A human ancestor thought to be 3.7 million years old, should be younger than 2.8 million years, says the newest research from South Africa and Australia, published on 29 March.
The age of “Little Foot”, an extremely rare, almost complete skeleton of the human ancestor referred to as Australopithecus prometheus, has been hotly debated ever since its discovery at the Sterkfontein Caves, in the Cradle of Humankind world heritage area, South Africa. Estimates have ranged from 2 to 4.2 million years, and the fossil is often touted as the oldest Hominid in southern Africa.
Little Foot was named for the four ankle bones which were found in 1980. The remainder of the skeleton was uncovered from 1997 onwards. In the years since, researchers from many countries have tried to determine Little Foot’s age and its place in the species classification of hominids, the early ancestors of humans who lived millions of years ago.
3.7 million years result
In 2015, Prof D Granger of Purdue University, USA and co-authors published an age of 3.7 million years for Little Foot in the authoritative science journal Nature. They used a pioneering technique to estimate the age of quartz grains falling in from the surface into the cave sediments where Little Foot was found.
Granger and co-workers determined the age of Little Foot with a burial age method based on the cosmogenic nuclides aluminium-26 and beryllium-10. These are produced in quartz at surface by the action of cosmic rays. Once the quartz grains fall into a cave production of the cosmogenic nuclides stops. Both nuclides are radioactive and gradually decay after burial, but their half-lives are different and therefore their abundance ratio changes with time. This allows the time of burial to be determined.
Little Foot is embedded in sandy and blocky sediment called breccia. Granger et al. determined how long the quartz in that breccia had been underground and assumed that the age of the fossils must be the same as the age of the quartz.
Prof Granger’s result was soon accepted as the definitive age of Little Foot.
Human ancestry implications of 3.7 million years result
However, this result if correct, would make Little Foot older than Australopithecus afarensis hominids from east Africa, which include the 3.2 million year old fossils of “Lucy”.
Lucy is generally considered to be our ancestor. If 3.7 million years is correct, Little Foot would be an alternative candidate species from which we could have evolved, with implications for the exact location of our birth place in Africa. In addition, the fossils of Little Foot would then be at least a million years older than other dated hominin fossils in the Cradle of Humankind, contradicting earlier age estimates.
2.8 million years result
Using the same data as Granger, in this latest research Kramers and Dirks have now shown with new calculations and interpretations that Little Foot cannot be older than 2.8 million years. In their analysis, they focused on quartz fragments that accumulated directly around the fossils, but falling in from a now-eroded cave chamber higher up, not from the surface. The research was published in the South African Journal of Science in March 2017.
Kramers and Dirks considered differences in cosmogenic nuclide concentrations between these fragments to work out when the grains came together inside the cave to form the sedimentary deposits surrounding Little Foot. The maximum age that resulted from this exercise was 2.8 million years, indicating that the fossil cannot be older than this.
To explain the apparent contradiction in age, Kramers and Dirks point out that results are only contradictory if the fossil and the quartz grains had fallen into the cave from surface together. But there are many examples in caves, including Sterkfontein cave, where material originating from high in the cave is eroded and redeposited in chambers deeper down.
Kramers and Dirks suggest that a cave chamber with sediment existed 2.8 million years ago above the cave chamber in which Little Foot was found and that there was an opening between the two chambers. If Little Foot had wandered into the upper cave, and in the dark had fallen into the lower chamber through a death-trap, the apparent age contradiction can be solved if the fossil was covered by sediment eroded from the upper cave (as opposed to sediment derived from surface).
Kramers and Dirks provide supporting evidence for this scenario by pointing out a large, tilted block of dolomite that lies at surface above the chamber where Little Foot was found, and represents the roof of a collapsed cave chamber.
Human ancestry implications
If Kramers and Dirk’s are correct, this means that the generally accepted sequence of hominids remain unchanged.
Birthplace of humans – South Africa or East Africa?
In 2015, National Geographic magazine published an article giving background on the battle between South Africa and East Africa as the birth place of humanity, discussing Granger’s results.
Other hominid research – Homo naledi
Both Kramers and Dirks are contributing to research about another hominid found in the Cradle of Humankind, Homo naledi, which was announced in 2015.
Prof Kramers’ research spans the ages of hominids through to analysing diamonds and comet fragments.