Bi, can you stop justifying your ignorance?
According to the context , we can hardly assume the very intention of human being is to encounter more prey, nor should we justify the usage of "HAD done" .
In case of any refutation, here is the origin.
The research, which was published in the Feb. 9 issue of the journal Science, has implications both for the modern-day debate over reintroduction programs and for a historical debate over what happened 10,000 to 50,000 years ago, when one particular predator, humans, expanded its territory and encountered large numbers of native prey. Some experts think that a ''blitzkrieg'' of killing by advancing humans contributed to the extinctions of many large animals in that era.

See, It's an article from NY Times. what I wanna say is stop offering your misguidance and misinterpretation!