Ancestral gals roamed, guys stayed home

Analyses of 2-million-year old hominid teeth reveal sex differences in lifestyle

Way back in the day, females came from far away and males didn’t stray — not far, anyway.

FEMALE RANGERS New dental evidence suggests that a 1.8-million-year-old hominid species called Paranthropus robustus, represented here by a skull of indeterminate sex, consisted of females that left their birth groups at sexual maturity while males stayed put. Darryl de Ruiter

That’s the implication, with apologies to Dr. Seuss, of a new study of members of two ancient species in the human evolutionary family. Adult females in both hominid lineages often moved from the places where they were born to distant locations, presumably to find mates among unrelated males, say anthropologist Sandi Copeland of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and her colleagues.

Most males in both hominid species spent their entire lives in a home region that covered no more than about 28 square kilometers, or about half the area of Manhattan, Copeland’s team proposes in the June 2 Nature.

These ancient “home boys” might have occasionally gone further afield, exploiting resources along wooded areas atop bands of bedrock that extend about 30 kilometers in opposite directions from the South African cave sites where the fossils were found.

It’s not clear how far females traveled to reach new groups, only that they did not grow up where they died.

“We have the first direct glimpse of early hominids’ geographic movements,” Copeland says. “Ranging differences between males and females were surprising.”

Her investigation measured a chemical marker of childhood diet in teeth from 19 hominids found in two caves about 1 kilometer apart. Specimens represented 11 Paranthropus robustus individuals that lived 1.8 million years ago and eight members of Australopithecus africanus dated at 2.2 million years old.

Some researchers regard A. africanus as a direct ancestor of the Homo genus, which includes living people (SN: 5/7/11, p. 16). P. robustus belonged to a dead-end branch of hominid evolution.

Copeland’s group measured levels of two forms of the element strontium in hominids’ tooth enamel and in plants and animals now living within 50 kilometers of the fossil sites. Strontium is a naturally occurring element in rocks and soils. Specific strontium signatures characterize different landscapes. Strontium signatures in A. africanus and P. robustus teeth, defined by their diet, developed by age nine, the scientists estimate.

Collective strontium data for both hominid species indicate that eight of nine large-toothed individuals — presumably males — grew up in the area where they died, whereas at least five of 10 small-toothed individuals — thought to be females — grew up elsewhere.

Since chimpanzee and gorilla females leave their birth groups upon reaching reproductive age, some researchers have long argued that early hominids did the same. Copeland’s team “came up with an innovative way to test this model, and in the process, developed the first direct evidence of early hominid social organization,” remarks anthropologist Peter Ungar of the University of Arkansas in Fayetteville.

A. africanus and P. robustus probably consumed all sorts of savanna delicacies, including fruits, nuts, seeds and grasses, although questions remain about ancient hominids’ favored foods (SN: 6/4/11, p. 8). If male hominids foraged limited areas, it’s unclear how they avoided predators such as saber-toothed cats while competing for food with baboons and other animals, comments anthropologist Margaret Schoeninger of the University of California, San Diego.

Bruce Bower has written about the behavioral sciences for Science News since 1984. He writes about psychology, anthropology, archaeology and mental health issues.

More Stories from Science News on Humans