Out of Africa Hypothesis - Did All Humans Evolve in Africa?

What Do the Discoveries of Neanderthal and Denisovan DNA in Us Mean?

Early Modern Human Klipgat Cave, Western Cape, South Africa
View from 70,000 Year Old Human Klipgat Cave Site, Western Cape, South Africa. Michael Langford / Getty Images

The Out of Africa or African Replacement Hypothesis is a well-supported theory that argues that every living human being is descended from a small group in Africa, who then dispersed into the wider world displacing earlier forms such as Neanderthal and Denisovans. Early major proponents of this theory were led by Chris Stringer. The Out-of-Africa theory was bolstered in the early 1990s by research on mitochondrial DNA studies by Allan Wilson and Rebecca Cann which suggested that all humans ultimately descended from one female: the Mitochondrial Eve.

Today, the vast majority of scholars have accepted that human beings evolved in Africa and migrated out; recent evidence has shown that happened in multiple waves. The number and timing of the waves is still being debated.

Leaving Africa

Scholars largely agree that our modern species (Homo sapiens) originated in east Africa by 195-160,000 years ago. The earliest known pathway Out of Africa probably occurred between Marine Isotope Stage 5e, or between 130,000-115,000 years ago, along the Nile Corridor and into the Levant, evidenced by Middle Paleolithic sites at Qazfeh and Skhul. That migration (sometimes confusingly called "Out of Africa 2" because it was discovered more recently than the next) is generally regarded as a "failed dispersal" ​because only a handful of Homo sapiens sites have been identified as being this old outside of Africa. However, fossil evidence of any kind this old is pretty rare and it may be too early to completely rule that out.

A later pulse from northern Africa, which was recognized at least thirty years ago, occurred from about 65-40,000 years ago [MIS 4 or early 3], through Arabia: that one, scholars believe, eventually led to the human colonization of Europe and Asia, and the eventual replacement of Neanderthals in Europe.

The fact that these two pulses occurred in the past are largely undebated today. A third, and increasingly convincing, human migration is the southern dispersal hypothesis, which argues that an additional wave of colonization occurred between those two better-known pulses. Growing archaeological and genetic evidence supports the existence of this earlier southern route into South Asia.

Denisovans, Neanderthals and Us

Over the past decade or so, evidence has been piling up that although pretty much all paleontologists agree that humans did evolve in Africa and move out from there, we did meet other human species--specifically Denisovans and Neanderthals--as we moved out into the world. All living humans are still one species--but it is now undeniable that we share differing levels of admixture of species which developed and died out in Eurasia. Those species are no longer with us--except as tiny pieces of DNA.

The paleontological community is still somewhat divided on what that means to this ancient debate: John Hawks (2010) argues "we are all multiregionalists now"; but Chris Stringer recently (2014) disagreed: "we are all out-of-Africanists who accept some multi-regional contributions".

Three Theories