1.12 – More mothers than Mitochondrial Eve

On January 11, 1988, Newsweek’s cover depicted “The Search for Adam and Eve” (Tierney 1988). The article reported on a paper entitled Mitochondrial DNA and human evolution” (Cann et al. 1987). The research purported all contemporary humans could be traced to relatively recent African origins, maybe even with a common mother. This mother became known as “Mitochondrial Eve” or “African Eve.” The model postulated all contemporary humans derived from a lineage of anatomically modern humans, evolved in Africa around 150,000 years ago and then spreading all over the world. They named this the “replacement hypothesis” or “out-of-Africa II” because these anatomically modern humans seemed to have fully replaced the earlier hominid populations who had migrated from Africa about 1.8 million years ago, such as the Neandertals and Homo erectus populations in Asia. The replacement hypothesis said there was little or no interbreeding, with no genetic evidence for contributions from Neandertal or other early hominids.

Initially anthropologists resisted a model derived purely from genetic evidence. Anthropologists studied fossils, and favored a model that emphasized gene flow and interconnection across the hominid species range. Known as the “multiregional model,” it was first proposed in 1946 by Franz Weidenreich, and then later developed and defended by Milford Wolpoff (Templeton 2007:1509). In this model, early hominids and anatomically modern humans are all part of a single interbreeding species, with evolution occurring across the range.

However, after initial resistance, scientific consensus and most anthropologists shifted toward the replacement hypothesis. The replacement hypothesis became dominant in textbook models of human evolution. Most students either never learned about the multiregional model, or if they did it was an impoverished and inaccurate version of it. “The out-of-Africa replacement hypothesis became the model of human evolution, particularly in undergraduate textbooks and the popular science literature” (Templeton 2007:1508). Ironically, the replacement hypothesis replaced all other accounts.

There were a number of reasons why anthropologists gravitated to the replacement hypothesis:

a) It was winning scientific consensus

Following the original research, more studies seemed to disprove any genetic connection between contemporary humans and Neandertal or other archaic groups. The genetic evidence seemed more dazzling, more precise, and more computational than messing with fossils. Robert N. Proctor observed in 2003: “It seems pretty clear that the Out-of-Africanists are winning the field. Multiregionalists have no technical wonder comparable to sequencing, and the original mitochondrial evidence has been joined by other evidence” (2003:225).

b) It got rid of the dreaded candelabra

Since the 1960s anthropology has been haunted by the candelabra model proposed by anthropologist Carleton Coon. The candelabra hypothesis asserts Homo erectus dispersed from Africa about 1.8 million years ago, and then Homo sapiens evolved separately in Asia, Africa, and Europe. This model is scientifically wrong but persists in popular imagination and supports racist thinking. Many anthropologists were eager to fully excise and deny the candelabra model–the out-of-Africa replacement hypothesis offered a simple way to dispense with an unfortunately persistent idea.

c) It allied anthropology with Lewontin and Gould

As noted in the Race revival section, the work of geneticist Richard Lewontin was very helpful for arguing against the genetic separation of human groups. Lewontin and especially Stephen Jay Gould explicitly embraced the replacement hypothesis. Many anthropologists had already promoted Gould’s more fluid and open-ended view of evolution. The replacement hypothesis helped cement the alliance.

d) Anthropology likes Africa

Some of the best anthropology documents human and natural richness in Africa. But for the rest of the world, Africa gets ignored, misrepresented, left off the map. Afflicted by poverty, inequality, military conflicts, and HIV-AIDS, sub-Saharan Africa has been sadly omitted from a globalizing world, seen as a hopeless basket case. By stressing the out-of-Africa replacement hypothesis, anthropologists could re-assert African primacy, contributing to ideas that “we are all African” or that everyone is indigenous to Africa. As Proctor notes, “Some people seem to like the fact that we are all ‘Africans under the skin’” (2003:225).

Surprise!

In 2010 the tide turned. An innocuously-titled study “A Draft Sequence of the Neandertal Genome” documented Neandertal contributions to contemporary humans (Green et al. 2010). Then, perhaps even more dramatically, researchers found that the archaic human remains found in Denisova Cave in Siberia also found some genetic match in contemporary humans (Reich et al. 2010; Zimmer 2010). Homo sapiens interbred with both Neandertals and Denisovans, and the results show up in contemporary genome sequences. “‘It’s hard to explain how good I feel about this,’ says Wolpoff, who says that seeing complete replacement falsified twice in 1 year was beyond his wildest expectations. ‘It was a good year’” (Gibbons 2011:393-4). Of course, these studies do not prove a lot of interbreeding–it took some very sophisticated techniques to demonstrate any interbreeding at all–but the idea of no contribution from Neandertals, or that archaic populations are entirely separate species from modern humans, is no longer defensible. Moreover, as of summer 2011, the Neandertal admixture findings continue to come in–see Rosemary Joyce, “A Neanderthal Tragedy of Shakespearean Dimensions” for an interesting summary.

These findings will take some time to filter into the textbooks, as they have not yet been absorbed into the geneticist scientific journals. Genetic anthropologist John Hawks writes on his blog that “a large-scale reorganization of the science of human origins is upon us.” However, “the shifting landscape has caught many geneticists off their footing” (Hawks 2011).

The 2010 studies, which might have been a boon to anthropology if we had defended the multiregional model, put some standard anthropological accounts in an awkward position. Embracing the replacement hypothesis–without further developing the multiregional model–leaves anthropology needing to reconfigure a response.

In retrospect, embracing the replacement hypothesis was very problematic for anthropology. Although these problems should have been more obvious earlier, they are painfully clear in 2011:

a) Talking about Eve, even Mitochondrial Eve, was just not good for promoting evolution.

This aspect may not be on the map for those who are debating scientific hypotheses, but I grew up with people who used the 1988 Newsweek cover to assert that scientists were going to eventually come around and prove the Biblical account of divine creation. Again, this may seem strange to those steeped in DNA sequencing, but the part about hominid and Homo sapien evolution is the most difficult for many people to swallow. Talking about any kind of Eve does not help. Moreover, the acrimonious debate let a lot of people tune out, contributing to an idea that scientists simply have no idea what was going on.

b) Anthropology denied a richer, home-grown model of human evolution

The replacement hypothesis is a simple model of human evolution. It is easy to explain, and it eliminates the candelabra justification for race-thinking. Multiregional evolution is more complex and difficult to diagram. There is more room for misinterpretation. However, as Alan Templeton convincingly explains:

Weidenreich (1946) argued that regional populations could display differences, and some local differences could persist through time in the same locality, but there is no assumption of independent, parallel evolution. Instead, humanity consists of a single evolutionary lineage with no subbranches because humanity’s geographically dispersed populations were and are interconnected by gene flow and lines of recent, not ancient, common descent due to this gene flow. (2007:1509).

Moreover, by embracing the replacement hypothesis, anthropology went along with the trend of reducing all biological evolution to genetics. Anthropology had fossil evidence and forms of analysis different from the dazzle of genetic computation. What genetic computation cannot deliver is the full range of the evolutionary process, where evolutionary change can occur with or without genetic modification. Indeed, by changing the conditions of the environment for later migrations, those early hominids migrating from Africa were contributing to the evolutionary selection process.

c) The replacement hypothesis looks like colonization and domination

The replacement hypothesis is perhaps less racially charged than the candelabra model, but the idea that anatomically modern humans set out from Africa and then replaced all other populations is one of colonization and domination. Advocates for replacement have been quick to say replacement could occur without any warfare or interaction–it may only depend on a small difference in reproductive success–but it is nevertheless congruent with ideas of dominance and subjugation:

It is striking how accurately this [out-of-Africa] hypothesis mirrors the story of global colonial conquest by White Europeans so much favoured by Darwin and his contemporaries. The story may have been turned upside down, but the structure is the same: one dominant race, equipped with superior intelligence, supersedes the rest. (Ingold 2006:278)

d) Stephen Jay Gould seriously distorted the multiregional model

The original authors of the Mitochondrial Eve research in 1987 lumped Wolpoff and Weidenreich’s model together with the candelabra model from Coon. Textbooks perpetuated this error, portraying multiregional evolution as candelabra with some gene flow. “There is no doubt that Cann et al. (1987) equated multiregional evolution to Coon’s model of parallel evolution of isolated archaic populations. . . . However, as is clear from Weidenreich (1946) and many subsequent papers by Wolpoff and his associates, parallel evolution was never part of the multiregional model, much less its core” (Templeton 2007:1510).

Stephen Jay Gould never understood the multiregional model, and he repeatedly and unfairly jabbed at the model and at Wolpoff:

Multiregional evolution should be labeled iconoclastic, if not a bit bizarre. How could a new species evolve in lockstep parallelism from three ancestral populations spread over more than half the globe? (Gould 2002, cited in Templeton 2007:1510).

By allying with Gould, anthropology did not defend one of our own from unfair characterizations. (See blog-post Mismeasuring Gould in “The Mismeasure of Science.”)

e) Out-of-Africa became escape-from-Africa

Anthropologists may have hoped the out-of-Africa hypothesis would bring much-needed attention to sub-Saharan Africa, as a celebrated cradle of humanity. It did not. Instead, people began to imply that leaving Africa was a good thing. “I call this ‘Out-of-Africa: Thank God!’ to point to the presumption that hominids became human in the process of leaving Africa–a slight that seems always unintentional, yet is surprisingly common. . . . Leaving Africa has become a troubling focus of a great deal of research and popular celebration” (Proctor 2003:225-226).

New York Times reporter Nicholas Wade raised this slight to an insult, writing about “when and how modern humans escaped from their ancestral homeland” (2011). Modern humans did not escape! There were migrations out of Africa, within Africa, and people also migrated back into Africa.

Extreme versions of the replacement hypothesis are dead. It is time to recapture the complexities of anthropological models. It is time to go back to Wolpoff’s statement from the 1988 Newsweek article: “we have a long history of people constantly mixing with one another and cooperating with one another and evolving into one great family.”

Next: 1.13 – Human Biologies

To cite: Antrosio, Jason, 2011. More mothers than Mitochondrial Eve. Living Anthropologically, http://www.livinganthropologically.com/anthropology/mitochondrial-eve/. Last updated August 2, 2011.

Sources

Cann, Rebecca L., Mark Stoneking, and Allan C. Wilson, 1987. Mitochondrial DNA and human evolution. Nature 325(1):31-36.

Coon, Carleton S., 1962. The Origin of Races.

Gibbons, Ann, 2011. A New View of the Birth of Homo sapiens. Science 331(January 28).

Gould, Stephen J., 2002. The Structure of Evolutionary Theory.

Green, Richard E., et al., 2010. A Draft Sequence of the Neandertal Genome. Science 328(5979):710-722.

Hawks, John, 2011. Population structure within Africa: has “modern human origins” become a non sequitur?. John Hawks Weblog, March 15.

Ingold, Tim, 2000. The Perception of the Environment: Essays on Livelihood, Dwelling and Skill.

——, 2006. Against human nature. In Evolutionary Epistemology, Language and Culture: A Non-Adaptationist, Systems Theoretical Approach.

Onion, The, 2011. Anthropologists Trace Human Origins Back To One Large Goat. February 17.

Proctor, Robert N., 2003. Three Roots of Human Recency: Molecular Anthropology, the Refigured Acheulean, and the UNESCO Response to Auschwitz. Current Anthropology 44(2):213-239.

Reich, David, et al., 2010. Genetic history of an archaic hominin group from Denisova Cave in Siberia. Nature 468(7327):1053-1060.

Templeton, Alan R., 2007. Genetics and Recent Human Evolution. Evolution 61(7):1507-1519.

Tierney, John, 1988. The Search for Adam and Eve. Newsweek, January 11.

Wade, Nicholas, 2011. Tools Suggest Earlier Human Exit From Africa. New York Times, January 27.

Weidenreich, Franz, 1946. Apes, Giants, and Man.

Zimmer, Carl, 2010. Siberian Fossils Were Neanderthals’ Eastern Cousins, DNA Reveals. New York Times, December 22.