The scientists examined the teeth of Paranthropus boisei, an ancient hominin that lived between 2.3 million and 1.2 million years ago.

Since the first specimen was reported by Mary and Louis Leakey in 1959, scientists have believed P. boisei fed on nuts and seeds or roots and tubers because the teeth, cranium and mandible appear to be built for chewing hard objects.

Now University of Arkansas Professor Peter Ungar and colleagues have used microscopy and fractal analysis to determine that what it actually ate doesn't correspond with the size and shape of its teeth. The finding, said Ungar, suggests evolutionary adaptation for eating might have been based on scarcity, rather than on an animal's regular diet.

"These findings totally run counter to what people have been saying for the last half a century," said Ungar, "We have to sit back and re-evaluate what we once thought."

Ungar, Frederick Grine of Cambridge University and Mark Teaford of Johns Hopkins University report their findings in the online journal PLoS One.