Genetic perspectives on human origins and differentiation.

Area of Science:

• Human evolutionary genetics
• Paleodemography
• Population genetics

Background:

• Debate exists regarding ancient human population size: a bottleneck (Garden of Eden model) versus a large, widespread ancestral population (multiregional evolution model).
• Genetic markers provide insights into human origins and diversity, but ascertainment bias in early studies (using European samples) complicated interpretation.

Purpose of the Study:

• To review genetic evidence concerning ancient human demography and the origins of global human diversity.
• To evaluate the evidence for and against a population size bottleneck in human ancestors.

Main Methods:

• Analysis of genetic evidence from various marker loci, including nuclear loci and microsatellites.
• Assessment of diversity gradients across global populations, particularly the gradient away from Africa.

Main Results:

• Some genetic markers suggest recovery from a small ancestral population, while others indicate widespread balancing selection.
• Microsatellite data, free from ascertainment bias, show a clear diversity gradient decreasing away from Africa.
• New World populations exhibit approximately 15% less diversity than African populations.

Conclusions:

• The observed diversity gradient is incompatible with a single, large population expansion model.
• Findings support a model of gradual diversity loss through successive colonization bottlenecks as humans spread globally.
• The evidence challenges the strict Garden of Eden model and suggests a more complex demographic history.