On one hand, a small difference in the reproduction rate of two similar species competing for the same resources leads to extinction of the slower reproducing o

On one hand, a small difference in the reproduction rate of two similar species competing for the same resources leads to extinction of the slower reproducing one in not so many generations. Even without hostilities. On the other hand, Spainiards and Portuguese killed around 90-95% of the indigenous population when they conquered South America. Contemporary Homo sapiens seems to be a rather aggressive monkey horde. Choose your odds. I think it was a mixture of both.