(i) Preamble

Our own species, Homo sapiens, is the subject of several zoogeographic oddities and uncertainties. The major ones involve the place and time of origin of modern humans (anatomically-speaking) and how we have come to disperse and reside in virtually all regions of the world. These subjects of study have in many cases been controversial because they often represent a sometimes unhappy marriage of diverse fields, e.g., genetics and evolution, comparative anatomy, physical and cultural anthropology, archaeology, and paleontology.

Human evolutionary tree (hypothesized).

The figure below depicts estimates of dispersal routes and times of colonization of modern Homo sapiens from a, presumably, African origin some 200,000-400,000 years ago. We'll focus on three "controversies": (i) the "out of Africa" theory of human dispersal, (ii) the timing and number of "invasions" of North America by aboriginals from northeast Asia, and (iii) island biogeography of humans colonizing the south Pacific.

In addition, the biogeography of humans has had profound effects on the biogeography of other life forms (and vice versa). Anthropogenically induced extinctions are perhaps the best (worst?) example.

(ii) Out of Africa theory and "mitochondrial Eve" (detailed notes below OPTIONAL FOR 2010- focus on section (iv) coming soon)

 Humans are hominids that belong to the Primate order which itself is at least 65 million years old. About 35 million years ago, a subgroup descended from ancestral hominids which eventually gave rise to the great apes or hominoid suborder (gorillas, chimps, orangutans, and humans). Hominids (i.e. the family Hominidae) split from the other great apes about 5-7 million years ago in central Africa with the oldest representative being Australopithecus fossils.

Australopithecus eventually gave rise to the genus Homo (with four major species, we are the only extant one) in the savannas of east and central Africa. The oldest fossil Homo spp. is H. erectus at about 1.8 million years of age. H. erectus was probably the first Homo species to extent its range outside of Africa, but it was apparently supplanted by the eventual evolution of H. sapiens (likely originated in Africa also) by about 300,000 years ago.

Within this general context, there have been three hypotheses, based on anthropological analyses, to explain the origin and dispersal of Homo sapiens: (i) the "candelabra" hypothesis, (ii) the multiregional hyopothesis, and (iii) the out of Africa or African replacement hypothesis. Each is pictured below in terms of the phylogenetic pattern of lineage interrelationships that are predicted under each theory (from Avise 2000).

The candelabra hypothesis posits that modern humans stem from multiple groups of completely isolated hominid populations that have persisted for up to 2 million years in scattered locales throughout the old world. This predicts that extant populations should be characterized by deep lineage divisions each localized in distinct areas of the new world (where each group originated).

The multiregional hypothesis posits a similar origin of humans in multiple areas of the old world, but also that these populations were only semi-isolated over the last 2 million years, and there were many instances of interregion dispersal and mating. This model also predicts deep lineage divisions, but that each region should containa mix of these lineages.

Finally, the out of Africa idea suggests that modern humans originated in a single region in central Africa (or perhaps the Middle East) only about 200,000 years ago then dispersed out of Africa (primarily eastward into Asia) where they "replaced" Homo erectus and archaic H. sapiens (perhaps through competitive displacement). This hypothesis predicts a phylogeny of humans that has very "shallow" branchs (because they arose so recently) that clearly stem from a deeper lineage found in Africa.

These models are "naturals" to test with molecular phylogenetic data that can generate phylogenetic trees to test the patterns expected under each of these hypotheses.

In 1980, W. Brown conducted a study of 21 humans of diverse ethnicity and their mitochondrial DNA and found a very low (<0.5%) degree of sequence divergence among the sample. Such low diversity suggested to many that modern humans could have been derived from a single mating pair of ancestral humans that existed in Africa 200,000-300,000 years ago. Because mtDNA is inherited from the female parent only in most organisms, this was interpreted by some to mean that all modern humans are derived from a single female that existed in the savannas of central Africa less than 0.5 million years ago. The mythical female was dubbed "mitochondrial Eve" by the popular press and her place of residence and the bottleneck that must have led to this situation as the "Garden of Eve" hypothesis.

A second study by Cann et al. (1987) of a larger sample (147 humans) found essentially the same result – very low sequence divergence among the human mtDNAs assayed. The data from both these studies appeared to be consistent with the "out of Africa" theory of modern human dispersal. The relationships of the mtDNA types found by Cann et al. is shown below. Note: (i) the ancestral (most divergent) haplotypes all are found in Africa (solid dots), (ii) there is (marginally) greater diversity (the lengths of the branches are longer) in the African mtDNAs (the lineage marked as "A"), and (iii) some "African" mtDNAs are "sprinkled" in the "B" lineage and indicate that they are more similar (and inferred to be more related) to mtDNAs found in other regions of the world.

These observations all suggested that, indeed, modern humans have had an African origin with repeated dispersal out of Africa accounting for colonization of other regions of the world. This early work has since been broadly corroborated by "population genomics", i.e. sequence analysis of the entire mitochondrial DNA genome in diverse groups of humans (see Ingman et al. 2000).

In other respects, however, further analysis including looking at other sequences from the nuclear genome has largely discredited the "mitochondrial Eve" idea, i.e., the bottleneck was likely never as severe as a single female, perhaps more like several thousand to several tens of thousands of females. A subsequent and pronounced population expansion in numbers of humans appears to have taken place sometime in the last Pleistocene. This expansion was accompanied by tremendous dispersal as can bee seen in the figure below. Take note of some the variability in the timing of human arrival in different parts of the world.

For instance, humans crossed the Arabian Peninsula first by about 100,000 years ago and fairly rapidly dispersed to southeast Asia (75,000 years ago) and were in Siberia by about 30,000 years ago.

By contrast, humans did not arrive in central and western Europe until later, perhaps 40,000 years ago and many island areas were colonized much later (e.g. Hawaii 1,400 years ago, New Zealand 1,000 years ago). These later colonization likely reflect the proximity of the Arabian Peninsula to eastern dispersal corridors that may have been more hospitable during late Pleistocene glaciations (remember the question in class???) relative to more western and northern areas that would have to provide access to western Europe. In addition, as we have come to appreciate, islands are problems in access and the isolation of the Pacific islands likely contributed to their late colonization by humans.