One possible way such signals could be functional is if climate changes were a persistent survival problem for humans. There is evidence that “during the present (Holocene) interglacial…cold and dry phases…[occurred]… on a 1500-year cycle, and with climate transitions on a decade-to-century timescale” [101]. On a smaller timescale, over the last millennium of Chinese history, climate changes to cold phases have been associated with decreased harvests, increased warfare, decreasing population and dynastic changes [102]. While very speculative, it is possible that physiological and behavioral plasticity based on birth environment allows better survival through such turbulent changes. Among early humans living predominantly in a tropical environment such a signal for plasticity may not have been obscured by the more marked seasonal variations now experienced farther from the equator.
