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."