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 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
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.
You done a superb job outlining this in easy to grasp terms and structure – thank you for the time you spent pulling this together and being open to editing it.
But like how did those develop like I understand the need for a hard egg but how did sex drastically change from external fertilization to internal like there’s no gradual way to evolve into that.
Good question, Nicholas! It’s harder to say, because this question involves the soft parts of animals that aren’t so well preserved in fossils. But probably internal fertilization does evolve slowly, the same way eyes evolve. It’s more efficient to fertilize just as the eggs are coming out (or before!), before anything can happen to the eggs, so any mutation that favored that would catch on quickly. Here’s our explanation of how eyes evolved, to compare: https://quatr.us/biology/eyes-work-evolution-biology.htm
You’ve also forgotten the Echidna! That and the platypus are the only monotremes in the world.
Thank you, I’ll amend the sentence!
whats up my doods
Not ALL mammals keep there eggs inside of them until birth. We can’t forget the platypus!
You’re right, Austin! I’ve added that to the article, thanks.