New York, Nov 23 (IANS): Human ancestors played little to no role in driving mammal extinction in ancient African ecosystems, instead it is related to environmental change, say researchers overturning decades of thinking on ancient hominin impacts.

Because human ancestors were present in Africa for nearly seven million years -- much longer than any other region of the world -- it was argued that they likely caused extinctions earlier in Africa than anywhere else.

Researchers from the University of Massachusetts-Amherst showed that the decline of megaherbivores -- largest mammal species over 2,000 lbs in size -- in Africa over the last seven million years occurred independently of any milestone in human evolution to which it might be linked.

In fact, the loss of megaherbivores begins 4.6 million years ago, a time when the only known hominin is Ardipithecus -- a small-brained and ape-like human ancestor that lacked stone tools and at best hunted small prey like modern-day chimpanzees.

"If hominins were responsible for the extinction of megaherbivores, we would expect their decline to be more step-like and to track behavioural or adaptive milestones in human evolution," said postdoctoral scientist John Rowan from the varsity.

"It is instead gradual -- playing out over nearly five million years -- long before the appearance of any hominin remotely capable of taking down rhino- or elephant-sized prey," he added.

The study, published in the journal Science, argued that falling atmospheric carbon dioxide (CO2) and the replacement of large shrubs and trees by grasslands caused the decline of megaherbivores.

Tropical grasses have a greater ability to cope with low levels of atmospheric CO2 in comparison to trees.

Importantly, many of the extinct megaherbivores fed on the leaves of trees, suggesting they disappeared alongside their food sources as grasses came to dominate African savannas over the last five million years, the researchers explained.

"Human impacts on Earth's biodiversity have been significant and sometimes catastrophic," Rowan said. "In this case, at least, our ancestors are not to blame. This is very clearly a long-term, bottom-up loss of diversity related to broader events in Earth's climate and environments over the last several million years."