Recent African Origins or Regional Evolution?


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Compares the two major models of human migration and the means of testing them.

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Recent African Origins or Regional Evolution?

  1. 1. Recent African Origins or Regional Evolution? Where did Modern Humans Come From, and When?
  2. 2. Human Origins: An Acrimonious Debate <ul><li>There is a longstanding debate as to how we got to be we, namely become modern Homo sapiens? </li></ul><ul><li>Why do the experts, whom we are supposed to respect for their careful research, differ so intensely that sparks fly? </li></ul>
  3. 3. Why the Debate: Several Reasons <ul><li>The evidence itself is fragmentary, as you’ve seen in the past lessons; we just don’t have complete skulls or postcranial skeletons handed to us on a platter </li></ul><ul><li>Therefore we have to make inferences from our bone fragments, and different experts make different inferences from those bone fragments </li></ul><ul><li>Every expert comes to the field and the lab with her or his own assumptions </li></ul>
  4. 4. Points of Agreement <ul><li>There is one agreement: almost all paleoanthropologists agree that we evolved from Homo erectus </li></ul><ul><li>They also agree that Homo erectus migrated from Africa into Asia and Europe </li></ul><ul><li>This is where the experts split into two camps—When and where did we evolve from Homo erectus ? </li></ul>
  5. 5. The Recent African and Multiregional Evolution Models <ul><li>Did our ancestors evolve at different places around the Old World from Homo erectus? </li></ul><ul><li>That is known as the Multiregional Evolution Model </li></ul><ul><li>Or did they all evolve from Homo erectus in Africa? </li></ul><ul><li>That is known as the Recent African Origin model, because we originated from Africa only in the past one or two hundred thousand years BP (before the present) </li></ul><ul><li>This is also known as the Out of Africa model. </li></ul>
  6. 6. The Two Models Compared <ul><li>Left: Multiregional Hypothesis: Homo erectus migrates to Asia and Europe and evolves in each region into Homs sapiens . </li></ul><ul><li>The little arrows indicate interbreeding across the regions, keeping our species intact.. </li></ul><ul><li>Right: Recent African Origin (Out of Africa) Hypothesis: Homo erectus evolves into Homo sapiens, then migrates to the rest of the Old World—and the new one </li></ul>
  7. 7. Multiregional Evolution Model <ul><li>This map depicts humans evolving from regional points and migrating from there </li></ul><ul><li>What prevents speciation, of humans from becoming new species? Milford Wolpoff (lower left) has an answer: </li></ul><ul><li>Whatever else hominins may do, they always interbreed. </li></ul><ul><li>Thus they maintain control over, or prevent, speciation </li></ul><ul><li>Alan Thorne (lower right) traces Homo erectus ‘s transition to Homo Sapiens in Southeast Asia </li></ul>
  8. 8. Recent African Origin Model <ul><li>This map places modern human origins, according to one conception, somewhere in the pink shaded area of S. and E. Africa </li></ul><ul><li>Then humans migrate in waves: the darker the red, the more recent the waves. </li></ul><ul><li>Ian Tattersall (with bony colleague) argues the following; </li></ul><ul><li>Modern Homo sapiens evolved in Africa around 200,000 BP </li></ul><ul><li>They migrated to Europe and Asia </li></ul><ul><li>They displaced archaic H. sapiens including H. neanderthalensis </li></ul><ul><li>Modern and archaic forms did not interbreed, extinguishing the latter </li></ul>
  9. 9. Partial Replacement Model: A Middle Ground? <ul><li>Gunter Br äuer, Univ. of Hamburg, Germany </li></ul><ul><li>Modern H. sapiens arose in Africa around 100,000 BP </li></ul><ul><li>They both replaced and interbred with archaic sapient forms </li></ul><ul><li>Through interbreeding, modern populations gradually replaced the premodern hominins </li></ul><ul><li>Fred Smith: replacement occurred as much through gene flow as through migration: they let the genes do the walking </li></ul>
  10. 10. Testing the Models <ul><li>It is very well to speculate on and interpret the two models </li></ul><ul><li>How do we test them? </li></ul><ul><li>We offer one set of tests to compare the hypotheses generated by the two models </li></ul><ul><li>We then show how that one test alone is fraught with ambiguities of the data—the fragmented hominin remains. </li></ul>
  11. 11. Out-of-Africa Model: Test Expectations I <ul><li>Oldest modern sapient fossils should be found only in Africa (see model to the right) </li></ul><ul><li>Transitional forms (e.g. H. Heidelbergensis) should be found only in Africa </li></ul><ul><li>Elsewhere, emigrant modern humans should coexist with archaic humans until the latters’ extinction </li></ul>
  12. 12. Out-of-Africa Model: Test Expectations II <ul><li>There should be a break between premodern (H. heidelbergensis and earlier forms) and modern fossil humans outside Africa </li></ul><ul><li>Modern human material cultures (e.g. tools) should make a sudden appearance outside Africa, with no transitional forms </li></ul><ul><li>Modern humans should be genetically distinct from premodern humans outside Africa </li></ul>
  13. 13. Multiregional Evolutionary Model: Test Expectations I <ul><li>Early modern human fossils should be found across all or many regions, none much older than the others (left model) </li></ul><ul><li>Intermediate humans should be found across the regions because evolution occurred everywhere. </li></ul><ul><li>Premodern features should grade into modern forms everywhere as modern genes replace premodern ones (e.g. reduction of prognathism) </li></ul>
  14. 14. Multiregional Evolutonary Model: Test Expectation II <ul><li>Local skeletal traits should show continuity between modern and premodern forms everywhere </li></ul><ul><li>There should be a continuous development between premodern and modern material cultural remains </li></ul><ul><li>There should be genetic continuity between modern and premodern forms in every region </li></ul>
  15. 15. Shifting Evidence <ul><li>Klases River Mouth, South Africa yielded the following finds: </li></ul><ul><li>Fragments of modern skulls and a jaw with a modern chin (upper left) </li></ul><ul><li>The jaw and fragments were dated 90,000 years BP, the oldest up to that time (1970s) </li></ul><ul><li>The find would confirm the RAO hypothesis </li></ul><ul><li>Then a modern skull was found in Liujiang, China (lower left) in 1958, dated 20 to 30,000 BP </li></ul><ul><li>An analysis in 2002, however, dated the skull 100,000 years BP, favoring the Multiregional Evolution model </li></ul><ul><li>Later finds place a modern find in Omo, Ethiopia, at 195,000 years BP, favoring the RAO Model—for now. </li></ul>
  16. 16. Current Status of the Models: Modern Homo sapiens <ul><li>At the moment, multiple sites indicate that African sapient sites are older </li></ul><ul><li>Homo sapiens skulls are oldest in Africa </li></ul><ul><li>They range between 100,000 and 200, 000 years BP, including the Omo find </li></ul><ul><li>Israel has remains ranging between 92,000 and 120,000 BP </li></ul><ul><li>European skulls range between 10,000 and 27,000 BP </li></ul><ul><li>China’s range is 10,000 to 100,000 BP </li></ul><ul><li>Australian skulls range from 40,000 upward </li></ul><ul><li>So far these data support the Recent African Origins Model </li></ul>
  17. 17. Current Status of the Models: “Archaic Homo sapiens ” <ul><li>The same pattern applies to Homo heidelbergensis , or Archaic Homo sapiens </li></ul><ul><li>African remains vary between 400,000 and 700,000 BP </li></ul><ul><li>European remains vary between 160,000 and 475,000 BP (780,000 in Spain) </li></ul><ul><li>China and India: 130,000-200,000 BP </li></ul><ul><li>These fit the pattern of the Recent African Origins model. </li></ul>
  18. 18. Typological Ambiguities <ul><li>Also open to controversy is what constitutes modern Homo sapiens </li></ul><ul><li>Harold Dibble argues that typologies often lead us down blind alleys </li></ul><ul><li>We often find differences that aren’t really there , creating even more squabbles—like this one or the one about the human status of Neanderthals </li></ul><ul><li>Artist’s conception of Homo heidelbergensis (lower left)—or is it Homo sapiens ? </li></ul>
  19. 19. Taxonomic Questions <ul><li>This comparison of a Neanderthal (left) and human skeleton reflects a major controversy about human typology </li></ul><ul><li>Wolpoff goes so far as to suggest that Homo erectus (lower left) and Homo sapiens (lower right( could be one species </li></ul><ul><li>As the clichés have it, you be the judge: </li></ul><ul><li>From the two sets of pictures, does Wolpoff make a prima facie case for his one-species argument? </li></ul>
  20. 20. Testing the Model <ul><li>Are the oldest modern forms found in Africa or are they also found in Europe and Asia? </li></ul><ul><li>Are transitional forms found only in Africa or are they also found in Europe and Asia? </li></ul><ul><li>What’s the evidence from genetic mutation? </li></ul><ul><li>Mitochondrial DNA in the female lineage> </li></ul><ul><li>Y Chromosomes? </li></ul><ul><li>Is the transformation from archaic to modern forms sudden or gradual outside Africa? </li></ul><ul><li>What do the archaeological finds say? </li></ul>
  21. 21. Case Example: Mitochondrial DNA <ul><li>Principles of mitorchorndral DNA (mDNA) tests </li></ul><ul><li>mDNA samples only from living persons </li></ul><ul><li>mDNA is used to retrodict past mutations </li></ul><ul><li>mDNA is passed only by women </li></ul><ul><li>Sperm leaves behind all its mDNA </li></ul><ul><li>Fetus inherits mDNA only from ovum </li></ul><ul><li>Number of mDNA mutations indicates antiquity of species </li></ul>
  22. 22. Out of Africa: Procedures <ul><li>Rebecca Cann and colleagues: </li></ul><ul><ul><li>Sampled 147 women </li></ul></ul><ul><ul><ul><li>mDNA shows little diversity </li></ul></ul></ul><ul><ul><ul><li>Sample was to be worldwide </li></ul></ul></ul><ul><ul><ul><ul><li>Africans should show the most mutations </li></ul></ul></ul></ul><ul><ul><ul><ul><li>New Guinea (NG)/Australian, dated 80K BP, included </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Africans showed 3 times mutation of NG./Australians </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Other populations similarly sampled </li></ul></ul></ul></ul><ul><ul><li>Results tended to support claim </li></ul></ul><ul><ul><ul><li>Mutations traceable to a single African female </li></ul></ul></ul><ul><ul><ul><li>Hence, “Mitochondral Eve </li></ul></ul></ul>
  23. 23. Out of Africa: mDNA a flawed methodology <ul><li>Nature of the flaws </li></ul><ul><ul><li>Sample was too small: 147 out of 2 billion </li></ul></ul><ul><ul><li>All “African” women were American </li></ul></ul><ul><ul><ul><li>Potential admixture with Europeans, Native Americans, and Asians </li></ul></ul></ul><ul><ul><li>Order of data input influenced results </li></ul></ul><ul><li>Alan Templeton reran the tests </li></ul><ul><ul><li>Found mutation rates equal for Africans, Europeans, and Asian </li></ul></ul><ul><ul><li>Supports mulitregional theory </li></ul></ul>
  24. 24. Out of Africa Theory: Retests of mDNA <ul><li>Laurence Excoffier & Andre Langanay </li></ul><ul><ul><li>Tested larger sample </li></ul></ul><ul><ul><ul><li>Africans from Africa exhibited less diversity </li></ul></ul></ul><ul><ul><ul><li>Than European and Asians. </li></ul></ul></ul><ul><li>Cann and colleagues ran a retest </li></ul><ul><ul><li>Sample much larger--5,000 </li></ul></ul><ul><ul><li>Africans from African included </li></ul></ul><ul><ul><li>Support more modest </li></ul></ul><ul><ul><ul><li>Africans showed more diversity </li></ul></ul></ul><ul><ul><ul><li>But the variations were not statistically significant </li></ul></ul></ul>
  25. 25. Mitochondrial DNA: General Results, If Any <ul><li>There might have been several migrations out of Africa, not just one. </li></ul><ul><li>The migrations could have taken place various times from 2 billions years BP—to the present </li></ul><ul><li>Thus it is not surprising that Templeton found varied and even contradictory results </li></ul><ul><li>The genetics of the world population isn’t all that diverse in the first place. </li></ul><ul><li>For full argumentation, see pp. 331-344. </li></ul>
  26. 26. Conclusion <ul><li>Fragmentary data makes the controversy less than conclusive. </li></ul><ul><li>It depends on our typology, from Tattersall’s extreme splitter taxonomy to Wolpoff’s lumper’s imagination. </li></ul><ul><li>And how we interpret our finds in relation to the typology we have accepted. </li></ul><ul><li>The artists’ conceptions involve knowledge of human/hominin anatomy with a great deal of subjective interpretation. </li></ul><ul><li>Finally, DNA results are too varied to be conclusive—mtDNA or Y chromosomes </li></ul>
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