The most popular theory for the cause of this mass extinction was first put forward by American scientist Paul Martin, nearly 40 years ago.
He put it down to the arrival of Homo sapiens. In both America and Australia these mass mammal extinctions followed shortly after the arrival of the first humans. In Australia they began about 40,000 years ago, in the Americas about 13,000 years ago. According to Martin, because animals in these continents had never come across humans, they were vulnerable.
Read the diary of any explorer who encounters a natural habitat where no man has ever been, for example Charles Darwin on the Galapagos Islands, and you will find that they always comment on the lack of timidity of the wildlife. It is still like this today in the few parts of the world which have no humans living near them. So when the first human wanderers arrived, flint weapons, bows, arrows and spears in hand, the animals they came across were fearless. They may have looked on with curiosity at these half-hairy, two-legged apes clambering ashore, but chances are that the horses would have just munched on. Even the lions, provided they weren't too hungry, would probably have just fallen back to sleep. Thus they were easy prey for hunter-gathering man with his sharp spears – so much so that in less than a thousand years most of the big game had been slaughtered, and many species were on the verge of extinction.
The theory also explains why in North Africa, Europe and Asia many similar animals survived the presence of mankind. Animals here had evolved alongside human species for over 2 million years, and had grown used to their appetite for meat and hunting. The experience of their ancestors had evolved into a well-honed instinct that allowed them to survive in sufficient numbers, avoiding contact with humans by running away and hiding. This meant that the mass extinctions seen in Australia and the Americas simply never occurred. So, the theory goes, in just a few years Homo sapiens single-handedly deprived nearly half the world's landmasses of all their large creatures by hunting them to oblivion.
Recently this theory, called the Pleistocene Overkill, has itself come under attack. For example, it doesn't explain why some species not generally eaten by humans (for example, sloths) became extinct, while others that were hunted (such as bison) survived. Mass slaughter by humans also doesn't explain why beavers, bears and bison all became so small.
The best theory seems to be one that blends the arrival of humans with the effects of natural, cyclical climate change. It goes like this: when humans first arrived on the virgin continents of Australia and the Americas, they indeed found big game were easy prey. Many of the key predator species, such as lions, tigers and wolves, were killed off in massive numbers by the two-legged hunter-gatherers. At the same time, temperatures rose rapidly, causing the glaciers to melt and the seas to rise. What was once a rich American landscape of parkland trees and pastures gave way to huge stretches of arid inland savannahs with dried-up waterholes that turned into thick conifer forests near the much wetter coasts.
Because humans killed off so many of the larger, carnivorous predator species, the populations of these animals' prey – herbivores such as bison, deer, sloths, horses and camels – grew uncontrolled because there was nothing left to eat them. They became so numerous that there simply wasn't enough food to go around. Combined with the changes in vegetation caused by rapid climate change, the effect was catastrophic. Herbivores were wiped out in their millions through starvation because the landscape couldn't support them any more, and only small species which could endure long periods consuming little food and water survived.
The intensive grazing of these huge overpopulations also contributed to the effect of climate change, accelerating the transition from parkland to grassland, making the landscape even less suitable for supporting future generations of large animals.
How fragile are nature's ecosystems. Add a new bit of something over here (humans), and see them remove something else over there (lions and sabre-toothed tigers). Now throw in a bit of random climate change, and devastation sets in on a massive scale. The role of humans in the annihilation of the large herbivorous marsupials and placental mammals of Australia and the Americas between 40,000 and 12,000 years ago – at the beginning of the last second to midnight on the 24-hour clock of Earth history – was humanity's first big impact on the Earth's fragile, changing natural environment. It would not be the last.