Apparently a large atmospheric oxygen surplus can make all different kinds of life forms grow much larger very quickly (on an evolutionary time scale). Life on this planet is like a fire. We're the flames. - Jared Jacobs

"It happened primarily in two great leaps, and each time, the maximum size of life jumped up by a factor of about a million," said Jonathan Payne, assistant professor of geological and environmental science at Stanford. - j1m

Oh wow, I hope there's not another one anytime soon. - Jim Norris

Don't worry, they theorize that we're already nearing the maximum organism size that the planet can handle. That's why the mass extinction and global warming and stuff. - j1m

"Why do the size leaps seem to hinge on the amount of oxygen in the air? "There are a few things that could be going on," Payne said. "The first thing is that eukaryotic cells require oxygen for metabolism. So if they want to take organic matter and burn it up to have energy in their cell, they need oxygen. That sets the first and probably most important limitation."" - j1m