Zoo 317 Heredity, Evolution and Society |
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| Lecture 38 | Cummings: pp 440-445 |
| HOMINID ORIGINS | |
II. Australopithecine ancestors.
I. The relationships of fossil species to each other and to living species is based on anatomical variations that have changed during the evolution from an ape-like ancestor to modern humans. These variations also can tell us much about the activities and lifestyles of the fossil species.
B. Bipedalism is recognized by the location of the foramen magnum (where the spinal cord enters the skull). The shape of the pelvis is also informative. In humans, internal organs are supported by the pelvis from below. In apes, organs are suspended from the pelvis.
C. The volume of the brain has increased steadily in the evolution of humans.
B. The major species that followed was A. africanus, from which the earliest Homo are thought to have arisen some 2 million ybp.
C. Two later species of Australopithecus are recognized: A. boisei, ca. 2.6-1.2 million ybp, and A. robustus, ca. 2-1 million ybp. These two species are sometimes put in a separate genus, Paranthropus. Both groups lived too late to be ancestral to humans. They eventually became extinct.
B. H. erectus arose some 1.8 million ybp and survived until about 300,000 years ago. Fossils of H. erectus are widespread in Africa and Asia. Thus H. erectus appears to have been the first hominid to migrate out of Africa and occupy large areas elsewhere.
C. The earliest fossils of our own species, H. sapiens, were formed about 400,000 ybp. These archaic H. sapiens occurred in Europe and Asia as well as in Africa. They had clearly larger brains than H. erectus, but not too much is known about other parts of the body.
D. H. neanderthalensis fossils are found in Europe and western Asia and date from 150,000 to 30,000 ybp. Until recently, they were thought to be early forms of H. sapiens. However, several lines of evidence argue against this, and very recently analysis of mitochondrial DNA from a neanderthal bone strongly supports the idea that the neanderthals diverged from early H. sapiens long before the emergence of modern H. sapiens. It is thought that their evolution was driven in part by adaptation to ice-age conditions in Europe.
E. When and where modern H. sapiens arose is currently a matter of considerable argument. The probable date is 100-200 thousand years ago. The most likely place is Africa, but some argue for Asia. An African origin is supported by the greater genetic diversity within African populations as compared to non-African populations. It would thus appear that migration from Africa to the Middle East and thence to the other land masses occurred about 100,000 ybp and involved only a subset of the African populations in existence at that time.
F. Neanderthal and modern humans coexisted in time and must have had contact. Did they interbreed? Although the genetic evidence is minimal, it appears more likely that modern humans, who presumably were smarter and better hunters, drove the neanderthals to extinction with little or no incorporation of neanderthal genes.
B. Sexual dimorphism may decrease.
C. Language and communication may improve.
D. In humans, the gestation period is longer than for other hominoids, perhaps allowing more time for protected development and maturation of the central nervous system. Also, humans are the only mammal to have a post-reproductive period. What is the adaptive value of this? Perhaps these trends will continue, with 12-month pregnancies and longer postreproductive life!