Ancient African extinctions

A long-held view that many of Africa’s large-bodied mammals – including relatives of today’s elephants, hippos and giraffes – went extinct at the hands of our tool-wielding forebears has been challenged.

A changing environment is the more likely culprit, according to a new analysis published in the journal Science.

As Homo sapiens fanned out across the world, a wave of megafauna extinctions in Europe, the Americas and Australia followed close behind. Wooly mammoths, massive armadillos and cave bears, as well as giant kangaroos and wombats have all met their end in the last 50,000 years.

But Africa is an anomaly. The vast majority of its megafauna – animals weighing more than 1000 kilos – died out before H. sapiens came on the scene.

One explanation is that earlier relatives of modern humans drove the massive mammals to extinction. With its huge appetite for meat and its ability to fashion stone hand-axes, the finger is often pointed at Homo erectus, a species that arose around 1.9 million years ago.

But, says palaeoecologist Tyler Faith from the University of Utah, “nobody really tested those ideas in any great detail, or explored any alternatives.”

To get a handle on how much of an impact ancient hominins had on the extinction of megaherbivores, Faith and his colleagues took a long view. They gathered data from over 100 fossil assemblages unearthed in modern-day Kenya, Tanzania and Ethiopia spanning the past seven million years.

Eastern Africa is especially rich in prehistoric animal bones, including those from some of the earliest known hominins, 3.2 million year old “Lucy” (Australopithecus afarensis) among them.

Most eastern African fossils are packed into well-dated layers of rock. This enabled Faith and his team to build a timeline of when different species went extinct.

Twenty-eight lineages of megaherbivores have met their demise over the past seven million years. This includes several species of elephant, hippos, rhinos, giraffes and a camel.

The picture that emerged was one of a steady decline in diversity that really got going around 4.6 million years ago, “two and a half million years before Homo erectus ever shows up,” says Faith.

There was also no acceleration in the rate of extinctions when H. erectus emerged. “If hominins are playing some role in this diversity loss, we’d see an acceleration of diversity loss,” notes Faith. “And we just don’t.”

Looking at global climate records, changes on soil chemistry, and trace elements in animal teeth, the team could overlay information about climate and vegetation changes onto this sequence of extinctions.

When they did this, another culprit emerged.

A global drop in the levels of atmospheric carbon dioxide led to a shift in vegetation where tropical grasses – much like those in modern-day African savannahs – flourished at the expense of trees.

Many of the megaherbivores that went extinct were species that relied on woody vegetation. As grasslands expanded, these herbivores lost their source of food. This likely had a knock-on effect for carnivores, such as sabre-toothed cats, that lost their prey.

“It’s really a long-term ecological process that’s really difficult to pin on increased carnivory by the tool-using, meat-eating hominids,” says Faith.

“It’s pretty convincing,” says ecologist Chris Johnson from the University of Tasmania, who studies megafauna extinctions. But, he adds, looking at megaherbivores is just one part of the puzzle.

“If we really want to come to grips with the impacts of early hominins, we probably need to start by looking at their impacts on carnivore assemblages,” he says.

Our ancestors might be off the hook, but anatomically modern humans should remain in the cross-hairs.

“In the search for ancient hominin impacts on ancient African ecosystems, we must focus our attention on the one species known to be capable of causing them – us, Homo sapiens, over the last 300,000 years,” he says.