A reconstruction of the early hominid Paranthropus.
Credit: Walter Voigt
SYDNEY: Fossilised teeth have for the first time shown how the diets of early hominids varied in individuals, and contradicted theories about their extinction.
"We've never before been able to see dietary change within a single individual's lifetime," said Matt Sponheimer, who led the team from the University of Colorado in Boulder. "It's like having a motion picture running over months and years instead of just having one still image."
In the study, which is published today in the U.S. journal Science, the researchers used a technique known as laser ablation to analyse the carbon isotope composition of 1.8 million year old fossil teeth from the early hominid Paranthropus robustus.
By examining the carbon profile of different layers of enamel from a single tooth, they could determine what sort of diet Paranthropus had and whether it changed over time.
"By analysing tooth enamel, we found that they ate lots of different things, and what they ate changed during the year," said co-author Ben Passey.
The findings came as a surprise, since Paranthropus had previously been thought to have a fairly unvaried diet, especially when compared with the emerging hominids from the genus Homo - the line we are descended from.
Their finicky eating had been cited as one of the reasons Paranthropus eventually went extinct in the face of a drying African continent. Meanwhile, the varied tastes of Homo meant it was better equipped to handle changing environmental conditions.
"Since we have now shown Paranthropus was flexible in its eating habits over both short and long intervals, we probably need to look to other biological, cultural or social differences to explain its ultimate fate," said lead author Matt Sponheimer.
Laser ablation has previously been used to shed light on the eating habits of prehistoric horses, rhinos and elephants, but the smaller teeth of humans had posed a much stiffer challenge - until now.
As Passey explained, "What I did was fine-tune the method to handle very small samples like human-sized teeth. If you tried the previous method on a human tooth, you would blast a hole clear through the enamel, and museum curators wouldn't like that."
The reason researchers can infer so much about the diet of long-extinct creatures is because of the way different plants metabolise carbon.
In tropical environments, virtually all trees and bushes, including forest fruits and leaves, use the C3 photosynthetic pathway, whereas savanna-based grasses and some sedges use the C4 photosynthetic pathway. Animals eating C3 plants end up with a different carbon isotope profile in their body - including their teeth - to those eating C4 plants.
When Sponheimer and colleagues looked at the teeth from Paranthropus, they found the signature patterns of both types of plants. What's more, the patterns changed between different layers of enamel, which are added to the tooth over a period of a week or so.
"One possibility is that they were migrating seasonally between more forested habitats to more open, savanna habitats," said Passey.
Since animals that eat C4 plants also produce the C4 'signal' in the teeth of their predators, it is possible that Paranthropus ate these animals, rather than the plants directly. "We cannot tell if they were carnivores or scavengers, but it is possible their diet included animals," said co-author There Cerling.
