Will the “most complete skeleton ever” transform human origins?
The public unveiling of a remarkable discovery highlights new potential in a changing science
Last week, paleoanthropologist Ron Clarke and the University of the Witwatersrand revealed a momentous discovery that has been more than 37 years in the making. Under excavation for the last 20 years, the “Little Foot” fossil may provide the most important evidence for human origins ever found earlier than 2 million years ago. As Clarke announced, “She is the most complete Australopithecus skeleton ever discovered from anywhere”.
The announcement did not provide new scientific data, which Clarke says will be published next year. Still, it appears likely that this discovery will be much more informative than any more fragmentary fossil, including the famous “Lucy”.
As an anthropologist, I’ve thought a lot about this discovery and how it may impact the study of human origins. The completeness of a fossil skeleton matters. This one is going to affect the way we look at many branches of our family tree.
The Little Foot discovery was a story of serendipity and detective work. In 1980, many ancient mammal fossils were extracted from a pile of fossil-bearing rock at Sterkfontein, a cave northwest of Johannesburg, South Africa. Sterkfontein has been a key fossil site ever since 1936, when the paleontologist Robert Broom first identified remains of an ancient human relative, or hominin, now recognized as Australopithecus africanus. But the Dump 20 fossils came from an area of the site deep underground, in which no hominin fossils had ever been identified. In 1994, Ron Clarke saw that four of the bones extracted from Dump 20 belonged to the foot and ankle of an ancient hominin. Together with the late Phillip Tobias, he published a description of this fragmentary foot skeleton, which Tobias named “Little Foot”.
After describing these pieces, Clarke felt that the cave might hold more of the skeleton somewhere. The foot and ankle included a broken piece of tibia, or shin bone. Two of his assistants, Stephen Motsumi and Nkwane Molefe, took a replica of this broken tibia into the cave, to see if they could find a place where a matching piece was exposed on the surface of the fossil-bearing rock. After two days they found a matching cross-section of broken tibia deep in the Silberberg Grotto, a part of the cave where few had looked for fossils.
It took time for Clarke to confirm that a large fraction of the Little Foot skeleton was there in the rock. A special gate was constructed within the cave to lock the chamber away as he worked with hammer and chisel at the hard rock surrounding the fossil bones. Last week, he described the work as “like digging a cake with puff pastry”.
From the beginning, Clarke and Tobias thought that Little Foot must be very old. The bones came from much deeper in the site than the geological layers that contained most of the hominin fossils, suggesting that these new bones might be older than 3 million years ago — maybe as old as the famous “Lucy” skeleton from Ethiopia, 3.2 million years. Later, by looking at the way that cosmic radiation had affected small grains of sand that fell into the cave around the time the skeleton did, Clarke and a team of geologists would conclude that the Little Foot skeleton was deposited deep in the cave around 3.67 million years ago.
As part of this same work, Clarke and colleagues assigned the Little Foot skeleton to the species Australopithecus prometheus. This was an old name that Raymond Dart once gave to fossils from another South African site, Makapansgat. Clarke has suggested that the species was also present at Sterkfontein.
These findings have been controversial. Some geologists are skeptical about whether the tiny grains actually entered the cave at the same time as Little Foot’s remains. They suggest that the fossil might be less than 2.8 million years old. Others question the concept of “Australopithecus prometheus”, dismissed by most specialists more than 50 years ago.
Over the years, anthropologists have discovered abundant fossil evidence of human evolution (“How much evidence have scientists found for human evolution?”) But most of this evidence is fragmented, overrepresenting teeth, jaws, and skulls. Partial skeletons that include different parts of the body are vanishingly rare, but they provide crucial evidence of how different parts of the body are correlated.
Back in the 1990s, anthropologists observed that the Turkana Boy skeleton of Homo erectus is profoundly different in some ways from the few partial skeletons of Australopithecus, including the “Lucy” skeleton. Focusing on these differences, scientists worked to fit other isolated, fragmentary bones into the same puzzle frame. It seemed possible that many parts of the skeleton all evolved in concert with each other to make Homo different from any hominin that went before.
But skeletons uncovered during the last twenty years have shaken the idea that Homo made such a clear evolutionary break with the past. Two partial skeletons of the new species Australopithecus sediba, together with the new “Kadanuumuu” skeleton attributed to Australopithecus afarensis, showed that some australopiths were like Homo erectus in ways that nobody expected. New skeletal evidence from Dmanisi showed the Turkana Boy skeleton did not tell the whole story about variation in H. erectus. Meanwhile the discoveries of Homo floresiensis and Homo naledi each demonstrated unpredicted ways that some species of our genus look a lot like Australopithecus.
What once seemed like one of the major transitions of evolution is now a series of halting hiccups. But that shouldn’t come as a surprise. All fossil hominin species were close relatives that shared much more than 99 percent of their DNA. As natural selection adapted them to their immediate circumstances, they often developed very similar skeletal features, a process called “parallel evolution”. They also may have hybridized, mixing genes and features directly with each other.
In the 1990s, the origin of Homo was a missing persons problem. Anthropologists thought they knew how our genus evolved, and tried searching for fossils at “the right time and right place” to be its ancestor.
Twenty years of new discoveries from the wrong times and wrong places have made the science a lot more interesting. Figuring out how all these new skeletons fit in the picture will give us a shot at learning how our genus took the first steps toward humanity.
So the Little Foot fossil makes its public debut at a crucial moment.
By all accounts, the Little Foot skeleton is not within our genus, Homo, nor is it confounded at the base of our genus, as Australopithecus sediba may be. Whatever its identity, whether it is Australopithecus africanus or “Australopithecus prometheus”, the Little Foot skeleton has an essential role for testing hypotheses of how species are related within Homo. For such hypotheses, Litttle Foot is what biologists call an “outgroup”, the most complete and closest one to our genus ever discovered.
Will it confirm old ideas, or overturn them? Obviously this one skeleton won’t answer every question. But now, no study will be sufficient without it.
The identity of Little Foot will likely prove to be a harder problem. No matter what data emerge from the skeleton itself, scientists will likely continue to argue about whether to revive “Australopithecus prometheus” or whether instead to recognize Little Foot as part of the better-known Australopithecus africanus. Scientists may also continue to dispute the skeleton’s age. Answering the questions about this one particular skeleton will likely require even more new discoveries.
There’s no doubt it will be fascinating to see the story continue to unfold.