A model of the amino acid molecule glycine
(red is oxygen, black is carbon, yellow is hydrogen,
and blue is nitrogen atoms)
Amino acids are molecules made out of hydrogen, oxygen, carbon, and nitrogen atoms (and a little sulphur). The earliest amino acids formed in space, about 14 billion years ago, before the planets even existed. Probably they formed inside chunks of ice, when ultraviolet radiation (energy) from stars shot through the ice and encouraged molecules to form. Astronomers have found evidence that amino acids are still out there floating around in nebulae in space.
When amino acids first came to Earth, they probably came inside these chunks of ice. When the ice later turned to water on a warmer Earth, the amino acids floated around in the water, and eventually linked up to become bigger molecules called ribonucleic acid and proteins. These proteins are what all living cells are made of.
There are twenty different kinds of amino acids. Plants make all of these amino acids themselves, out of the atoms that are their ingredients. Animals (and humans) can only make ten of the twenty different kinds of amino acids, but we need all twenty kinds in order to live. So we have to get the other ten kinds by eating plants, or animals that ate plants.
Different amino acids are in different foods, but if you eat milk, eggs, chicken, and bread you will get all of them. If you're a vegetarian, you can get amino acids from bread, lentils, bananas, peanuts (or peanut butter), and chocolate or red wine. We can't store amino acids in our bodies, so we have to eat these foods pretty much every day.
To find out more about protein, check out these books from Amazon.com or from your library: