The multiregional hypothesis for the human species holds that the evolution of humanity throughout the Pleistocene has been within a single widespread human species, Homo sapiens, in response to the normal forces of evolution: selection, mutation, genetic drift, and gene flow. Multiregional evolution contrasts with two theories. According to the "Eve theory," human evolution was a consequence of many cases of species replacement, as newer species replaced older ones across the human range. Modern human origins, according to the "Eve theory," is the most recent example of species replacement. The other theory is Polygenic evolution, a multiple=origins theory in which the different human populations or races had independent origins and evolved in isolation from each other. Held by many scholars of the 19th century such as Haeckel and Klaatsch, and even some of the 20th, such as Carleton Coon, it is biologically impossible since all populations of a species must have the same, single origin.
Polygenism is sometimes mistaken for Multiregional evolution, because they are both hypotheses of evolution within a single species. However, Polygenic evolution depends on isolation of populations while Multiregional evolution requires population interactions and interbreeding so that genetic changes can spread throughout the human range, especially when they are promoted by natural selection. According to the Multiregional hypothesis, geographic differences between human populations are the results of climatic variation, isolation by distance, and historical accidents (genetic drift).
A graph detailing the origin of modern humans using the Polygenism theory of human evolution
The multiregional hypothesis was originally developed from the fossil evidence, but more recent work has focused on molecular data, in which DNA is sequenced. Because this hypothesis posits that human evolution has been within a single species, and not between species as one species replaced another (Eve Theory), it makes genetic predictions. These predictions have been met with the continued study of non-recombining DNA such as mitochondrial DNA and the Y chromosome, as well as regions of Nuclear DNA] where recombination is usual. These genetic studies show an ancient history of population mixture throughout the world, perhaps as far back as 2 or more million years. Since for most of that time the majority of humans lived in Africa, more genes moved out of Africa than into it, but gene movements were always multidirectional. The genetic studies also show the independent evolution of different genes; their evolution does not reflect a recent species replacement, which would have affected all genes the same way because of the associated period of very small population size causing a genetic bottleneck.
Studies on past population bottlenecks that can be inferred from molecular data have led Multiregionalists to conclude that the recent single-origin hypothesis is untenable because there are no population size bottlenecks affecting all genes that are more recent than the one at the beginning of the species, some 2 million years ago. Discovery of a possible hybrid Homo sapiens X neanderthalensis fossil child at the Abrigo do Lagar Velho rock-shelter site in Portugal in 1999 further supports the Multiregional hypothesis, by reflecting the inter-mixture of diverse human populations.
A recent Australian study of an ancient Aboriginal skeleton known as Mungo Man supports the multiregional hypothesis. Genetic tests show the mitochondrial DNA of Mungo Man to be from a mtDNA lineage with no descendants today. Yet Mungo man is an anatomically modern human and has been dated to be at least 40,000 years old. The study suggests that mtDNA does not reflect ancestry or divergence times, and this interpretation is supported by the discovery that the gene is subject to natural selection.
A recent, non-fossilized discovery of one metre-tall, small-brained (350 cc), Homo floresiensis, on the Indonesian island of Flores, might imply populations of Homo erectus survived very late, and gave rise to even later, physically dwarfed isolated "erectus" groups. However, this possibility does not address the Multiregional hypothesis, which is only about the human species, and the evidence is marred by the possibility that the single dwarf cranium found on Flores might have been pathological.
Proponents of Multiregionalism
Besides Milford H. Wolpoff, paleoanthropologists most closely associated with the multiregional hypothesis include James Calcagno, Rachel Caspari, David Frayer, John Hawks, Andrew Kramer, Alan Mann, Janet Monge, Jakov Radovcic, Karen Rosenberg, Lynne Schepartz, Fred Smith, Alan Thorne, Bernard Vandermeersch