Only five species of massive herbivores are left and some say humans killed off the others:
Writing in the journal Science, Tyler Faith, from the Natural History Museum of Utah, and colleagues argue that long-term environmental change drove the extinctions.
This mainly took the form of an expansion of grasslands, in response to falling atmospheric carbon dioxide (CO ) levels.
“Despite decades of literature asserting that early hominins (human relatives) impacted ancient African faunas, there have been few attempts to actually test this scenario or to explore alternatives,” said Dr Faith. “Humans ‘off the hook’ for African mammal extinction” at BBC
Now that Dr. Faith mentions it, the claim is more of an accusation than a hypothesis, and it is one no living person can easily refute. Except to make the obvious point that humans were not very numerous or powerful back then.
According to the numbers, 28 species of large herbivores went extinct; the decline began about 4.6 million years ago, and evidence for humans is found from about 3 million years ago but there was no change in the pattern as a result.
Of course, if evidence is found for humans earlier than 3 million years ago, it might change the picture.
Abstract: Megaherbivore extinctions in Africa
Human ancestors have been proposed as drivers of extinctions of Africa’s diverse large mammal communities. Faith et al. challenge this view with an analysis of eastern African herbivore communities spanning the past ∼7 million years (see the Perspective by Bobe and Carvalho). Megaherbivores (for example, elephants, rhinos, and hippos) began to decline about 4.6 million years ago, preceding evidence for hominin consumption of animal tissues by more than 1 million years. Instead, megaherbivore decline may have been triggered by declining atmospheric carbon dioxide and expansion of grasslands. (paywall) More.
From Eurekalert,
Faith and his team quantified long-term changes in eastern African megaherbivores using a dataset of more than 100 fossil assemblages spanning the last seven million years. The team also examined independent records of climatic and environmental trends and their effects, specifically global atmospheric CO2, stable carbon isotope records of vegetation structure, and stable carbon isotopes of eastern African fossil herbivore teeth, among others.
Their analysis reveals that over the last seven million years substantial megaherbivore extinctions occurred: 28 lineages became extinct, leading to the present-day communities lacking in large animals. These results highlight the great diversity of ancient megaherbivore communities, with many having far more megaherbivore species than exist today across Africa as a whole.
Further analysis showed that the onset of the megaherbivore decline began roughly 4.6 million years ago, and that the rate of diversity decline did not change following the appearance of Homo erectus, a human ancestor often blamed for the extinctions. Rather, Faith’s team argues that climate is more likely culprit.
“The key factor in the Plio-Pleistocene megaherbivore decline seems to be the expansion of grasslands, which is likely related to a global drop in atmospheric CO2 over the last five million years,” says John Rowan, a postdoctoral scientist from University of Massachusetts Amherst. “Low CO2 levels favor tropical grasses over trees, and as a consequence savannas became less woody and more open through time. We know that many of the extinct megaherbivores fed on woody vegetation, so they seem to disappear alongside their food source.”
The loss of massive herbivores may also account for other extinctions that have also been attributed to ancient hominins. Some scientist suggest that competition with increasingly carnivorous species of Homo led to the demise of numerous carnivores over the last few million years. Faith and his team suggest an alternative.
“We know there are also major extinctions among African carnivores at this time and that some of them, like saber-tooth cats, may have specialized on very large prey, perhaps juvenile elephants” says Paul Koch. “It could be that some of these carnivores disappeared with their megaherbivore prey.”
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See also: Mystery: Extinct birds as well adapted for flight as surviving modern ones
and
Assumed extinct, tree kangaroo reappears