Science

Little Foot skeleton reveals a brain much like a chimps

An ancient hominid skeleton dubbed Little Foot possessed a brain largely similar to that of modern chimpanzees and an inner ear with a mix of apelike and humanlike features, two studies suggest. These findings, along with other analyses of the adult females 3.67-million-year-old skeleton, point to the piecemeal evolution of humanlike traits in close relatives of our species, scientists say.

The research is part of the first formal analyses of Little Foots skeleton, which was discovered more than 20 years ago in a South African cave but was recently removed from its rocky encasing. Other analyses of trunk and limb bones indicate that Little Foot, who lived perhaps a million years before the emergence of the human genus, Homo, already walked upright about as well as people today do (SN: 1/19/19, p.13).

Although Little Foot consists of a nearly complete skeleton, her evolutionary identity is controversial. Paleoanthropologist Ronald Clarke of the University of the Witwatersrand in Johannesburg — Little Foots discoverer and a coauthor of the two new studies — assigns the find to Australopithecus prometheus, an early extinct hominid species that many scientists dont regard as valid. Other researchers regard Little Foot as an early member of Australopithecus africanus, a species previously known from fossils discovered at several South African sites (SN: 1/19/19, p. 13).

In one of the new studies, Witwatersrand paleoanthropologist Amélie Beaudet and her colleagues compared a 3-D digital reconstruction, or endocast, of Little Foots brain surface with digital endocasts of 10 other South African hominid specimens dating to between roughly 1.5 million and 3 million years ago.

Little Foot had a small brain, only about one-third of the volume of a modern adult womans brain, the analyses showed. And overall, Little Foot had a more chimplike brain than any other southern African hominid, including specimens from species such as A. africanus and Paranthropus robustus, Beaudets team reports in the January Journal of Human Evolution. Thats not surprising, the investigators add, since Little Foot is also the oldest known southern African hominid.

Chimplike positioning of a tissue groove toward the back of the brain indicates that Little Foot had a proportionately larger neural area devoted to vision than people do, the researchers conclude. In human brains, a relatively smaller visual area makes room for an expanded neural section involved in integrating sensory and spatial information.

Still, a complex, humanlike set of blood vessels clustered on part of Little Foots brain surface could have fueled brain expansion observed in later hominids, the researchers say. “Even if Little Foots brain was different from ours, the vascular system that allows for blood flow and may control brain temperature was possibly already present,” Beaudet says.

Whats more, the left side of Little Foots brain protruded out slightly more in the back than the right side, the researchers say. Present-day humans and chimps possess such asymmetrical brains, as did hominids that evolutionarily followed Little Foot. Signs of brain asymmetry roughly 3.7 million years ago strengthen the possibility that this trait characterized the last common ancestor of hominids and modern great apes, which may have lived 7 million years ago or more, the investigators say.

But scientists disagree on how to interpret these endocasts. For example, paleoanthropologist Dean Falk of Florida State University in Tallahassee contends that 2-million- to 3-million-year-old hominid endocasts from eastern and southern Africa preserve grooves on the brains surface — created by adjacent tissue folds — positioned like those of present-day chimps. Other researchers argue that those particular furrows of brain tissue appear in humanlike positions on the same endocasts (SN Online: 3/26/18).

Falk sees no substantial differences between Little Foots endocast and those of A. africanus and P. robustus, which belonged to a large-jawed, small-brained hominid lineage that died out around a million years ago. And, she says, neural landmarks and blood vessels preserved on the inner surface of Little Foots skull wont illuminate her particular species. “Endocast studies havent gotten that refined.”

Analyses of Little Foots inner ear seem to show more of a mix of chimplike and humanlike features, a second study finds. A computerized reconstruction of the ancient South African hominids inner ear bones reveals chimplike dimensions of a set of hoop-shaped structures, called the semicircular canals, that help control balance and motion, Beaudets team reports in the February Journal of Human Evolution. That configuration is consistent with Little Foot having engaged in a mix of two-legged walking and careful movement along tree branches, the researchers contend.

But a spiral-shaped inner ear cavity called the cochlea, which translates sound vibrations into nerve signals sent to the brain, looks more humanlike than chimplike in Little Foot, the researchers say. Inner ear similarities of Little Foot to other Australopithecus specimens leave unclear whether those hominids could hear the same range of sounds as people do today.

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