The Homo habilis Debate
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It was once thought that the evolution of the genus Homo was an example
of anagenesis, the continual and gradual change of one parent species into
its daughter species in a linear fashion. As the fossil record began to expand
and more early human fossils were found dating to the period between 2 million
and 1 million years ago, some questions as to the validity of this hypothesis
were raised. Below are two fossils, shown to scale. KNM ER 1813 is to the left, and KNM ER 1470 is to the right.
Originally, both were assigned to the species Homo habilis, with ER 1470 thought of as male and the smaller ER 1813 a female in a strongly dimorphic species. However, the anatomies of the two skulls differ considerably. Note that there is a strong supraorbital torus (horizontal bar of bone above the eye sockets) on 1813, whereas the supraorbital torus of 1470 is slight at best, and does not have the depression behind it that is seen in 1813. The face of 1470 is longer than 1813's and 1470's upper jaw is square instead of rounded-off. There is a great discrepancy between the cranial capacities of the two individuals as well. ER 1470 has a cranial capacity of 775cc, where ER 1813 has a cranial capacity of only 510cc (which is above the australopith average, but well below the accepted 600cc cutoff for Homo) One debate in paleoanthropology today is whether or not ER 1470, and several other fossils previously identified as H. habilis, should be grouped into a new species, Homo rudolfensis. This classification would acknowledge that ER 1470 and the other members of Homo rudolfensis differ more from Homo habilis, sensu stricto ("in the strict sense," meaning: as originally defined), than could possibly be accounted for by variation within a population or between sexes. This would place two species of the genus Homo in Africa during the same time period in addition to two members of the genus Paranthropus, and, possibly, late surviving members of the species Australopithecus africanus. Far more complicated than the original neat, linear model. Most, but not all researchers are convinced that 1470 should be placed in a separate species. Some, such as Richard Leakey, claim that 1470 should be placed within H. habilis, while excluding 1813 from H. habilis. As yet, the debate is not satisfactorily settled. In addition, it is not yet entirely clear which of the two species Homo habilis and Homo rudolfensis led to the later species in Homo. The larger cranial capacities of the H. rudolfensis individuals lead some to think that later humans evolved from this species. Morphology of the facial bones of H. habilis, such as the shape of the cheekbones and the browridges, suggests that this species was the ancestor of later humans. (But see Homo habilis concerning the post-cranial paradox). (Click to see a representation of the two implied phylogenies) Another debate centered around Homo habilis and Homo rudolfensis is whether or not these two species belong in the genus Homo or would be better suited in one of the other hominid genera. Some researchers feel that all species within the genus Homo should have characteristics, such as locomotor patterns, diet and body proportions, that make them more like modern humans than like the australopiths. These researchers feel that the characteristics of H. habilis and H. rudolfensis are more ape-like than modern, a conclusion that would remove them from our genus. This would make Homo a monophyly (all species evolved from a common ancestor), rather than a polyphyly (the species evolved from more than one ancestor) as it is now thought to be. Other reseearchers think, however, that moving the two species out of the genus Homo does not solve the problem since the specimens do not easily fit into the genus Australopithecus as currently defined. Return to the Human Family Tree  |
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