3.67 million-year-old fossil shows how early human ancestors used their arms

A nearly complete fossil skeleton 3.67 million years old provides new insight into how the hominin ancestors of man used their arms.

The Australopithecus fossil is called "Little Foot" because the first of its bones, identified in the 1990s, were those of a small foot. The skeleton was discovered in the Sterkfontein Caves site in South Africa. 

The fossil is whole and well preserved because it fell into the cave and became mummified in the cave's dry conditions. The near-complete 4-foot skeleton is being analyzed by teams of scientists worldwide, each examining a different part of the skeleton.

A team at the University of Southern California's Keck School of Medicine has focused on the shoulder area, called the pectoral girdle, which includes the collarbones, shoulder blades and joints. Their research is featured in the April 20 issue of The Journal of Human Evolution, along with articles from other "Little Foot" research teams.

"Little Foot" is not as well-known as the younger Australopithecus skeleton, Lucy, discovered in East Africa in the 1970s, but it is the oldest and most anatomically complete Australopithecus skeleton found. It provides important clues about how ancestors of humans moved, and how humans developed.

A Rosetta Stone for early man

Current Science Daily interviewed the lead author of the article on the pectoral girdle, Kristian Carlson, associate professor of clinical integrative anatomical sciences at the Keck School of Medicine of USC in Los Angeles.

Carlson said he called  “Little Foot” the Rosetta stone for early human ancestors, "Because of the completeness of the skeleton, its condition, and the period of time it represents (3.67 million years ago.)" 

He added, "'Little Foot' is like a corner piece of a jigsaw puzzle. It is the rare discovery that makes a lot of other pieces fit into place, even if it alone cannot reveal the entire complicated picture of our evolutionary history."

The importance of the pectoral girdle

Analysis of the pelvis and lower limbs of "Little Foot" indicated that it walked upright on two feet, like humans, but the research team wanted to know how its upper limbs were primarily used. Did it use trees, like living chimpanzees and gorillas?

"The fundamental question for us is whether the upper limb of 'Little Foot' provided substantial weight-bearing support during a range of arboreal activities," Carlson said. "The pectoral girdle of 'Little Foot' provides a deeper understanding of how human ancestors at this time may have been using their upper limb to exploit trees for their survival, for example, for acquiring food resources, or for protection from terrestrial predators."

The researchers compared the dimensions of the pectoral girdle of "Little Foot" to those of humans and apes, to learn whether it was "more chimpanzee-like, gorilla-like, orangutan-like, or human-like."

"We know from studies of muscle activity in chimpanzees, for example, when certain muscles are recruited during activities such as climbing," Carlson said. "We see signs of structural reinforcement or orientation of muscle attachments in the apes that would be beneficial during climbing types of behaviors, or other behaviors when the hand is placed above the head and the upper limb bears weight.

"What 'Little Foot' shows us, based on some similarities to apes in these reinforcements and orientations, is that there was still active selection in human ancestors 3.67 million years ago for exploiting arboreal resources to survive. In this sense, the upper limb of 'Little Foot' reflects some balance of selection pressures on skillfully utilizing and  moving through both arboreal and terrestrial habitats, and quite possibly so do other anatomies of 'Little Foot.'" 

Carlson stressed, "There is no earlier evidence of a complete pectoral girdle closer to the divergence of humans and African apes than that provided by 'Little Foot.' While some older fossils, like the famous Ardipithecus, offer glimpses of a critical dependency on arboreal resources based on other anatomies, there is no earlier evidence on the shoulder than that of 'Little Foot.'"

The evolutionary timeline

Carlson said that based on its skull and dental anatomy, "Little Foot" was thought to be in the taxon (group) Australopithecus prometheus on the evolutionary tree.

"This makes it older than all members of the genus Australopithecus, the same genus to which ‘Lucy’ is attributed, except the slightly older species, Australopithecus anamensis (3.8-4.2 million years ago), known from fossils in Kenya and Ethiopia," he said.

"'Little Foot' belongs to a species that is among the older representatives in the genus that gave rise to our genus, Homo. It is the most complete skeleton of a single individual among early human ancestors, that is, those pre-dating genus Homo." 

He added, "Slightly younger human ancestors, for example a partial skeleton from the Woranso-Mille area of Ethiopia, offer evidence of more human-like features in a less complete pectoral girdle." 

In the future, Carlson said, researchers look forward to "expanding comparisons of 'Little Foot' with contemporary human ancestors and even younger ones. After all, as special as 'Little Foot' is, it is only one individual."

"We must continue to appreciate the growing morphological variability in the shoulder region that appears to exist within the fossil record of early human ancestors, for example in Australopithecus," he said. "This variability is important to understand, for we do not yet know the extent to which it reflects variation within one group, or perhaps different adaptations in separate groups?"

Advanced scanning technology crucial

The researchers were able to make use of advanced scanning technology to examine the bones in great detail. 

"Simply put, without the use of microCT scanning and computer software to assist with our digital reconstruction of this extraordinarily delicate bone [fossil], we would not have had a precise reconstruction of the `Little Foot' pectoral girdle," Carlson said.

"We would have been able to estimate aspects of the bony morphology--for example, the form of the shoulder joint in particular--to some extent, but we could not have attained the same level of confidence in our results. Accordingly, we would have had less confidence in our understanding of how the shoulder blade of  `Little Foot' was capable of moving on its torso, and thus how its upper limb could have been used while interacting with the environment in which `Little Foot' lived."

As to the future, Carlson said, "Hopefully, we’ll have some new discoveries that offer more direct comparisons of the shoulder of these earlier human ancestors with that of 'Little Foot.'"