Even Epicurus just tried to explain according to his surviving works. As for Lucretius preceding Darwin by 2 millennia, then consider that the idea of survival

Even Epicurus just tried to explain according to his surviving works. As for Lucretius preceding Darwin by 2 millennia, then consider that the idea of survival of the fittest by itself is not a scientific theory in the modern sense that makes predictions that can be verified. Basically one can explain anything with it by defining what “fittest” means post-factum. The science begins when one tries to predict with it things like speed of evolution or the existence of species not yet observed in paleontological records. There is nothing like that in Lucretius works AFAIK.