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Scholars have wondered how mammals emerged as the dominant class of animals following the cataclysmic impact of an asteroid on Earth that wiped out the dinosaurs.

Previous theories suggested that the animals lived in the shadows of the giant lizards until the asteroid hit the planet around 66 million years ago.

But a new study has found that mammals were constantly evolving and adapting before the impact which allowed them to weather the mass extinction event, New Scientist reported.

Researcher Jorge García-Girón and his team studied more than 1,600 fossils of different North American fauna, including dinosaurs, fish, reptiles, amphibians and mammals. They created an estimation of the niches each extinct species occupied and entered the data into a computational model developed to track how food webs change over time.

Their findings showed that dinosaurs living 66 million years ago in North America closely resembled their ancestors 18 million years earlier, which represented a form of ecological stability.

But the mammals were playing the evolution card: Despite living among larger dinosaurs, they continuously adapted and evolved into a vast array of creatures able to climb, glide, and swim among other features during this period.

“It not only amazed me how mammals managed to thrive in the highly complex, and probably dangerous, dinosaur-dominated ecosystems,” said García-Girón, but also how rapidly our ancestors moved into vacant niches after the asteroid hit, he added.

Contrary to past studies, the researchers also found dinosaur numbers were not declining prior to the impact.

Maybe our mammal ancestors were just waiting for the right time to rule the planet.