'Little Foot' and Lucy: Two Human Ancestors May Have Co-existed

Scientists have finally agreed upon the age of an ancient South African pre-human fossil named "Little Foot," dating at 3.67 million years old. Now, they believe that it and the world-famous Lucy skeleton were two human ancestors that may have co-existed on Earth, helping to shed light on our evolutionary timeline.

Little Foot is a rare, nearly complete skeleton first discovered 21 years ago in a cave at Sterkfontein, in central South Africa. It is likely a member of the species Australopithecus prometheus, while Lucy, discovered in Ethiopia, is the best-known example of Australopithecus afarensis. Each species featured both ape-like and human structures, although the two varieties of our ancestors differed significantly from one another. It is thought that Australopithecus is an evolutionary ancestor to humans that lived between 2 million and 4 million years ago.

But nailing down Little Foot's birthday is something scientists have struggled with for years. So a team of researchers led by Purdue University studied 11 samples from the Little Foot fossil to accurately determine the age of the artifact. Instead of studying the bones directly, investigators examined the ratios of aluminum and beryllium isotopes in quartz crystals surrounding the specimen - a novel technique called isochron burial dating. Isotopes - chemical elements with differing numbers of neutrons - are formed by interactions with cosmic rays, allowing the atoms to be used for dating.

"If we had only one sample and that rock happened to have been buried, then re-exposed and buried again, the date would be off because the amount of radioisotopes would have increased during its second exposure," Darryl Granger, a professor of earth, atmospheric and planetary sciences at Purdue, who helped lead the research, explained in a news release. "With this method we can tell if that has happened or if the sample has remained undisturbed since burial with the fossil."

Researchers believe the individual who formed the Little Foot fossil fell down a narrow shaft, leading to her death. Four tiny foot bones of Little Foot - for which the specimen was named - were discovered in 1994, followed by the discovery of her shin bone three years later. For years, anthropologists and biologists argued about which species Little Foot belonged to, as well when she lived.

"The original date we published was considered to be too old, and it wasn't well received," Granger said. "However, dating the Little Foot fossil as 3.67 million years old actually falls within the margin of error we had for our original work. It turns out it was a good idea after all."

The new findings suggest Little Foot walked the Earth at around the same time as the famed Lucy, who dates back 3.2 million years ago. Though, both pre-human species differed in their physical features. For example, Little Foot (A. prometheus) probably had a rounder skull, larger brain and smaller teeth than Lucy and other members of A. afarensis.

Nonetheless, the discovery suggests significant diversity of humanity's ancestors across of Africa.

"It demonstrates that the later hominids, for example, Australopithecus africanus and Paranthropus did not all have to have derived from Australopithecus afarensis," argued paleoanthropologist Ronald J. Clarke, Little Foot's original discoverer. "We have only a small number of sites and we tend to base our evolutionary scenarios on the few fossils we have from those sites. This new date is a reminder that there could well have been many species of Australopithecus extending over a much wider area of Africa."

The findings are described in further detail in the journal Nature.

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