Ancient Grandmothers, African Savannas

Human life history evolved from an ancestral condition shared with the other great apes whose infants begin to feed themselves on the fruits and leaves their mothers are eating while still nursing, then feed on their own at weaning so mothers can commit to their next offspring. The savannas that spread in ancient Africa would have presented foraging opportunities with novel tradeoffs. Lessons come from modern people foraging in African savannas now. Hadza women earn predictable returns from plants that flourish in savanna seasonality, but youngsters are too small to be effective at exploiting the deeply buried tubers that are year ‘round staples, so they depend on their moms. Yet mothers have next babies long before the one can feed itself as weaned dependents are subsidized by the daily productivity of grandmothers.

Those lessons contributed to a grandmother hypothesis to explain the evolution of our distinctive life history, which modeling and evidence now link to other features that distinguish us from our great ape cousins. Postmenopausal longevity brought along all those old men, biasing mating sex ratios toward males - a likely basis for our distinctive pair bonding habits. Greater longevity delays development, slowing neural maturation and expanding brain size. So ancestral infants and toddlers immature brains wired to a world without the full maternal commitment enjoyed by other ape infants. Survival depended on actively engaging relationships from babyhood onward, which could make our distinctive life-long social appetites a legacy of our grandmothering life history.