyclopaedia Britannica, Inc. All Rights Reserved

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This page was last edited on: June 08, 2000