Scientists estimate modern-day ants first evolved about 120 million years ago. But the fossil record suggests that ants at this time weren't the prevalent insect that they are today. Not until 60 million years later, when some ants adapted to the new world of flowering plants and diversified their diets, did the critters achieve ecological dominance. Since then they've had a successful run of the planet. Scientists estimate that about 20,000 ant species crawl the Earth. Taxonomists have classified more than 11,000 species, which account for at least one-third of all insect biomass. The combined heft of ants in the Brazilian Amazon is about four times greater than the combined mass of all of the mammals, birds, reptiles, and amphibians, according to one survey.