The anthropic principle says that the universe is how it is because it must allow for the eventual creation of us, as observers. The anthropic principle was thought of in 1974, by the astronomer Brandon Carter.
Douglas Adams explains this concept quite well using a puddle as an analogy:
“If you imagine a puddle waking up one morning and thinking, 'This is an interesting world I find myself in — an interesting hole I find myself in — fits me rather neatly, doesn't it? In fact it fits me staggeringly well, must have been made to have me in it!" 
There are two different kinds of anthropic principles : the weak anthropic principle, and the strong anthropic principle.
The original thoughts that Brandon Carter had were:
Weak anthropic principle: "We must be prepared to take into account the fact that our location in the universe is necessarily privileged to the extent of being compatible with our existence as observers."
Strong anthropic principle: "The Universe (and hence the fundamental parameters on which it depends) must be such as to admit the creations of observers within it at some stage." 
References[change | change source]
- Stenger, Victor, The Anthropic Principle The Anthropic Principle, http://www.colorado.edu/philosophy/vstenger/Cosmo/ant_encyc.pdf The Anthropic Principle, retrieved 2009-9-11
- Adams, Douglas (September, 1988). "Douglas Adams' speech at Digital Biota 2". biota.org. http://www.biota.org/people/douglasadams/index.html.