The birds we see around us today are the descendant of giant dinosaurs, according to a study published in the journal Science. A branch of dinosaurs continued to shrink consistently for more than 50 million years to evolve into today’s birds. Michael Lee, a professor at the University of Adelaide and lead author of the study, said that birds out-evolved and out-shrank their dinosaurian ancestors. It allowed them to survive where their less evolvable, giant relatives could not.

Flying Dinosaurs

Only a specific class of dinosaurs evolved into birds

But not all dinosaurs evolved into birds. Only a specific class of meat-eaters, two-legged beasts, called theropods, that kept getting smaller and smaller. Theropods included Velociraptor and Tyrannosaurus Rex. In many previous studies, theropods have been linked to modern birds. They have many similar characteristics such as wishbone, features and hollow bones.

Theropods evolved four times faster than any other class of dinosaurs. Being lighter and smaller, along with evolving anatomical adaptations, gave them new opportunities such as the ability to climb trees and fly. When their “shrink cycle” began, there were few small species to compete against theropods. Researchers examined fossils of 120 different species and more than 1,500 anatomical features of dinosaurs to construct a detailed family tree.They found that theropods survive to this day as birds.

Dinosaurs went from 600 pounds to just 2 pounds

The evolution began about 225 million years ago. At the time, the theropods weighted around 600 pounds. About 200 million years ago, their size shrank drastically, and they weighed 360 pounds. Researchers said the shrink cycle became faster and more prolonged. In another 25 million years, these dinosaurs were slimmed down to just 100 pounds. By 163 million years ago, the first birds appeared that had a weight of about two pounds.

Their evolutionary flexibility, new shapes and capabilities helped the early birds survive the asteroid impact about 66 million years ago. The asteroid wiped out almost all larger dinosaurs from our planet, researchers say. Scientists used complex mathematical models to trace the evolution.