Out of Africa Hypothesis

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

Jebel Irhoud (Morocco)
View looking south of the Jebel Irhoud (Morocco) site. The remaining deposits and several people excavating them are visible in the center. At the time the site was occupied by early hominins, it would have been a cave, but the covering rock and much sediment were removed by work at the site in the 1960s. Shannon McPherron, MPI EVA Leipzig

The Out of Africa (OoA) or African Replacement Hypothesis is a well-supported theory that argues that every living human being is descended from a small group of Homo sapiens (abbreviated Hss) individuals in Africa, who then dispersed into the wider world meeting and displacing earlier forms such as Neanderthals and Denisovans. Early major proponents of this theory were led by British paleontologist Chris Stringer and in direct opposition to scholars supporting the multiregional hypothesis, who argued that Hss evolved several times from Homo erectus in several regions.

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: Mitochondrial Eve. Today, the vast majority of scholars have accepted that human beings evolved in Africa and migrated outward, likely in multiple dispersals. However, recent evidence has shown that some sexual interaction between Hss and Denisovans and Neanderthals occurred, although at present their contribution to Homo sapiens DNA is considered fairly minor.

Early Human Archaeological Sites

Probably the most influential site for paleontologists' most recent change in understanding evolutionary processes was the 430,000-year-old Homo heidelbergensis site of Sima de los Huesos in Spain. At this site, a large community of hominins was found to encompass a wider range of skeletal morphology than was previously considered within one species.

That has led to a reassessment of species in general, and what scholars should call the species identified within the site is still under review. In essence, Sima de los Huesos allowed paleontologists to be able to identify Hss with less stringent expectations on what an Hss looks like.

A few of the archaeological sites associated with early Hss remains in Africa include:

  • Jebel Irhoud (Morocco). The oldest known Hss site in the world to date is Jebel Irhoud, in Morocco, where the skeletal remains of five archaic Homo sapiens have been found alongside Middle Stone Age tools. At 350,000–280,000 years old, the five hominids represent the best-dated evidence of an early 'pre-modern' phase in Homo sapiens evolution. The human fossils at Irhoud include a partial skull and lower jaw, which although they retain some archaic features such as an elongated and low braincase, are thought to be more similar to Hss skulls found at Laetoli in Tanzania and Qafzeh in Israel. Stone tools at the site are Middle Stone Age in age, and the assemblage includes Levallois flakes, scrapers, and unifacial points. The animal bone at the site shows evidence of human modification, and charcoal indicating the likely controlled use of fire.
  • Omo Kibish (Ethiopia) contained the partial skeleton of an Hss who died ~195,000 years ago, alongside Levallois flakes, blades, core-trimming elements, and pseudo-Levallois points.
  • Bouri (Ethiopia) is located within the Middle Awash study area of East Africa and includes four archaeological and paleontological-bearing members dated between 2.5 million and 160,000 years ago. The Upper Herto Member (160,000 years BP) contained three hominin crania identified as Hss, associated with Middle Stone Age Acheulean transition tools, including hand axes, cleavers, scrapers, Levallois flake tools, cores, and blades. Although not considered Hss because of its age, Bouri's Herto Lower Member (260,000 years ago) contains later Acheulean artifacts including finely made bifaces and Levallois flakes; no hominid remains were found within the Lower Member but it will likely be reevaluated given the results at Jebel Irhoud.

    Leaving Africa

    Scholars largely agree that our modern species (Homo sapiens) originated in East Africa by 195–160,000 years ago, although those dates are clearly undergoing revision today. The earliest known pathway out of Africa probably occurred during Marine Isotope Stage 5e, or between 130,000–115,000 years ago, following 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 more recently proposed than the original OoA theory but refers to an older migration) 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. One still controversial site reported in early 2018 is Misliya Cave in Israel, said to contain an Hss maxilla associated with full-fledged Levallois technology and dated between 177,000-194,000 BP.

    Fossil evidence of any kind this old is 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 65,000–40,000 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 is 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 this migration from southern Africa following the coasts eastward and 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. It is possible that the later Hss interacted with the descendants of the earlier pulse as well. All living humans are still one species—but it is now undeniable that we share different levels of the 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: in 2010 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

    The three main theories concerning human dispersal were up until recently:

    But with all the evidence pouring in from around the world, paleoanthropologist Christopher Bae and colleagues (2018) suggest there are now four variations of the OoA hypothesis, ultimately incorporating elements of all three of the original ones:

    • A single dispersal during MIS 5 (130,000–74,000 BP)
    • Multiple dispersals beginning MIS 5
    • A single dispersal during MIS 3 (60,000–24,000 BP)
    • Multiple dispersals beginning MIS 3

    Sources

    There is an enormous amount of scientific literature on the Out of Africa model, and the following is a partial bibliography covering the last few years.