Page 64 - Flipping book The Adam Paradox Hypothesis - Second Edition.pdf

The Ādam Paradox Hypothesis 41
Strand One: Bottlenecks — The Narrowing of the Species
Genetic Evidence for Bottlenecks
Modern humans carry signatures of a severe bottleneck ~70–50 ka. Studies of
mitochondrial DNA and whole genomes suggest effective population sizes
shrank to as low as 1,000–10,000 breeding individuals (Li & Durbin, 2011;
Prüfer et al., 2014).
Li and Durbin’s genomic modeling concluded:
“The history of effective population size shows a dramatic reduction around 70–50 thousand
years ago, with recovery following the out-of-Africa dispersals.
” (Li & Durbin, 2011, p.
196)
This means that all living humans descend from a tiny founder population,
squeezed through an evolutionary bottleneck.
Toba Super-Eruption Hypothesis
One proposed cause is the Toba volcanic eruption (~74 ka) in Sumatra, which
released 2,800 cubic kilometers of ash, triggering a volcanic winter. Stanley
Ambrose (1998) suggested Toba reduced human populations to a few
thousand.
“The Toba super-eruption may have plunged early Homo sapiens into near-extinction,
creating a genetic bottleneck that shaped modern diversity.
” (Ambrose, 1998, p. 623)
This hypothesis is debated. Some argue the bottleneck predated Toba, but all
agree on dramatic population reductions.
The Evolutionary Effect of Bottlenecks
Bottlenecks can accelerate evolution by:
Concentrating rare variants. Alleles that were rare in larger populations can
fix quickly in small groups.
Eliminating diversity. Genetic drift sweeps away competing lineages,
simplifying the genome.
Creating founder effects. Populations that expand after bottlenecks carry
distinctive signatures.