When anthropologists imagine the environments in which humanity's ancestors evolved, they tend to think of either the forests or grasslands that dominate Africa today. However, an overview of Africa's fossil record indicates things were much different 7 million, or even 1 million, years ago than we usually acknowledge. We evolved in environments with more giant animals than we see today.
Dr Tyler Faith of the University of Utah examined the collected fossil records for East African herbivores over the last 7 million years, taking in 305 fossil communities. Until 700,000 years ago, almost all of these were outside the bounds of what we see today, with a much greater frequency of so-called megaherbivores weighing more than a tonne.
Even before cattle farming, the grasslands of Africa were dominated by ruminants such as wildebeests and giraffes. One elephant may eat as much as many antelopes, but doing so along with other large non-ruminant herbivores, such as rhinoceroses, is relatively rare. When it comes to food sources for carnivores, or dung production to fertilize the soil, it is the ruminants that shape the continent.
However, Faith and colleagues show in the Proceedings of the National Academy of Sciences that this wasn't true until around a million years ago, followed by a transition lasting some 300,000 years.
Before that time, there were many more megaherbivore species and fewer creatures weighing 18-80 kilograms (40-180 pounds). Unsurprisingly, this also meant more big carnivores. Today, the remaining megaherbivores need fear only lions and humans, but Africa used to also support giant hyenas, saber-toothed cats, and predominantly carnivorous bears. Although there were also swings in the richness of medium-sized animals, and in whether they preferred grasses or leaves, over the same period, these generally occurred within modern ranges.
It is easy to jump to the conclusion humans are responsible for the disappearance of these fantastic beasts. After all, on other continents and islands, humanity's appearance has been bad news for the largest animals.However, the story is not so simple in Africa.
Ecosystems similar to the ones we see today in East Africa emerged hundreds of thousands of years before modern humans. The species that represented our direct ancestors around 700,000 years ago when the change took place may have had most of our intelligence, but their tools were far less advanced and combined with their small numbers probably had little to do with the megaherbivores' decline.
Instead, Faith and co-authors believe increasingly dry climate intervals, combined with C4 grasses displacing C3, shifted the advantage to more drought-tolerant ruminants. This sent ripples through the food web that changed Africa's ecosystems and probably induced some of the behavioral and technological changes that made us what we are today.