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fish eggs - small orange balls, gloppy

Fish eggs: Egg evolution starts with soft eggs like these

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.

More about mitosis
And about meiosis
The first cells evolve
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As time went on, these creatures evolved into plants and animals, but both the plants and the animals continued to use specialized cells as eggs.

a bird's nest with three small blue eggs in it

A robin’s nest with three eggs in it

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.

More about reptiles
And about birds
What is calcium carbonate?

Egg evolution: eggs inside the mother’s body

About a dozen fossilized eggs in a circle

Dinosaur eggs

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.

More about Eggs
Some projects with eggs

Bibliography and further reading about eggs:

Duck-billed Platypus
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