Chapter Twelve: The Debate over Modern Human Origins

1. Chapter Twelve: The Debate over Modern Human Origins

2. The Debate over Modern Human Origins • What have been the major competing models regarding the origin of modern Homo sapiens? • What evidence has been offered-- from the fossil record, from genetics, and from evolutionary theory-- for and against each model? • Is there a possible solution to this debate?

3. The Debate over Modern Human Origins • The Two Standard Models • Do modern traits accurately define and distinguish a new species of hominin, Homo sapiens, • or is the collection of traits we think of as modern just the latest set of variable features in the evolution of a much older species?

4. The Debate over Modern Human Origins • The Recent African Origin (RAO) model • Although various supporters of this model recognize different numbers of species within genus Homo, they all share the view that modern Homo sapiens is a separate species that branched from a preexisting archaic Homo species in Africa around 200,000 to 150,000 ya.

5. The Debate over Modern Human Origins • The Multiregional Evolution (MRE) Model • If the MRE model is correct, we should find no clear evidence that modern H. sapiens is a separate species from any of the so-called premodern groups. • In other words, there should be no biologically meaningful definition of modernity-- no set of traits that is found among all populations classified as modern that is lacking among all premodern populations.

6. The Debate over Modern Human Origins • The Key Requirements of the RAO Model • Requires an anatomical definition of modernity that clearly distinguishes modern humans from premodern ones. • Requires genetic distinctions of a degree that indicates modern humans are a separate species. • Requires that transitional forms appear only in the single region in which modern humans evolved.

7. The Debate over Modern Human Origins • The Key Requirements of the MRE Model • Requires that there be no biologically meaningful definition of modernity; that is, no set of “modern” traits that is lacking among all premodern populations. • Requires that populations with transitional sets of traits be found in many locations. • Requires a regional continuity of traits: that is, feature characteristics of a given geographic area should appear in premodern and modern populations within that region.

8. The Debate over Modern Human Origins • The Evidence • In reviewing the extensive literature on modern human origins, we find that different authorities have very different interpretations of the same data. • Indeed, the same general data have been convincingly used to support both the MRE and RAO models, but proponents differ in which specific pieces of data they emphasize.

9. The Debate over Modern Human Origins • The Fossil Record • The RAO model would say that forms transitional between archaic species and H. sapiens are found only in Africa. • Modern forms appear first in Africa and then show up at increasingly recent dates as one moves away from that continent.

10. The Debate over Modern Human Origins • The Fossil Record • The MRE model would contend that modern traits did not all arise in one location but in many and that they spread throughout the species through gene flow, to be expressed differently in different geographic locations.

11. The Debate over Modern Human Origins • Genetic Evidence • In general, living human beings exhibit very little genetic variation. • This suggests a relatively recent, common source for all living humans, which is consistent with the RAO hypothesis.

12. The Debate over Modern Human Origins • Genetic Evidence • The MRE model would say that as a mobile species with a network of constant genetic exchange among populations, regional genetic differences would become increasingly lessened as the species increased its population and improved its ability to move around. • In such a species, even a very old one, we would also expect relatively little modern genetic variation.

13. definitions • Nuclear DNA • The genetic material in the nucleus of a cell. • Mitochondrial DNA (mtDNA) • The genetic material found in the cell’s mitochondria rather than in the cell’s nucleus. • Symbiosis • An adaptive relationship between two different species, often, but not necessarily, of mutual benefit.

14. The Debate over Modern Human Origins • Evolutionary Theory • A major issue of the previous genetics argument is the plausibility of the gene flow required by the MRE model. • Could such gene flow have taken place, and if so, how?

15. The Debate over Modern Human Origins • The RAO Model • Until recent times hominin populations were too thinly spread across the three continents of the Old World to be connected by gene flow.

16. The Debate over Modern Human Origins • The MRE Model • The apparently thin spread of early human populations across the Old World may be in part attributable to the nature of the fossil record. • Most individual hominins who ever lived did not leave fossilized remains, nor have we found but a fraction of all the fossil hominin remains that were left.

17. The Debate over Modern Human Origins • The RAO Model • There were too many geographic barriers. There were mountain, desert, and water barriers. • The MRE Model • Geographic barriers to human habitation and movement certainly existed and still do. They did not, however, prevent the spread of human populations.

18. The Debate over Modern Human Origins • Mostly-Out-Of-Africa: A Model from the Data • This model proposes more than one major expansion out of Africa, the first during H. erectus times and the latest around the time proposed by RAO for the origin and spread of modern H. sapiens. • There may have been other expansions as well, not necessarily “out of Africa”; people migrate in all directions. These expansions would have spread collections of genes and traits across geographic space.

19. The Debate over Modern Human Origins • Thus, the evolution of some features of modern human anatomy, and the genes that coded for them, could have occurred first in Africa, but their spread around the world did not necessarily result in the replacement of an existing species by a new one.

20. The Debate over Modern Human Origins • Summary • The data from paleontology, archaeology, and genetics are ambiguous on the issue of the origin of modern H. sapiens. • Either model (RAO or MRE) may be supported by the same basic facts. • But now a new model, the Mostly-Out-Of-Africa model, or multiple expansion model, seems to reconcile all the evidence into a more satisfying scheme.

21. Questions for further thought • Aside from the alleged race implications, why do you think professionals are so emotional about this debate? • Do you see any issues other than purely scientific ones-- philosophical ones, perhaps-- that could be involved?

22. Questions for further thought • Some claim that the Multiregional Evolution model is potentially “racist.” • How would you respond to such an accusation?