(CNN) — Humans are picky eaters, and not just because we're the only species that reviews restaurants. A new study suggests that our ancestors' diets may have been different from our close primate relatives much earlier than we thought.
The human ancestor in question is called Australopithecus bahrelghazali. Remains of it were found in Chad at the Koro Toro fossil site. Researchers looked at fossils are more than 3 million years old.
Researchers examined the ratios of carbon isotopes present in the teeth of this early hominin, a word paleontologists use to talk about human ancestors. They reported their results in the journal Proceedings of the National Academy of Sciences.
Based on the carbon isotope ratios, it appears that Australopithecus bahrelghazali was focusing its noshing on a special category of plants that includes tropical grasses and sedges. These grassland plants have a distinct carbon signature in an animal's teeth.
Apes, on the other hand, such as chimpanzees, do not eat that stuff, said Mark Teaford, co-editor of the Journal of Human Evolution and professor at High Point University, who was not involved in the study.
Although primates have a wide range of plant foods in their diet, their meals broadly fall into the category of leaves, fruits and nuts -- in other words, more "woody" plants.
Considering the similar anatomy of early humans to other primates, Teaford says it doesn't necessarily follow that they would be eating a lot of grasses and sedges.
"To be able to look at the isotope work and say, this thing is eating a diet fairly similar to some more recent ones, it's surprising," Teaford said.
Australopithecus bahrelghazali is the earliest example of a human ancestor who may have been eating grasses and sedges, and it's more than 1.5 million years older than a previously-identified example of a hominin with this diet.
"Here you have this early human ancestor and you say, 'Wow, what's allowing it to do this?' " Teaford said. "Maybe we're naïve in terms of our perspective on what's required to process these foods efficiently."
Teaford, independent of this new study, does work on the microscopic wear on the teeth of ancient human ancestors. There's evidence in that line of research also that the diet of our early humans was not quite like that of other primates such as chimpanzees and baboons.
It's possible that these results reflect, instead, that human ancestors were eating animals that were eating grasses and sedges. However, Teaford said that based on the strength of the carbon isotope signal from the fossils, it is probably not based on this indirect consumption of the plants. Also, different kinds of teeth probably would have been required to cut up tough meat.
More research should be done to firm up these theories, however.
This study is only based on two individuals, and the researchers could only investigate what these early humans ate during the time period when their teeth were forming -- i.e. when they were young, notes Peter Ungar, anthropologist at the University of Arkansas who co-authored a book on the human diet with Teaford.
But the results are interesting, and shows that our puzzle of the food choices of early hominids gets more complicated as we add pieces to it, he said.
"I think it's a wake up call to say, how well do we know what our ancestors were eating and how do we know what's required to be able to eat these foods?" Teaford said.