The ecosystems of the Omo Valley in southern Ethiopia changed about 2.8 million years ago, causing a shift in the diet of herbivores towards C4 grasses. Such grasses exhibit a more efficient photosynthetic pathway than their more common C3 cousins.

Faysal Bibi of the Natural History Museum in Berlin and his colleagues found increased amounts of the isotope carbon-13 in the tooth enamel of two herbivores, the antelope Tragelaphus nakuae and the pig-like Kolpochoerus limnetes, after the 2.8-million-year mark. This points to a diet high in C4 plants, which accumulate more carbon-13 than do C3 plants.

The findings tally with evidence of a major environmental shift in Africa at the time, the authors say. However, they add that it is not yet clear whether the incoming C4 grasses replaced woodlands or C3 grasses.

Biol. Lett. http://dx.doi.org/10.1098/rsbl.2012.0890 (2012)