[Back to Topic] Molecules and Modern Human Origins
Several models to explain the evolution of anatomically modern humans are currently being investigated. These models also have distinctive geographic implications that require exploration.
A single origin hypothesis has been supported based on initial analyses of human mitochondrial DNA diversity. These studies, undertaken in the late 1980's, suggested that the patterning of DNA diversity in our species could be explained by a phylogeny much like the one pictured. Since mtDNA is maternally inherited, diversity in mtDNA can theoretically only be maintained as mutations are passed from mother to daughter. This implies that that all anatomically modern humans possess mtDNA that originat ed from a single female. The figure at right shows this idea, with mtDNA lineages terminating each time a female fails to have a female offspring. The early studies of mtDNA indicated that the highest levels of mtDNA diversity could be found among mode rn Africans. This suggested that the African lineages were the most ancient.
More recent analyses have questioned the assumption that greatest diversity in mtDNA implies the greatest time depth. Specifically, high diversity may more accurately reflect large ancestral population size. Consequently, recent analyses have focuss ed on statistical analyses of mtDNA diversity in an attempt to account historical changes in population size and structure while analyzing phylogenetic issues.
Image source: Lewin, R (1991) The biochemical route to human origins. Mosaic 22 (3):46-55.
This model predicts a fair degree of independence among the major regions of the Old World (broadly defined as Europe, Africa, and Asia). Despite some level of independence, the model predicts gene flow among various regions (otherwise, it is possible th at reproductive isolation [speciation] could preclude gene flow among the various regions). If this model is accurate, then we should observe general continuity in skeletal features within regions coupled with a few shared features among regions. In p articular, areas between these areas should show mosaics of skeletal features. Genetic data should show approximately equal quantities of DNA diversity. The level of diversity should be comparable because each region has long lineages dating back to ei ther the origins or migration of H. erectus in each area. In addition, the maintenance of gene flow should ensure comparable levels of diversity in each region
Paleontological support for this model is difficult to assemble, largely as a result of problems with the fossil record. It is likely that Europe can be discounted--Neandertals seem to persist there so late that they likely had little contribution to sub sequent populations. Africa seems to have the earliest anatomically modern fossils, but we know so little about Asian fossils that adequate tests of this model are very difficult to undertake.
Like the multiregional model, the single-origin (or Strong Garden of Eden) model has proven difficult to adequately test. This model implies the origins of anatomically modern humans in Africa (or other region) at some point between 125,000 and 200,000 y ears ago. The establishment of anatomically modern populations in Africa is followed by migration out of Africa, and subsequent replacement of archaic or late H. erectus populations. This implies that existing non-African populations made little or no contribution to subsequent generations.
Paleontological evidence for this model is mixed. There do seem to be some very early skulls in Africa. Anatomically moderns are clearly in the Levant along with Neandertals. As noted, Neandertals seem to have been replaced. Thus, the model cannot be ruled out on paleontological grounds when we consider Europe. Again, Asia is problematic. Genetic evidence seemed to compellingly support this model. However, certain inferences may have been based on faulty assumptions. Specifically, mtDNA diversity may be more a function of effective population size (=number breeding individuals in a population) than a function of time (for mutations to build-up). Thus, initial mtDNA results may simply have suggested that African populations have been the largest t hroughout prehistory. This is a very tenable idea, given that Africa obviously provides a very favorable environment to hominins.
The Weak Garden of Eden Model is very complicated, and seems to combine some elements of the previous models. The model predicts that anatomically modern humans evolved in Africa, then spread to other regions of the Old World. Populations were establis hed in these regions, but probably were fairly small. Genetic evidence suggests that populations that originated in Africa experienced a dramatic reduction in size (or a bottleneck--shown by the constriction of lineages in the figure). The severity of the bottleneck seems to have varied, with populations in Africa least affected. Following the bottleneck, the model predicts expansion of anatomically modern populations, with subsequent extinction of archaic or late erectus populations.
This model raises an important question: What caused the bottleneck? Prof. Stanley Ambrose here at the Univ. of Illinois along with others have argued compellingly that the eruption of a huge volcano (at about 72,500 years) was the ultimate cause of the bottlneck. This eruption may have been the Toba volcano super-eruption in Indonesia.
The Weak Garden of Eden Model is very difficult to test either paleontologically or genetically. Genetic evidence seems to provide the strongest evidence for this model. These analyses compare mtDNA variation within regions to variation among regions. The patterning of variation can be explained by this model. Similarly, modern skeletal variation is not inconsistent with the model. Paleontological evidence is too scant for robust tests of the model, although the coexistence (or parallel existence) of Neandertals and anatomically moderns in the Levant shows that some aspects of the model are at least testable at the paleontological level.
[Geographic views of these models]
[ Links to anatomically modern Homo sapiens]