390 million years ago, during the sultry days of the Devonian period, some adventurous fish

started crawling out of swamps to explore the wide open frontier of dry land. These fish were

the ancestors of the earliest amphibians, which evolved some millions of years later. Amphibians were much better at getting around out of the water,

what with the legs and lungs and all that. But most amphibians, even to this day, can't venture too

far from watery habitats, if for no other reason than because their eggs won't survive unless they're immersed in water.

Some amphibians eventually evolved into reptiles, about 320 million years ago.

One of the defining evolutionary inventions of reptiles was the ability to lay enclosed eggs

that can survive on dry land. Your standard issue reptiles was the ability to lay enclosed eggs that can survive on dry land. Your standard-issue

reptile egg has a leathery shell that protects the growing embryo inside, protects it from

things like harmful microbes, solar radiation, and drying out. As early reptiles evolved into the first dinosaurs, somewhere along the

way, these animals started laying eggs not with leathery, flexible shells, but with hard shells.

Dinosaur eggs with their hard shells were an upgrade from early reptile eggs. One lineage of dinosaurs became the

birds, of course. Birds with their warm, feathered bodies incubate their hard-shelled eggs at an

optimal temperature. This brooding behavior in birds, combined with the structure of their eggs, opened even more

opportunities for them. They could raise their young just about anywhere. So, even though the

fossil record of this story is very incomplete, we can look back and see the evolution of vertebrate

eggs. From the delicate, slimy eggs of fish and amphibians

to the self-contained eggs of reptiles, dinosaurs, and birds. Mammals are another story.

They went down a different evolutionary path. Most mammals stopped laying eggs over 100 million years ago. Although, I'm not sure what's

going on with the Easter bunny. It's a giant rabbit, okay, so that means it's a mammal, but it lays

these crazy, colorful eggs and carries them around in a wicker basket. Oh, wait, wait, what if?

Maybe the Easter bunny isn't actually a rabbit,

...