April 20, 2021 During 2.4 million years of existence, a total of 2.5 billion Tyrannosaurus rex lived on Earth, and 20,000 individual animals would have been alive at any moment, according to new calculations. Tyrannosaurus rex spanned all of ancient North America, and about 20,000 lived at once. Image via 1Ado123/ Wikimedia Commons. During 2.4 million years of existence on Earth, a total of 2.5 billion Tyrannosaurus rex ever lived, and 20,000 individual animals would have been alive at any moment, according to a new calculation method we described in a paper published on April 15, 2021 in the journal Science. To estimate population, our team of paleontologists and scientists had to combine the extraordinarily comprehensive existing research on