The anthropic principle is merely the observation that characteristics of the universe, from the size of galaxies to the workings of subatomic particles, are peculiarly suited to life on Earth. It is an expression of a fact which science is then obliged to explore. There are several variations of the principle, each attempting to delve into the issue of the existence of life without diving too far into philosophy. Although the word "anthropic" refers to humans, its use is a misnomer; the principle applies equally to any conscious life form.
The Weak Anthropic Principle: "In as much as we can observe the universe, the universe must be conducive to our existence and our ability to observe it." Although the logic of this variation is circular, this is the most respected version. It explains we are able to observe our universe because only those who have adapted the ability to observe their surroundings could do so.
The Strong Anthropic Principle: "The universe had to have such a character as to allow life to develop at some point." The "strong" in this case doesn't mean that the argument is strong, but that the "anthropic" emphasis is strong. Scientifically, this version makes no sense; it's what is called a teleological statement—it suggests a predetermined goal defined the development of the universe. The only way the universe could have been endowed with the precondition for life is if Someone outside the universe made it so.
The Participatory Anthropic Principle: "The universe must be conducive to living observers because it can only exist if it is being observed." This principle attempts to apply the quantum theory to all of reality. Unfortunately, the apparent influence of observation on character and position has only been observed in subatomic particles, not objects as large as stars or planets or water droplets.
The Final Anthropic Principle: "Intelligent information processing must come into existence in the Universe, and, once it comes into existence, will never die out." This is part of a larger theory that intelligent life will take over the entire universe, force its collapse into a single point, and use an ensuing infinite ability to complete calculations to resurrect the dead. It is not generally accepted by most scientists.
However the principle is described, it does highlight one incredible mystery: time and time again, scientists learn new ways in which a microscopically slight variation in physics, astronomy, or geology would alter the Earth and make life impossible. Here are a few of the many ways the universe appears to be tailor-made for life on Earth.
The Earth: The axial tilt of the earth ensures not only an optimal temperature, but that the temperature will be normalized over the surface of the planet. If the rotational period of the earth were longer, the difference in temperature between night and day would be too drastic; if shorter, atmospheric wind would be too fast. If the Earth's gravity were stronger, the atmosphere would retain too much ammonia and methane; if weaker, it would lose too much water. The "shininess," or albedo, of the Earth's atmosphere is also vital. If the earth reflected too much sunlight, the surface temperature would cool, and extensive glaciers would form; if it retained too much sunlight, the atmosphere would fall into a greenhouse effect.
The Oceans: The moon is just the right distance away from the earth to affect the tides. If the moon were farther, the stagnant water wouldn't be able to support plant life; if closer, the tides would cover too much of the land. Underwater earthquakes occur in just the right amount to disperse the nutrients lying on the seabed without destroying too much of the ocean life. The salinity of the ocean is also a key characteristic; if the ocean wasn't salty, ice would form more easily, reflecting heat back out into space and keeping the planet in a permanent ice age.
Water Cycle: The Earth is situated at just the right distance from the sun to ensure that water can exist as solid, liquid, and vapor. Any closer, the water would boil, possibly creating a greenhouse effect that would heat the temperature on the surface like that of Venus. Any farther, the water would freeze, making plant life all but impossible. The rate of precipitation is also finely tuned. A greater rate would erode away the arable soil needed by plants; a lesser rate would prevent water from reaching those plants, and prevent nutrients from eroding into the ocean where sea life depend on them.
The Solar System: If our sun were redder or bluer, plants would not be able to use sunlight for photosynthesis. The sun's magnetic field is perfectly aligned to protect us from cosmic rays, yet not generate too high of an x-ray flux. Even the positions and sizes of the gas giants are imperative. If they were closer or larger, they would catastrophically affect Earth's orbit around the sun; if they were smaller or farther away, they wouldn't cause as many comets and asteroids to detour away from the interior of the solar system.
These and hundreds of other examples of "fine tuning" show that our universe is particularly suited for the existence and continuation of life. Hundreds of constants, from the charge of an electron to the mass of the sun, are exactly where they need to be. Any deviation of even one of these constants, and life would not be possible.
There are several possible explanations for the happenstance of intelligent creatures living in a universe suitable for life. One is that it is chance that our universe developed in such a way. One is some yet unknown scientific law that made it inevitable, although this doesn't explain how this fundamental law is so anthropic. A very popular option in scientific circles is that there are actually billions and billions of universes that all differ in some way, and we happen to live in one conducive to life because the others are uninhabitable. The Participatory Anthropic Principle suggests that only universes with the potential to develop conscious life-forms can exist. Another theory is that none of this is real, and we all live in some kind of virtual reality (although this does nothing to describe the reality of the VR equipment).
Where were you when I laid the foundation of the earth? Tell me, if you have understanding. Who determined its measurements—surely you know! Or who stretched the line upon it? On what were its bases sunk, or who laid its cornerstone, when the morning stars sang together and all the sons of God shouted for joy?
Can you bind the chains of the Pleiades or loose the cords of Orion? Can you lead forth the Mazzaroth in their season, or can you guide the Bear with its children? Do you know the ordinances of the heavens? Can you establish their rule on the earth? (Job 38:4-11, 25-27, 31-33).
Romans 1:19-20 says that God's power and divine nature are clearly displayed in His creation. We don't need intellectual gymnastics to explain why the universe happens to be just right. God's invisible attributes are evident in His creation. He wants us to have abundant life (John 10:10). It's no surprise He would design a universe that makes that possible.