[Virginia-American]

...what would it take to "disprove" purely naturalistic evolution (with no intelligent design)? ...

Finding an ERV in both orangutans and chimps that was not also present in gorillas and people would disprove the currently-accepted family tree of the primates. Doing the same thing over and over, with, say, cows, hippos and whales, or dogs, cats and bears, etc. would destroy the theory of evolution, with or without some sort of guiding intelligence.

[so_real]

Finding an ERV in both orangutans and chimps that was not also present in gorillas and people would disprove the currently-accepted family tree of the primates.

You might find this study interesting. How about an ERV found in chimps, gorillas, baboons and macaques but not found in orangutans, siamangs, gibbons or humans?

Have a look at this outline of the primate phylogeny as deduced from human-ERV studies : http://www.talkorigins.org/faqs/comdesc/images/retrovirus.gif

Then consider that the primate-ERV studied is found in Old World Monkeys, not in gibbons or orangutans, and then in gorillas and chimps, but not in humans ... The primate phylogeny from the human perspective doesn't exactly jive with the same phylogeny from the Old World Monkey perspective. We know that not every ERV found in both gorillas and chimps is also found in humans, we just don't know the how's and why's yet.