When times were tough and food was scarce on the savannah, Australopithecus africanus toddlers returned to the boob.
An analysis of fossil teeth, published in the journal Nature, has found that mothers among these extinct human ancestors supplemented their infants’ diet with breastmilk long after they had started eating solids.
Lengthy periods of breastfeeding are common in great apes. Gorillas wean at around age four, and chimpanzees stop nursing at about five.
Humans wean comparatively early – usually by age three – but invest heavily in their young in other ways to see them through lengthy childhoods.
Studying breastfeeding in Australopithecus africanus – a hominin that lived more than two million years ago in southern Africa – is part of an effort to trace the origins of this short period of breastfeeding in otherwise late-maturing children.
Renaud Joannes-Boyau, from Australia’s Southern Cross University, and colleagues sliced through four teeth from two individuals that were entombed in the Sterkfontein cave site in South Africa between 2.6 and 2.1 million years ago.
Teeth, like trees, have growth rings that provide a record of an animal’s life.
By mapping the concentration of barium – an element that is absorbed and incorporated into growing teeth more readily with a diet of milk – the team worked out the breastfeeding patterns of the individuals.
It wasn’t what they were expecting.
“The breastfeeding of Australopithecs africanus was unusual,” says Joannes-Boyau. “We were expecting something quite long, similar to what we see in modern day great apes, like chimpanzees.”
Instead, they found that Australopithecus breastfed exclusively for just six to nine months, at which point solids were introduced. By about a year of age, breastfeeding had tapered off almost entirely.
But annual surges in breastfeeding continued for the next four or five years.
This cyclical pattern of breastfeeding was confirmed by levels of the element strontium. When breastmilk consumption ramped up, as indicated by barium, other sources of food, indicated by strontium, disappeared.
The strategy was probably the product of a harsh environment, with seasonal periods of food scarcity.
“When it’s the off season then the mother will supplement a lot of the calorie deficiency with breast milk,” says Joannes-Boyau.
The team also looked at the element lithium, which revealed that the hominins were briefly consuming a particular food – perhaps a less desirable fall-back food consumed in the off-season – right before the annual peak in breastfeeding.
The annual cycling of breastmilk consumption is not unlike that seen in orangutans.
Joannes-Boyau believes that instead of moving from place to place to ensure year-round food, Australopithecus africanus bet on a strategy of sticking it out in tough conditions with fewer predators.
“Perhaps this environment that is not offering much food and so on forces them to invest a little bit more in each offspring instead of having a lot of kids,” he adds.
The heavy investment in children more than two million years ago was an early sign of things to come, he says.
The team are analysing teeth from other extinct members of the human family tree to flesh out the picture of how our own childhoods were shaped by evolution.
Dyani Lewis is a freelance science journalist based in Melbourne, Australia.
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