The so-called "anthropic principle" in cosmology (or in the philosophy of human existence) seems devilishly clever and subtle, worthy of careful consideration and great respect. In fact, it is not--it is stupidly simple. Basically it says that any theory or way of looking at things that doesn't allow, as one conclusion, that human beings exist is wrong.
Evidence in science is sometimes hard to come by. But, yup, here we are; human beings exist (and the rest of biology, and houses, and oceans, and stars, too). So if whatever theory you're promulgating doesn't really allow for that, you'd better take it back to the drawing board.
In 1946 Fred Hoyle was working out the details of an elaborate theory called stellar nucleosynthesis. Basically it said that you can start with hydrogen, the simplest element with ONE proton. Hydrogen was spewed around the Universe when it was created in the Big Bang some 13.7 billion years ago. Little by little gravity pulled that together into clouds of gas. But gravity kept working and gradually pulled the hydrogen into tighter and tighter packages until the hydrogen was so compressed it mashed together into helium, the second smallest element--it has TWO protons. Basically, two hydrogens make a helium plus releasing a helluva lot of energy, so the star lit up, a sort of ongoing hydrogen bomb. When there got to be a lot of helium, gravity kept squishing it together and it made beryllium, the element that has FOUR protons (get it? Two plus two makes four). That rattled around, mashed up with another helium, and made a carbon atom--SIX protons. Mash that with another helium and you get oxygen--EIGHT protons. All this synthesis of atomic nuclei ("nucleosynthesis") is going on in the hot insides of stars ("stellar nucleosynthesis").
As these heavier elements are knocking around, things get kind of messy with bigger and bigger elements mashing into one another (and into leftover small elements, too). Also, this nuclear soup gets so hot and unstable and a lot of times it goes BOOM! A supernova. And the goodies it's been making are spewed all over the heavens. Where, once again, the slow but inexorable pull of gravity goes to work and gradually pulls them together into clouds and stars. But these stars are very different from the first series of stars--they've got a lot of weird elements in them like carbon and oxygen. This secondary star stuff is suitable for forming planets, like Earth, and life, like us.
So the theory of stellar nucleosynthesis says that when the first series of stars mashes hydrogen (from the Big Bang) together, that's how heavier elements like carbon and oxygen get produced. And then supernovae spew them around the heavens for second-series stars, like our Sun, and its planets, like Earth, to get made with the strange mixture of heavy elements needed for life.
It's a nifty theory. A lot of experimental observations fit right together nicely (like the characteristics of atomic hydrogen, and helium, and beryllium, and carbon, and oxygen, for example). But it has some problems, too. For one thing, there isn't much beryllium around and there's a lot of carbon, but the theory says you've got to go through beryllium to get carbon. Now beryllium, when you study it in the laboratory, is very unstable (it has a half-life of 10^-17 seconds). So how come you happen to get enough beryllium hanging around to mash up with helium to make carbon (because it sure does take a lot of carbon to make life possible)?
Fred Hoyle proposed that it just wouldn't happen--there just wouldn't be enough carbon around (via the process of stellar nucleosynthesis)--unless there was an energy level that was just right so that when it did happen to happen, the resulting atomic nucleus, carbon, sort of fell into an energy hole and was quite stable. Essentially Hoyle figured out that for the theory of stellar nucleosynthesis to work (and it had a lot of observations going for it) and to wind up with enough carbon to make us, the carbon nucleus must have an excited energy state (a sort of hole waiting to be filled) at 7.6 million electron volts. He said, "Look for it, boys." And they did. And there it was. And that's considered one of the great proofs of the anthropic principle.
To untangle the string of logic, Hoyle said essentially that since we humans are here, the element carbon must have an excited-energy hole at 7.6 million electron volts. Or else what? Or else the elaborate theory of stellar nucleosynthesis (with all its satisfying mathematics and validating experiments) wasn't right--at least, it couldn't explain how there got to be enough carbon around to make us.
In other words, "human beings exist" is a pretty strong position to argue from. That's the "anthropic principle."
Bun Gladieux, president of the Presssure Positive Company, has a blog with an interesting series of topics.
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