It's that time of the semester -- exam time -- and I'm getting a lot of questions from my students by e-mail. One of the most common is how to differentiate the Multiregional evolution hypothesis from the Out of Africa hypothesis. So I'm posting a nutshell version to help with studying.
To begin with, both hypotheses try to account for the evolution of today's humans from our Pleistocene ancestors. The difference between the hypotheses is in which Pleistocene people were our ancestors, and which were not.
Both hypotheses have to account for the same basic set of facts:
- Humans first left Africa and established populations in other parts of the world (first southern Asia, China, and Java, later Europe) by 1.8 million years ago.
- Humans today are quite different anatomically and behaviorally from archaic people (that is, most humans before 40,000 years ago) anywhere in the world. Recent people are called "modern" humans.
- Human populations today are genetically very similar to each other.
- African populations today are more genetically diverse than populations in other parts of the world.
- Recent humans in Europe and Asia share a few features with the ancient archaic people who lived in those places before 40,000 years ago.
Anthropologists consider many more detailed sources of evidence about human origins, but many sources of evidence fall into one or more of these basic categories. This combination of facts is a bit puzzling, and both hypotheses account for them a bit differently.
Out of Africa
Under the Out of Africa hypothesis, the first humans to leave Africa 1.8 million years ago divided into several different species during the Pleistocene. Species, of course, are defined by reproductive isolation, so the evolution of these several species of humans was separate. The fossil archaic humans that we find throughout the Old World belonged to these several species, but only one branch of this ancient family tree could give rise to today's humanity.
This branch was African. The origin of modern humans in Africa explains why today's Africans are more genetically variable than other populations --- they were the first human population to expand, and other populations (like those of Europe and Asia) were founded later. The recent origin explains why today's human populations are genetically similar -- they haven't had time to diverge very much.
The resemblances with archaic humans in some modern people are explained either as a result of parallel evolution --- the same selection in the same place leads to similar features --- or as a result of slight genetic contributions from archaic humans into today's populations.
Under the Multiregional evolution hypothesis, the first humans to leave Africa 1.8 million years ago never divided into different species. Instead, these populations always exchanged genes with each other through recurrent gene flow. Today, we are part of this same species, which has evolved greatly over time to a very different morphology and behavior from the first humans.
The low genetic differences among human populations are a result of a history of gene flow between ancient populations. Our present morphology and behavior have greatly changed from archaic humans because of natural selection in a global human population. Resemblances between archaic and modern humans in some parts of the world are the result of ancestry.
The greater genetic variation within Africa is a consequence of larger African population size, greater ecological diversity and local selection, or both. These factors gave Africa a dominant role in the ancestry of today's human population.