2 Million-year-old Teeth of Extinct Early Human Reveal Species Breastfed Infants to Make up for Seasonal Food Shortages

Renaud Joannes-Boyau, Southern Cross University,
Renaud Joannes-Boyau in the Geoarchaeology and Archaeometry Research Group (GARG) laboratory at Southern Cross University. SCU media

Scientists that studied the 2 million-year-old teeth of an extinct species of human believe they breastfed to deal with seasonal food shortages.

The authors of the study, published in the journal Nature, analyzed five fossilized teeth from two sets belonging to members of the Australopithecus africanus hominid species. The specimens were found in the Sterkfontein cave near Johannesburg, and are believed to be between 2.6 to 2.1 million years old.

Previous research suggested the early humans lived around 2 to 3 million years ago, and survived on a diet of roots, grasses, leaves and fruit.

Teeth are built-up growth rings of minerals, comparable to tree rings. By looking at the rings, the authors concluded these early hominids mostly drank breast milk in the first year of their lives. As babies who drink breast milk grow, barium, which is found in the liquid, collects in their teeth. Up until six to nine months, babies appeared to drink milk, and were slowly weaned until around the age of 12 months, the teeth suggest.

Tests also revealed that food seemed scarce for the Australopithecus africanus. During the summers, they likely had plenty of food on the savanna, but would have found it harder to eat in the dry winters—as suggested by lithium in the teeth. Breastfeeding could have helped early humans get around seasonal food shortages, the authors believe.

Renaud Joannes-Boyau, lead first author of the study and head of the Geoarchaeology and Archaeometry Research Group (GARG) at Southern Cross University in Australia, told Newsweek: "While our bones continue to change composition as they remodel during our lives, our teeth don't change after they form during childhood. Teeth are thus a perfect chemical time capsule of our childhood diet."

"Our study presents the first indication of the duration of breastfeeding in one of the oldest human ancestors," he said.

"What is a truly surprising result is that Au. Africanus appears to breastfeed cyclically after around one year of age. This indicates that Au. africanus mothers were supplementing their offspring with breastmilk for years after the initial weaning event took place. This has several major implications about how long infants were cared for and were reliant on breastmilk, how many offspring a mother could have in their lifetime, the strong seasonal difference in their diets that put stress on mothers (and limited food availability for their offspring), the social interactions within the species, and the kinds of environments these ancestors were occupying."

The teeth also provide an insight into why the species didn't survive, according to Joannes-Boyau.

"It also may directly indicate reasons why the group eventually went extinct relative to other hominin groups (like our own genus, Homo) if they struggled to acquire sufficient nutrition from the local habitats," he explained.

Recalling some of the difficulties the team faced while conducting their work, Joannes-Boyau said: "It was very challenging to obtain a clear and accurate signal of the elemental distribution in the teeth."

"The samples are very fragile 2 million-year-old fossil teeth so it is challenging to manage to prep the fossil properly and to extract the geochemical signal from it. On top of that, finding ways to recreate elemental maps of the fossil is even more challenging."

The team used a high-tech laser to take off tiny pieces of the fossil teeth's surface, then used an instrument to analyze the chemical composition. This is less destructive than traditional techniques, Joannes-Boyau explained.

Acknowledging a limitation of the work, Joannes-Boyau said the team only analyzed two individuals, "which is not enough to make confident extrapolation on the entire species. We need to analyze more samples and to compare it to other early hominins to see what are/could be the differences."

Christine Austin, a first author of the study and Assistant Professor of Environmental Medicine and Public Health at the Icahn School of Medicine at Mount Sinai, explained in a statement that the research could help parents today.

"Seeing how breastfeeding has evolved over time can inform best practices for modern humans by bringing in evolutionary medicine. Our results show this species is a little closer to humans than the other great apes, which have such different nursing behaviors."

She continued: "These are important findings from an evolutionary perspective, because humans have long childhoods and short breastfeeding periods while apes have longer breastfeeding periods than humans do. We're still in the dark about why or when we made that change and what the effect of more recent major changes in breastfeeding, with agriculture and industrialization, could have on mothers' and babies' health."

Professor Gail Kennedy, professor emerita at UCLA Anthropology who was not involved in the research, told Newsweek: "The biggest problem with this piece was with weaning itself. Weaning is perhaps the most critical of all life history events: too early and the infant misses the many advantages that its mother's milk and its mother (e.g. protection, learning, socialization) can provide, putting it at risk. And too late, the mother' s future reproduction can be compromised leading to lowered population size and even extinction.

"Therefore, there will be a clear selective advantage to weaning time, both for the youngster and for its group. This is the most serious flaw in this piece: There is no mention of the selective advantage of weaning around one year. The reason, I suspect, is there is not one."

Kennedy explained all apes wean late, from a few years in chimps and gorillas to as long as eight or nine years in orangutans.

"We wean earlier, on average, 2.5 years in natural fertility groups, because our rapidly growing brain requires a lot of nutrients that are not sufficient in mother's milk. So, comparing early weaning in Australopithecus africanus to Homo sapiens is wrong because africanus still had an ape- sized brain."

She concluded: "Until they explain why africanus weaned so early—earlier than any ape—their data will have little or no useful application."

This article has been updated with comment from Professor Gail Kennedy.

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