AFRICA’S great grasslands are one of that continent’s most famous features. They are also reckoned by many to have been crucial to human evolution. This school of thought holds that people walk upright because their ancestors could thus see farther on an open plain. Forest primates do not need to be bipedal, the argument continues, because the trees limit their vision anyway.

As “Just So” stories go, it is perfectly plausible. But some go further and argue that the transition took place when the savannahs themselves came into existence, replacing the pre-existing forest and forcing human ancestors to adapt or die out. Fossil evidence suggests humanity’s upright stance began to evolve between 6m and 4m years ago. So the question is, did that coincide with the formation of the savannah? A paper in Geology, by Sarah Feakins, of the University of Southern California, suggests not.

Dr Feakins studied sediment cores from the Gulf of Aden, a place where offshore winds deposit detritus from a goodly part of the east of the African continent. In these, she discovered plant molecules that date back between 12m and 1m years. Such molecules contain carbon, and carbon atoms come in various isotopes, whose ratios give away their history. In particular, the ratio of ¹²C to ¹³C can tell you what sort of plant made the molecule in question.

Plants in rainforests tend to discriminate against ¹³C. Those in modern African grasslands are less selective and ¹³C is thus more abundant in their molecules. Dr Feakins was therefore able to ask when these grasslands came about.

To her surprise, they seem to have been there even 12m years ago. Close examination of the cores shows that the nature of the grass changed over the millennia, as species that were adapted to dry conditions took over from those that prefer wetter weather, but savannah of some form there always was.

The climatic change she observed was already known about. It was the reason people suspected forests had given way to savannah. But, contrary to that suspicion, Dr Feakins has shown that early humanity’s east African homeland was never heavily forested, so the idea that people were constrained to walk upright by the disappearance of the forests is wrong.

Perhaps it was more pull than push—a pre-existing, but empty ecological niche crying out to be filled by an enterprising species that could make the transition. But perhaps those who seek an ecological explanation of this sort are, as it were, barking up the wrong tree.