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 questions 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.
UPDATE (2018-03-03): This post continues to get a lot of visits from students around the world. It was first posted in 2005, and the science has changed a lot since then. We now have ancient DNA evidence from Neanderthals, early modern humans in Europe, and a handful of ancient samples from Africa. Those have changed the picture substantially from the turn of the century. In today’s science, the “multiregional versus out of Africa” distinction is not really valid.
Modern humans originated as a population within Africa, with substantial input from diverse African populations of the Middle Pleistocene. Some scientists describe that early evolutionary process as “multiregional evolution within Africa”. That’s not an unfair description, although the presence of very primitive hominin populations like H. naledi during the origin of modern humans makes it unclear how much of Africa was occupied by modern human ancestors.
Before 100,000 years ago, there were genetic exchanges between Africa and Eurasia, which left marks of introgression in the genomes of Neanderthals.
Today’s modern humans of Eurasia derive most of their ancestry from a bottlenecked population that existed before 70,000 years ago. This population had originated from African ancestors within the last 150,000 years, but where it lived during the time of the bottleneck is not yet known. As they spread across Asia, the descendants of this bottlenecked population mixed with Neanderthals and with another archaic human population, the Denisovans. The modern human populations that ultimately arrived throughout Asia, Australia, the Americas, and Europe would all carry a small fraction of Neanderthal genes.
Some scientists describe the current picture as a multiregional evolution scenario, others describe it as an out of Africa scenario, and still others describe it as a blend or middle ground between the two.
In either case, the more detailed picture that we have today shows that the contradictions posed by datasets of the 1990s could indeed be resolved in a single picture of human origins. Humans have low genetic variation today, and this variation is highest in Africa, and much lower in other parts of the world. This shows that most modern human ancestors lived in a small population within Africa.
At the same time, modern humans from other parts of the world show some skeletal similarities and a small proportion of genetic similarities with earlier archaic human populations from those areas, including the Neanderthals. Today’s data from ancient DNA and the whole genomes of living people show that all these facts are part of a single picture. Anthropologists continue to work to test hypotheses about how and why these populations evolved.