Early Human Ancestors Not to Blame for Extinctions of Giant African Mammals

A team of researchers, including postdoctoral scientist John Rowan, overturns decades of thinking on ancient hominin impacts in Africa. Rowan is with the University of Massachusetts Amherst and a 2018 doctoral graduate of Arizona State University.

The team’s research, published this week in Science, shows that early hominin species played little to no role in driving mammal extinctions in ancient African ecosystems.

The research team focused their analyses on the very largest mammal species, so-called megaherbivores (species over 2,000 lbs). To test for ancient hominin impacts on megaherbivore diversity, the researchers analyzed a seven-million-year-record of extinctions in Africa and compared it to milestones in human evolution that have been previously implicated in these extinctions. This includes, for example, the earliest stone tools and mammal butchery (around 3.4 to 3.3 million years ago) and the first appearance of Homo erectus (around 1.9 million years ago), a species that made advanced stone hand-axes and is known to have consumed ample quantities of meat.

Rowan and colleagues showed that the decline of megaherbivores 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 chimpanzees do today. Going a step further, the researchers propose that most hominin species over the last 4.6 million years were unlikely to have contributed to the demise of megaherbivores, as the rate of extinction does not significantly change from Ardipithecus onwards despite the appearance of larger-brained and tool-bearing hominin species.

“If hominins were responsible for the extinction of megaherbivores, we would expect their decline to be more step-like and to track behavioral or adaptive milestones in human evolution” says Rowan. “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. This suggests to our research team that widespread impacts on the large mammal species and terrestrial ecosystems might be unique to us – Homo sapiens.”

As an alternative to ancient hominin impacts, the researchers argue that the decline of megaherbivores is instead related to environmental change. The main culprits of this change were falling atmospheric carbon dioxide (CO2) and the replacement of large shrubs and trees by grasslands. They note that CO2 decline and grassland expansion are linked phenomena because tropical grasses have a greater ability to cope with low levels of atmospheric CO2 in comparison to trees. They also showed that many of the extinct megaherbivores were browsers who 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.

“Human impacts on Earth’s biodiversity have been significant and sometimes catastrophic,” says Rowan. “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.”

This research is published in Plio-Pleistocene decline of African megaherbivores: No evidence of ancient hominin impacts. J. Tayler Faith, John Rowan, Andrew Du, Paul L. Koch. Science

Contacts and sources: 
John RowanArizona State University

Citation:   Plio-Pleistocene decline of African megaherbivores: No evidence of ancient hominin impacts. J. Tayler Faith, John Rowan, Andrew Du, Paul L. Koch. Science