Grass roots of Ice-Age extinctions

What killed off all the giant animals that roamed the earth during the ice ages? Was it… us? Probably. But if it was us, then almost certainly it wasn’t just us. And, according to a visiting anthropologist, we need to stop blaming some of our more distant great-grandparents on the evolutionary family tree altogether.

Earlier this week anthropologist and paleobiologist Tyler Faith from the University of Utah delivered a lecture on megafauna extinctions at the Wits Origins Centre, drawing attention to an increasing number of scientific papers that are stating as fact out-of-date assumptions that our hominin ancestors were responsible for major extinctions in Africa.

“This ancient hominin impact theory crept in under the radar, and now it’s being treated as the gospel truth,” Faith said, before going on to name his prime suspect in the ancient extinctions: lots and lots of grass.

It’s difficult to deny the complicity of Homo sapiens in the disappearances of megafauna elsewhere in the world. Mammoths, giant sloths, dire wolves, cave hyenas, woolly rhinoceroses, sabre-tooth cats and the okapi-like Sivatherium — their disappearances all track the rise and spread of humanity around the globe during the Pleistocene era, which lasted from about 2.58-million years ago to just over 11700 years ago.

Humanity’s dominance was driven by the ability to work with fire and employ co-ordinated group tactics, not to mention the technological edge of being pretty great at tying sharp rocks to sticks and then stabbing anything that moved.

This anthropogenic or people-powered explanation for megafaunal extinction is called “Overkill” — a dramatic name for a dramatic hypothesis coined by the American geoscientist Paul Martin in 1967. There are also other theories around, implicating everything from lethal solar flares and disease to extraterrestrial phenomena (comet and asteroid impacts, not aliens — steady on!).

And, of course, climate change — which isn’t especially surprising considering that during the Pleistocene global temperatures ricocheted between ice age and not-ice age, just like a South African refrigerator during a week of stage-two load-shedding.

For the most part, however, it’s accepted that humans are highly culpable.

This is where things get tricky for the African extinctions, though, because most of the megafaunal loss on the continent appears to have taken place well before modern humans came on to the scene — earlier by several million years, in some cases.

Nevertheless, in the absence of Homo sapiens, proponents of the Overkill hypothesis — including Martin himself — attributed these megafaunal declines to humanity’s predecessors. Homo erectus gets a lot of the blame, but by Overkill reasoning the extinction chronology would incriminate even poor old Ardipithecus, who was barely a step ahead of chimpanzees in the evolutionary game and not known to be especially good at stabbing things either.

Martin would later back away from the idea after more precise methods of dating fossils updated the chronology involved, but the assumption that ancient hominins caused the extinction of African megafauna would live on in academia, by and large unchallenged.

In his lecture at Wits this week, and in research with co-authors Andrew Du, John Rowan and Paul L Koch, published in the journal Science, Faith looked at some of the papers and studies that either underpin or help to propagate this idea.

Some of them fail in the face of new understandings based on better data. For instance, one paper that is often cited proposed that Homo erectus was responsible for whittling down the number of proboscidean or elephant-like species from 12 to just two over the course of a million years.

But this was based on data from 1978, and Faith highlights new data that has prompted a re-evaluation of the chronology, which in turn changes the loss of 10 elephant-like species over a million years, to just two species lost over two million years, with another two proboscideans still around when Homo sapiens came on to the scene. So it looks like Homo erectus is off the hook for that one, at least.

In a paper titled ‘“Plio-Pleistocene decline of African megaherbivores: no evidence for ancient hominin impacts”, Faith and his colleagues argued that the early megaherbivore extinctions should instead be attributed to loss of habitat brought about by the shift from forested terrain to grassy savannah, beginning about 4.6-million years ago.

A drop in carbon dioxide levels in the atmosphere prompted the expansion of grasslands at the expense of the trees and shrubs upon which the giant herbivores fed.

Fewer prey (and fewer trees from which to ambush them) led to fewer big carnivores. And so on.

So although anatomically modern humans are certainly implicated in extinction events at the end of the Pleistocene (and onwards), it’s unfair to dump responsibility for major extinction events in Africa at the feet of our ancient ancestors.

If anyone is to blame, said Faith this week, it’s us.

But mainly it’s down to a whole lot of grass.

Matthew Du Plessis
Before focusing on science journalism, Matthew was the Mail & Guardian's managing editor and chair of the Adamela Trust. He now writes about the environment, dinosaurs, particle accelerators, evolutionary anthropology, genomics and the occasional continent-spanning field of molten lava.
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