Independent: 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 …