Three million-year-old Australopithecus teeth have revealed that years after their owners started eating solid food they went back to the breast, probably when food was scarce. The finding shows the human trait of investing heavily in a few children, rather than having many in the hope a few will live, runs deep in our evolutionary history.
We deposit a daily layer of enamel on our teeth, and the composition of that layer can be very revealing of our diets. Breast milk is rich in a form of barium that is preferentially laid down in teeth. By vaporizing layer upon layer using a laser and assessing the released gas Dr Renaud Joannes-Boyau of Australia's Southern Cross University was able to measure the breast milk composition of the diet of two A. africanus who lived 2.6-2.1 million years ago over the first years of their lives.
In Nature, Joannes-Boyau reports both were breastfed exclusively until around the age of one, but experienced regular cycles where their diet was supplemented by breast milk until at least the age of five. This is not a pattern we see in modern hunter-gatherers, nor in Neanderthals who have previously been studied in this way, but it does resemble the diet of young orangutans.
Joannes-Boyau told IFLScience the pattern probably evolved in response to wide swings in food availability in A. africanus' southern African home. “We think the mothers put on a lot of fat during the rich season and when shortages came they drew on these fat reserves to feed their children as well as themselves.”
Considering the challenges of restarting stopped breast milk, Joannes-Boyau thinks A. africanus mothers, like orangutans, did not stop producing milk entirely until their children were well grown. Instead, volume would have dropped in times good enough for the baby to feed on its own, and revived when food was sparse.
Although he can't say for sure, Joannes-Boyau doubts such a regime was compatible with also feeding a second child, suggesting Australopithecines spaced their births widely, caring for their offspring intensively in between. “The strong bond between mothers and offspring for a number of years has implications for group dynamics,” he said in a statement. “The social structure of the species, relationships between mother and infant and the priority that had to be placed on maintaining access to reliable food supplies.”
“If you lined up all the animals and think which would be the most successful,” Joannes Boyau told IFLScience, “you would not pick the one without claws or big teeth or speed, but humans have a very long childhood and during that are very adaptable. This is why we were so successful, we were able to adapt and survive, with a broad diversification of diet.”