Why were animals bigger in the past?

All different kinds of giant prehistoric creatures used to walk the Earth, from 20-foot-tall sloths to sharks longer than a school bus.. They all seem huge in our imaginations, but were animals in the past actually bigger than animals on Earth now? We asked paleontologist Kristi Curry Rogers to help us find the answer.Got a BIG question? Send it to us at BrainsOn.org/contact, and we’ll help size up the answer!