Eggs
Fish eggs
The earliest living creatures, beginning a little less than four billion years ago, made babies by dividing themselves in half, so that one cell became two cells. But by about two billion years ago, some cells began to reproduce using meiosis instead of mitosis - a mother cell and a father cell combined their DNA to make a new cell. The mother and father each produced a half-cell, and the two half-cells merged together to make a baby. These half-cells were pretty much the first eggs.
By 600 million years ago, a few of these mother and father cells were combining to make creatures with more than one cell like hydras. These creatures also made eggs, and
Robin's eggs
By about 500 million years ago, some mother fish were laying thousands of eggs all at once. The eggs just lay on the floor of the stream or the ocean. Then the father fish came along and squirted sperm onto the eggs to fertilize them, and the eggs immediately started developing into baby fish.
When lungfish and then frogs began to come out of the water on to the land, about 400 million years ago, they had to go back to the water to lay their eggs. But soon reptiles evolved to have hard shells on their eggs, so the eggs could be safe on land. But if the egg had a hard shell, how could it mix with the sperm?
While reptiles and birds continued to lay eggs with shells, mammals evolved that kept their eggs inside their bodies, with a special place, the uterus, inside their body where the babies could grow until they were big enough to survive on their own, and then they were born. That's how mice, and cats and people make their babies.
To find out more about animals, check out these books from Amazon.com or from your library:
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