Human ancestors can't be held responsible for extinction of large, plant-eating mammals that roamed Africa millions of years ago, argues a new study carried out by researchers from the University of Utah. The researchers assert that it were actually the climatic and environmental changes that led to the wiping out of several megaherbivores from the continent.
Only five species of megaherbivores - giraffes, elephants, hippos, and white and black rhinos - live in Africa today. However, the continent was characterised by a greater diversity millions of years ago. Why many of the ancient megaherbivores became extinct in Africa has long been a mystery for paleontologists.
Most studies in the past have held evolution of tool-using hominins responsible for the extinction. The new research, however, challenges this view and argues that the mammal decline had started long before tool-bearing humans appeared in the region.
The research team was led by Tyler Faith, assistant professor in the Department of Anthropology at the University of Utah and curator of archaeology at the Natural History Museum of Utah, and also included researchers from the University of Chicago, the University of Massachusetts Amherst, and the University of California, Santa Cruz.
"Despite decades of literature asserting that early hominins impacted ancient African faunas, there have been few attempts to actually test this scenario or to explore alternatives," said Tyler Faith.
"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."
The results of the study show that an expansion of grasslands due to diminishing atmospheric carbon dioxide (CO2) levels likely caused extinction of several species of megaherbivores.
The team analysed records of plant-eating mammals extinction in eastern Africa in the past seven million years. They used a dataset of more than 100 fossil assemblages and focused on megaherbivores weighing more than 907 kilograms.
The results showed that 28 lineages of megaherbivores vanished in Africa in the past seven million years. The decline started about 4.6 million years ago, and there was no change in the rate of decline following the appearance of Homo erectus - the human ancestor that could have contributed to the extinction of megaherbivores.
The team also investigated records of environmental and climatic changes, including global atmospheric CO2 and stable carbon isotope records of fossil herbivore teeth and vegetation structure. They found that climate likely played a major role in vanishing of herbivores. Dwindling CO2 levels led to replacement of trees and shrubs by grasslands, resulting in lesser availability of food for herbivores.
"It follows that 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," said Faith.
The findings of the study are published in the journal Science.
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