Evolution of Evolution — Text-only | Flash Special Report
Minor Physical Variations More Than Evolution Noise
Anthropologist considers change within species over time
By Susan Antón
In “On the Origin of Species,” Charles Darwin spent little time discussing fossil species but a great deal of time discussing variation in living species. One of his key insights was recognition of the biological importance of individual variation while allowing that members of a specific species all share a ‘unity of type.’ Darwin reached the critical conclusion that individual, even minor, variation was not so much ‘noise’ around an ideal type, but rather was important stuff upon which an individual’s survival might depend. That is, the differential survival and reproduction of particular individuals with particular kinds of variations, could, given enough time, shift the average appearance of a species by changing the frequency of certain types of variation in the population.
There is evidence from living species that Darwin vastly underestimated the power of natural selection, and overestimated the duration of time needed to change the average morphological appearance of a species. For example, measurable morphological change can happen between just two generations due to natural selection. But even given this, is there evidence of gradual skeletal change in the human fossil record?
Although the fossil record forms a primary set of evidence that evolution has occurred, paleoanthropologists ironically spend infinitely more time focused on what characteristics constitute the ‘unity of type’ (or underlying plan) of a fossil species than they do on the all-important variation in the species. This is in large part due to the incompleteness of the geological record and the fragmentary nature of the remains that are preserved, limitations that Darwin rightly recognized. However, over the past 50 years, a few human lineages have yielded enough fossils that we can begin to consider individual variation and how change accumulates within a species over time. These lineages include Australopithecus afarensis, Neandertals, and to a lesser extent, Homo erectus.
Within each of these species we see evidence of significant individual variation, but also consistent change with time. For example, in the long-lived and geographically diverse species H. erectus, brain size varies quite a bit in each geographic region. Over the entire duration of the species (1.9 million years ago to perhaps 100,000 years ago), brain size varies from about 650 to 1250 cubic centimeters (cc). At any given period, the difference in brain sizes is about 400 cc between the smallest and the largest individuals, but the mean value for the species increases with time. The same is true of average brow-ridge size, which increases with time while still maintaining a fair degree of individual variation (smaller brained H. erectus also have smaller brows). This suggests that at least some important variables increased consistently, possibly gradually, through the duration of this species. Although this doesn’t preclude the possibility that other traits might change at different paces, such evidence is consistent with Darwin’s view of evolution as a slow and steady process.
Although Homo is a relatively well-represented genus, we have little evidence about the pace of change between species because early Homo fossils, dating before the origin of H. erectus, are precious and few.However, the lineage from Australopithecus anamensis to A. afarensis seems to suggest a gradual accumulation of change in these time-successive species.
Regardless of the pace of the origin of species, understanding—as Darwin did—intraspecies variation in the context of environmental and ecological (natural selective) factors is essential to understanding our evolutionary past, as well as to providing insight into the potentials and pitfalls in the evolutionary future of our world.
Susan Antón is the joint-editor of the Journal of Human Evolution, and associate professor at the Center for the Study of Human Origins in the Department of Anthropology at New York University (NYU). She is director of NYU’s MA program in Human Skeletal Biology and recently received NYU’s ‘Golden Dozen’ award for excellence in undergraduate teaching. Her field research concerns the evolution of genus Homo in Indonesia and human impact on island ecosystems in the South Pacific. Her published work includes papers on the evolution of the genus Homo in Asia and Africa, for which she was elected a fellow of the American Association for the Advancement of Science (AAAS)in 2008. The National Science Foundation supported her work on “Late Pleistocene Homo erectus in Java.”
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