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fish eggs - small orange balls, gloppy

Fish eggs

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 that could combine with the cells from another creature to make a new creature. These were the first eggs.

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

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. Many animals, like snakes and birds, still lay hard eggs today.

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.

More about Eggs
Some projects with eggs

Bibliography and further reading about eggs:

Duck-billed Platypus
Mammals
Chordates
Biology
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