Did our ancestors kill all the island megafauna?

Humans haven't always been great to nature. But at least our ancestors may not have killed off island megafauna in the distant past, so that's something. New research, published in the Proceedings of the Natural Academy of Sciences, suggests that there's not enough data to say that hominids in the Pleistocene—2.6 million to 11,700 years ago—were responsible for most of the extinctions on the islands they traveled to.

Overkill

The hypothesis that homo sapiens' distant ancestors killed off the world’s myriad ancient megafauna (not just on islands) dates back to 1966, with geoscientist Paul Martin's "overkill" proposal. But the idea has been floating around for far longer than the formal proposal. According to Julien Louys—associate professor of paleontology at Griffiths University in Australia and an author of the new research—the question of what caused the death of the world's megafauna dates back to the 19th century.

“It has, in certain circles, become very polarized,” Louys told Ars.

Louys and his colleagues' research argues against the overkill hypothesis. The work began in 2014, when Louys and a team looked into the Indonesian island Timor. Timor was once home to some species of megafauna that are now extinct—for instance, an elephant-like creature called a stegodon. The earliest archaeological records of the island peg hominid arrival to 45,000 years ago. But the stegodons likely went extinct before that, 130,000 years ago, implying that the hominids that reached the island didn't cause the species' decline.

Louys and his team were curious to see if Timor was a unique case in ancient island extinctions or if megafauna around the world managed to survive when their hominid neighbors first moved in. Or maybe the megafauna declined for other reasons.

To figure this out, Louys came together with a large group of archaeologists and paleontologists specializing in island ecosystems. The researchers sat down and compared the data and records they had for 32 islands. In the end, the team found that only on two islands were all of the extinctions associated with hominid arrival. These two islands were Kume in Japan and Cyprus in the Mediterranean.

Every island is unique

There are later cases in the Holocene—the last 11,700 years—in which humans arriving on an island clearly resulted in the untimely demise of its large creatures. New Zealand and Madagascar are prime examples. This isn’t to say that human-island contact didn’t result in any extinctions, just that not every island extinction came from it. “Some of the extinctions were coincident with human arrival. But by and large, most of the extinctions didn't seem to be correlated in time with human arrival,” Louys said.

The paper suggests several reasons why our Pleistocene kin weren’t responsible for the extinction to the island giants. For one, the populations of these island-goers were likely smaller. Louys said that technological advancement may have also played a role in determining the magnitude of humans' impact upon arrival, though the team did not explicitly study this idea.

Local ecology was another factor, as each island is unique. Some islands are quite large and have a similarly large carrying capacity for species. Others are isolated, so fewer species could have reached them to encroach on or overshoot this carrying capacity. (The first few species to sail or swim to a remote island aren't going to cause extinctions on it; they're going to populate it.) But some islands are also small and relatively accessible, meaning that their carrying capacity could be reached more quickly. This, in turn, could have made human contact more of a destabilizing factor for the island ecosystems.

“It might not have been humans. Or it might have been humans in conjunction with some other cause,” said Louys.

Ongoing disagreement

The researchers also looked into what those causes could have been. According to Louys, the extinctions were most likely “stochastic, random events.” The islands ended up reaching their carrying capacity, or some other random, yet-to-be-discerned environmental events caused the extinctions. Many of these islands are understudied when it comes to their paleo-environmental pasts, he said. The paper notes that changes in the climate could also have contributed in some cases.

“We're only barely scratching the surface as far as our understanding of what went on in these ecosystems in the past. All we can say, at this stage, is that there is no evidence for a mass wiping out of species as soon as humans arrive,” he said.

True to the controversial nature of the topic, Stuart Fiedel, a retired independent archaeologist, called the paper “a very confused piece.”

Fiedel brings up the case of two locations, both of which were home to dwarf mammoths: Wrangel Island near Siberia, and the Pribilof Islands near Alaska. However, while all terrestrial mammoths went extinct around 12,000 years ago, these isolated groups only kicked the bucket 5,000 years ago. And there is no evidence of human presence on the islands until recently, he said, adding, “If environmental change was the cause, why didn't the changed climate cause extinctions on these islands?”

Fiedel said there's not enough evidence to suggest that humans were not responsible for the megafauna extinctions around the world.

Louys takes a similar stance. “My understanding is that there's just not enough data for every species that goes extinct to say, unambiguously, what the cause of the extinction was,” he said.

PNAS, 2021. DOI: 10.1073/pnas.2023005118  (About DOIs).

Doug Johnson (@DougcJohnson) is a Canadian freelance reporter. His works have appeared in National Geographic, Undark, and Hakai Magazine, among others.