Classification and Human Evolution by Washburn Sherwood L.;

Author:Washburn, Sherwood L.;
Language: eng
Format: epub
ISBN: 1474550
Publisher: Taylor & Francis Group

DISCUSSION

The emphasis on behavior may be defended on theoretical grounds. If the direction of evolution is due primarily to natural selection, the taxa which are recognized today should be the ends of adaptive radiations. However, there are numerous practical difficulties in the application of the point of view. There are few behaviorally oriented studies. There is no agreement on the meaning of the terms, such as brachiation. The most important parts may not fossilize so that the dangers of reconstruction and speculation are greatly increased.

These points may be illustrated with reference to bipedal locomotion. When human bipedalism is under discussion, reference is usually made to other bipeds (birds, dinosaurs, and various jumping mammals). But human bipedalism is structurally unique, and it is not even mechanically convergent with these other forms of bipedalism. Human bipedalism is a gait which is carried on while the trunk is erect, and walking is dependent on a unique form of ilium and a pattern of muscles found in no other animal. In mammals the ischium is normally long and the main propulsion comes from pulling the leg back, primarily by the hamstring muscles. Two-joint muscles are the most important. In man the ischium is short, quadriceps extensor femoris is larger than the hamstrings, and one-joint muscles are large. A pattern of walking based on large gluteus maximus, quadriceps, and soleus is uniquely human, as is the importance of the first toe in the structure of the foot.

Considering Australopithecus from the point of view of behavior, it will be seen that the ilium is constantly described but the ischium is not, although both are essential to the analysis of gait. The use of the word “biped” does not distinguish the kind of bipedalism, whether like a bird, a galago, a gibbon, or recent man. And an estimate of locomotion has to contain a personal opinion on how the muscles should be reconstructed. Emphasis on behavior forces the consideration of a range of problems which are minimally considered at present. What sort of a sequence of behavioral events might lead to the ilium (and presumably the hip musculature) evolving to nearly the condition seen in Homo while the ischium remained almost as seen in the living Pongidae? One theory which will fit the facts is that bipedal running preceded efficient, long-distance, bipedal walking. In this stage of locomotor evolution the ischium was long (fact) and the hamstrings were relatively large (guess). From the point of view of evolution, this means that Australopithecus may have been able to run as fast as Homo. Since even some of the living apes can run faster than man, there may never have been a stage of slow, partial bipedalism. Whatever the truth proves to be, emphasis on behavior leads to making a distinction between bipedal running and walking, to postulating running as essential in the ape-to-man transition, and to see fully-erect, long-distance, bipedal walking as later. Casts of the foot from Olduvai were shown to the conference by Drs. Leakey and Napier.

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