When did the first eggs evolve?
The earliest living creatures made babies by dividing themselves in half, so that one cell became two cells. But about 1.4 billion years ago, some creatures with more than one cell began to develop specialized cells. These specialized cells could combine withthe cells from another creature. Together, they could make a new creature. These were the first eggs.
When did eggs get hard shells?
The next step in egg evolution came when reptiles began to lay eggs with hard shells made of calcium carbonate, a molecule combining carbon and calcium. Inside these shells, a baby reptile could grow safely until it was big enough to be born. That way, the reptiles didn’t have to lay their eggs in the water. The eggshell holds a miniature ocean for the tiny embryo to grow in. Many animals, like snakes and birds, still lay hard eggs today.
Egg evolution: eggs inside the mother’s body
About 200 million years ago, however, some reptiles evolved into mammals that kept their eggs inside them until the babies were big enough to be born. The earliest animals that kept their eggs inside them were the ancestors of the opossum. Today, all mammals (except the platypus and the echidna!) keep their eggs inside them until the baby is born – horses, dogs, cows, donkeys, sheep, pigs, chimpanzees, and people.