A study published on Thursday in the journal Science revealed that our earliest tool-bearing ancestors were not blamed for ancient mammal extinctions in Africa over the last several million years.

The findings, contrary to the long-held view over early hominins impacts on ancient African faunas, showed that long-term environmental change led to the extinctions, mainly in the form of grassland expansion likely caused by falling atmospheric carbon dioxide levels.

A group of American researchers compiled a seven-million-year record of herbivore extinctions in eastern Africa, focusing on the very largest species weighing over 900 kilograms.

Three-million-year-old hominin "Lucy" once shared her woodland landscape with a diversity of herbivores including three giraffes, two rhinos, a hippo, and four elephant-like species at Ethiopia. But these species disappeared afterward and the evolution of tool-using and meat-eating hominins getting most of the blame.

"Our analyses show that there is a steady, long-term decline of megaherbivore diversity beginning around 4.6 million years ago," said the study leader Tyler Faith, assistant professor in the Department of Anthropology at the University of Utah.

"This extinction process kicks in over a million years before the very earliest evidence for human ancestors making tools or butchering animal carcasses and well before the appearance of any hominin species realistically capable of hunting them, like Homo erectus," said Faith.

Their analysis revealed 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.

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.

Faith and his team also examined independent records of climatic and environmental trends and their effects like stable carbon isotope records of vegetation structure and eastern African fossil herbivore teeth.

"Low CO2 levels favor tropical grasses over trees, and as a consequence savannas became less woody and more open through time," said the paper's co-author John Rowan, a postdoctoral scientist from the University of Massachusetts Amherst.

"We know that many of the extinct megaherbivores fed on woody vegetation, so they seem to disappear alongside their food source," said Rowan.

Researchers said that the lacking of prey might also have caused the extinctions among African carnivores at that time like saber-tooth cats.

Previously, some scientist suggested that competition with increasingly carnivorous species of Homo led to the demise of numerous carnivores over the last few million years.

"We think our study is a major step towards understanding the depth of anthropogenic impacts on large mammal communities and provides a convincing counter-argument to these long-held views about our early ancestors," said Faith.

(Cover: VCG Photo)