Rapid population rise bad for our health?

As an alternative, Speakman has put forward a non-adaptive scenario — the drifty gene hypothesis. According to this idea, genetic drift — the change of gene frequencies due to random, non-selective processes— could explain the current incidence of obesity seen in modern, industrialized populations. Speakman has hypothesized that the threat of predation on early humans may have provided a strong selective pressure to keep maximum body weight in check. A human fleeing from a sabertooth cat, for example, would be at a clear disadvantage if they carried extra weight. The introduction of fire and cooperative social groups may have reduced the threat of predation, allowing upper body weight to wander through genetic drift. The effects of genetic drift are most profound in small populations, but Speakman argues that just such a process could explain the accumulation of mutations in the human population that affect body weight.