This is a quote used by the 4th Doctor in describing the human race. Sarah Jane repeats it back to the 10th Doctor to remind him of such words.
“Homo sapiens. What an inventive, invincible species. It's only a few million years since they crawled up out of the mud and learned to walk. Puny, defenseless bipeds. They've survived flood, famine and plague. They've survived cosmic wars and holocausts. And now, here they are, out among the stars, waiting to begin a new life. Ready to outsit eternity. They're indomitable.”-The Ark in Space [12.2]