Becoming Human

The New York Times, citing a paper in Nature,  reported last week “Researchers studying the diet of human ancestors who lived two million years ago in southern Africa have unexpectedly come across a crucial clue to their social structure. The males never strayed far from home, and the females dispersed after puberty to neighboring groups”

Dr Sandi R. Copeland, Department of Human Evolution, Max Planck Institute for Evolutionary Anthropology, led a team in examining the fossil teeth of 19 individuals representing two species living two million years ago in southern Africa.  Using a laser technique which removes negligibly small amounts of dental enamel, and analyzing trace amounts of isotopic Strontium, they found individuals with small teeth roamed more widely from their birthplace than did individuals with large teeth. The authors state, “Given the relatively high levels of sexual dimorphism in early hominins, the smaller teeth are likely to represent female individuals, thus indicating that females were more likely than males to disperse from their natal groups. This is similar to the dispersal pattern found in chimpanzees, bonobos and many human groups, but dissimilar from that of most gorillas and other primates. The small proportion of demonstrably non-local large hominin individuals could indicate that male australopiths had relatively small home ranges....”

Strontium exists in the rocks underlying soil and is taken up by plants and grass growing on the soil. When we ingest  this plant material, or ingest the meat of animals feeding on these plants and grasses, we take up Strontium as well.  Analyzing the ratios between two Strontium isotopes, ⁸⁷Sr and ⁸⁶Sr, researchers can determine differences from place to place on the landscape so uptake of Strontium over a wide area can be distinguished from uptake over a small area.

Passing small amounts of vaporized tooth enamel through a mass spectrometer enables researchers to determine the range of animals whose teeth have survived millions of years. (Note: a colleague is using this same methodology to see whether Neanderthals followed, and used as a food resource, migrating deer; we plan to make the results of her research available in a news article in  future.)

The Copeland team analyzed the teeth of 19 hominins: eleven Paranthropus robustus individuals from Swartkrans, fating to 1.8 million years ago and eight Australopithecus africanus individuals, 2.2 ma from nearby Sterkfontein.

How did this team conclude females dispersed and males stayed put? Since sexual dimorphism (males larger than females of the same species) existed within both these taxa, the scientists assumed small teeth were female and large teeth were male. The enamel of the smaller teeth showed a wider variety of isotopic Strontium than did the larger teeth.

Fellow scientists reading this report, however, suggest caution in evaluating these findings on two critical points: sample size and the sexing of the teeth(determining the gender of the individuals whose teeth were analyzed). A total of 19 individuals, comprising two separate species, is a small sample. As regards sexing,  Dr William Kimbel, director of the Institute of Human Origins and a professor at Arizona State University says, “My main caution concerns the sexing of the teeth; given the inherently high size variation in relatively non-dimorphic M3s and unimodally distributed canine dimensions in thee samples, their assignment of sex is methodologically risky. “

Attached is a downloadable pdf of the Nature paper. A less technically demanding article from the New York Times can be read by clicking here.