They were some of the strangest animals to walk the Earth: wombats as big as hippos, sloths larger than bears, four-tusked elephants and an armadillo that would have dwarfed a VW Beetle. They flourished for millions of years, then vanished from our planet just as humans emerged from their African homeland.

It is one of paleontology’s most intriguing mysteries, and will form the core of a conference at Oxford University this week where delegates will debate whether climate change or human hunters killed off the planet’s lost megafauna, as these extinct giants are known.

“Creatures like megatherium, the giant sloth, and the glyptodon, a car-sized species of armadillo, disappeared in North and South America about 10,000 years ago, when there were major changes to climates, which some scientists believe triggered their extinctions,” said Yadvinder Malhi, professor of ecosystem science at Oxford, one of the organizers of the conference, “Megafauna and Ecosystem Function.”

“However, it is also the case that tribes of modern humans were also moving into these creatures’ territories at these times, and many of us believe it is too much of a coincidence that this happened just as these animals vanished. These creatures had endured millions of years of climate change before then, after all.”

Modern humans emerged from Africa around 70,000 years ago, traveled across Asia and reached Australia 50,000 years ago, a time that coincides with a wave of extinctions of creatures there, including the diprotodon, a species of wombat that grew to the size of a hippopotamus.

By about 14,000 years ago, humans had reached North America by crossing the land bridge that linked Siberia and Alaska. Then they headed south.

By 10,000 years ago, Homo sapiens had conquered North and South America at a time that coincided with major megafauna extinctions.

“We think of Africa and Southeast Asia — with their lions, elephants and rhinos — as the main home of large animals today, but until very recently in our planet’s history, huge creatures thrived in Australia, North America and South America as well,” said professor Adrian Lister of the Natural History Museum in London. “The question is: Why did they disappear in the New World but survive in the Old World? Some believe it is because large animals in Africa and Southeast Asia learned to become wary of human beings and decided to avoid them at all costs. However, I also think climate change may have been involved in the Americas and Australia and that humans only finished off these big animals when they were already weakened by loss of habitats and other climate-related problems.”

The idea that humans were involved in any way in eradicating dozens of species of giant animal when we were still hunter-gatherers has important implications. It was thought, until relatively recently, that humans’ relationship with the natural world became unbalanced only when we invented agriculture several thousand years ago.

But if ancient hunter-gatherers played a part in wiping out these species of huge animals as long as 50,000 years ago, humanity’s supposed innate harmony with the living world appears misplaced.

More to the point, humanity is still paying the price for the disappearance of the megafauna of the Americas and Australia.

“There is now a lot of evidence to suggest that large herbivores like gomphotheres, a family of elephantlike animals that went extinct in South America around 9,000 years [ago], played a key role in spreading nutrition in areas like the Amazon. They would eat fruit in the forest, including avocados, and their excrement would then fertilize other areas. That no longer happens, and places like the Amazon are today affected by low nutrition as a result,” Malhi said.

Another example is provided by the giant wombat, the diprotodon, which some scientists have argued browsed bush across Australia and kept biomass levels very low. When the diprotodon vanished, plants and shrubs across the outback grew unhindered. The result was fires that, archaeologists have discovered, became a serious problem.

Similarly, creatures such as the mammoth played a key role in trampling tundra and maintaining healthy grasslands in high latitudes such as Siberia. When the mammoth became extinct, the tundra took over, to the detriment of the landscape.

“It is now becoming clear that lots of our understanding of contemporary ecology is incomplete because it does not take into account that ecosystems were adapted to having giant animals like the mammoth or the diprotodon,” added Malhi. “These are not natural systems today, because they are missing key components to which most plants had adapted.”

This awareness has led some scientists to propose moving populations of the planet’s surviving large animals into regions where they could help restore the ecologies to their previous healthy conditions.

One such experiment is being carried out by ecologist Sergoy Zimov at a nature reserve called Pleistocene Park in Siberia. Zimov has reintroduced musk ox, moose and other large animals and is attempting to find out if their browsing will restore the landscape to its previous healthy, grassy state.

Other researchers go even further and have proposed bringing extinct megafauna back to life. For example, several scientists have suggested that it could be possible to clone a mammoth from frozen remains found in Siberia using an Asian elephant as a surrogate mother.

The real lesson from the fate of the Earth’s megafauna is to appreciate how important surviving species are to our planet.

Oxford University ecologist Emily Read, a conference organizer, said: “We need to protect the megafauna that we have. . . . It’s not just the cultural value of these large animals that we need to think about, but the fact that removing them affects the whole ecosystem.”

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