Kenneth L. Feder

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C H A P T E R

The Evolution of Us: The Origins of Modern Humans

6

The Past in Perspective

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The Evolution of Us: The Origins of Modern Humans• Mitochondrial DNA and Molecular Archaeo

logy• Molecular Archaeology and the Evolution o

f Human Beings• The Replacement Model• The Multiregional Model• A Middle Ground: Genetic Replacement• Issues and Debates• Case Study Close-Up• Summary

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Mitochondrial DNA and Molecular Archaeology

• Nuclear DNA serves as the blueprint for an individual and a species and is present in most cells of an organism.– Most cells also contain mitochondrial DNA

(mtDNA).

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Molecular Archaeology and the Evolution of Human Beings• Preserved mtDNA has been recovered from

the preserved bones of a 40,000-year-old woolly mammoth (Pääbo, Higuchi, and Wilson 1989), a 14,000-year-old saber-toothed tiger (Grimaaldi 1993), and modern human beings.– Researchers have also recovered genetic

material from the bones of a few Neandertals and anatomically modern-looking human beings who lived during the same period (Adcock et al. 2001; Scholz et al. 2000).

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The Replacement Model

• The replacement model states that nearly all of the fossils of premodern humans represent extinct forms of human beings that contributed nothing to the evolution of modern humanity.– The first anatomically modern human beings

spread out from their African homeland, first into southwest Asia and then east to the rest of Asia and north and west to Europe.

• Premoderns could not successfully compete for resources with the modern humans and so became extinct.

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Molecular Archaeology and the Evolution of Human Beings

Schematic depiction of competing models of the evolution of anatomically modern Homo sapiens.

Insert Figure 6-1 (Old figure 7.1)

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The Replacement Model

The geography of the evolution of modern humans implied by the population replacement model.

Insert Figure 6.2 (old figure 7.2)

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The Multiregional Model

• In the multiregional (or regional continuity) model, the evolution of modern human beings was a geographically broad process, not an event restricted to a single place.– Enough contact between groups in Africa,

Europe, and Asia was maintained to allow for gene flow among them.

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A Middle Ground: Genetic Replacement

• Genetic Replacement– Gunter Bräuer (1992) believes that as the first

anatomically modern human beings spread from Africa.

• They mated with the archaics, producing hybrid populations that were pulled along to modernity.

– Fred Smith (1991, 1992) holds that modern human traits developed in a single population.

• They spread primarily through gene flow (as opposed to migration) into archaic groups.

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A Middle Ground: Genetic Replacement

• Biologist Alan Templeton (2002) sees major traits that characterize modern human skeletal anatomy as having originated in Africa.

• Spread by a population movement from there to Europe and Asia more than 90,000 years ago.

• Early African modern migrants mated with, rather than replaced, local human groups when they encountered them.

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Issues and Debates

• Replacement or Continuity?– What We Would Expect on the Basis of the

Population Replacement Model?1. The oldest anatomically modern human fossils

should be found in Africa and nowhere else.

2. There should be anatomical continuity only in Africa.

3. Outside of Africa, the emigrant moderns should be contemporaries of indigenous archaics.

4. There should be a distinct break in the form of archaic and modern human fossils outside of Africa.

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Issues and Debates

• Replacement or Continuity?– What We Would Expect . . . (continued)

5. Anatomically modern humans should be genetically distinct from premoderns.

6. The archaeological record is expect to show the sudden appearance of nonlocal, African-originating artifact types in Europe and Asia.

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Issues and Debates

• Replacement or Continuity?– What We Would Expect on the Basis of the

Multiregional Model1. Early versions of anatomically modern Homo

sapiens should be found in many different regions.

2. Intermediate forms—advanced premoderns—should be found in each region.

3. There should be no or very little chronological overlap between the archaic and modern forms.

4. Local skeletal traits should show continuity between archaic and modern humans.

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Issues and Debates

• Replacement or Continuity?– What We Would Expect . . . (continued)

5. DNA recovered from the bones of ancient, archaic looking human beings, would be as similar to the DNA of modern humans as the DNA recovered from the bones of ancient, modern-looking individuals.

6. The archaeological record should show a continuity in regional artifact types.

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Issues and Debates

• Replacement or Continuity?– What We Would Expect on the Basis of the

Middle Ground1. The oldest anatomically modern human fossils

should be found in Africa and nowhere else.

2. There should be chronological overlap between indigenous premodern humans and emigrant moderns.

3. There should not be a distinct break between premodern and modern forms.

4. There should be continuity of regional traits seen in local premodern groups.

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Issues and Debates

• Replacement or Continuity?– What We Would Expect . . . (continued)

5. DNA recovered from the bones of premoderns should show a gradual transformation into the modern form.

6. Cultural remains in Europe and Asia should show a gradual shift to more modern forms.

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Issues and Debates

• Replacement or Continuity?– Testing the Implications of Replacement and

Continuity1. Are the oldest anatomically modern human fossils

found in Africa and nowhere else?

2. Is there continuity only in Africa?

3. Were archaic and anatomically modern human beings contemporaries?

4. Is there a break in the form of premodern and modern human fossils outside of Africa?

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Issues and Debates

• Replacement or Continuity?– Testing the Implications . . . (continued)

5. Is the DNA recovered from the bones of premodern humans significantly different from that of their contemporary, anatomically modern-looking humans, as well as from the DNA of living people?

6. Does the archaeological record show the sudden appearance of nonlocal, African-originating artifact types in Europe and Asia?

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Issues and Debates

• How Can Modern Human Genetics Inform Us about Their Ancient Origins of Humanity?– The mtDNA sample of living human females

showed little overall diversity, so modern human beings must have evolved relatively recently.

• The oldest modern human mtDNA and, therefore, the source of the rest of the world’s mtDNA, was African.

• Based on estimated rates of mtDNA mutations, Cann and her colleagues calculated the African mtDNA lineage began accumulating these mutations 120,000 to 150,000 years ago.

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Issues and Debates

• Why Were the Neandertals Replaced?– Harris lines, cracks on the ends of long bones

(arm and leg) that result from dietary deficiency during the developing years, and enamel hypolasisa, zones of thin tooth enamel that result from unmet nutritional needs during early childhood, are present to a far greater degree in Neandertal remains than in the remains of anatomically modern human beings.

• Our anatomically modern human ancestors were not precisely adapted to the environmental conditions of Late Pleistocene Europe as were the Neandertals and this likely contributed to their survival.

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Issues and Debates

• Why Were the Neandertals Replaced?– Our brains are significantly bigger in the frontal

lobe than are those of the Neandertals, and this implies differences in intellectual capacities.

• May explain why the Neandertals could not compete successfully with the anatomically modern human beings.

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Issues and Debates

Fossil localities of early anatomically modern Homo sapiens.

Insert Figure 6.3 (Old figure 7.3)

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Case Study Close-Up

• The caves near where the Klasies River empties into the Indian Ocean may have been first explored scientifically in 1923 during a survey of caves and rock shelters along the South African coast.– An extensive array of tools was recovered.

• The technology represented at KRM is, in the vernacular of African Archaeology, a Middle Stone Age (MSA) industry, which means that is essentially Mousterian in it technology.

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Case Study Close-Up

• The caves . . . – An extensive array of tools . . . (continued)

• Compared to European Mousterian industries, however, KRM technology exhibits a number of more finely made long flakes, called blades.

• The inhabitants were among the first people in the world to exploit aquatic resources, including shellfish, seals, penguins, fish, and sea birds (Klein 1977).

• Richard G. Milo’s analysis indicates that anatomically modern human beings were hunting in a behaviorally modern way by 100,000 years ago in South Africa.

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Summary

• Two different models have been proposed to explain the evolution of anatomically modern human beings—the replacement model and the multiregional model.

• The data discussed in this chapter provide only ambiguous support for either extreme or for any intermediate formulation.

• The question of modern human origins continues to be a source of often-bitter debate among paleonanthropologists and geneticists.