Even if one were to believe that life evolved on its own, the physical laws are peculiarly conducive to life, apparently fine-tuned to an extraordinary degree. Stephen Hawking wrote in "A Brief History of Time": “The remarkable fact is that the values of these numbers [the constants of physics] seem to have been very finely adjusted to make possible the development of life.” Other prominent (non-believing) scientists who authored books on the subject of anthropic fine-tuning include Roger Penrose, Frank Tipler, and Paul Davies.There are four fundamental forces that define the subatomic world: gravity (which still isn't fully understood), electromagnetism (the attraction of opposite charges, enabling electron orbitals to remain in proximity to the nuclei, necessary for formation of chemical bonds), the strong nuclear force (binding atomic nuclei together), and the weak nuclear force (which allows protons to become neutrons and vice versa).

If gravity were much weaker, matter would not be sufficiently attracted to each other and planets and stars wouldn't be able to form. If it were only slightly weaker, stars would not explode and distribute the heavy elements formed in their cores, necessary for life. If gravity were stronger, smaller and thus shorter-lived stars would have formed, burned out faster, and likewise would not have been massive enough to explode and distribute heavy elements for life.

If the electromagnetic force were stronger, the electrons would collapse into the nucleus of an atom, rendering chemistry impossible. If it were weaker, electrons would not hover around the nucleus at all, likewise rendering bonding and thus, more complex molecules impossible.

If the strong nuclear force were 50% stronger, hydrogen (the simplest atom and starting point for nuclear fusion in stars) would have been consumed in the early universe. If it were 50% weaker, fusion would either not have occurred at all, or would not occur to the degree necessary to form heavier elements. In order to produce adequate carbon and oxygen for life, the strong nuclear force could not deviate from its present strength much at all. 

If the weak nuclear force were weaker, conversion of neutrons to protons would be much faster, and thus, hydrogen in stars would turn into helium too fast--ultimately causing the stars to burn up too quickly. In addition to these, the ratio of the masses of protons to neutrons is exactly as it must be for DNA to be possible.

The masses of neutrons relative to protons are also exactly as they must be to allow heavy elements to form, without causing all stars to collapse into black holes.

The convection in earth's core runs on radioactivity. If there were any less fuel, it might not have eventually formed iron, necessary for the production of earth's magnetic field which protects us from the sun's harmful solar wind, or charged particles that might otherwise destroy us. Any more radioactive fuel, and we'd be constantly beset by earthquakes volcanic eruptions, the ash of which would blot out the sun.

Along the same lines, if earth were less massive, the magnetic field would be correspondingly weaker. As a result, the solar wind could strip away our atmosphere and thus, our breathable air.  If it were more massive, earth's gravity would correspondingly increase, which would at a certain point cause a more uniform surface (no mountains or sea floors). This would distribute the oceans across earth's surface, making us a water world.

In order for water to be present on a planet at all, it must orbit its star at a precise distance, called the circumstellar habitable zone; too close and we would experience the same runaway greenhouse effect that is believed to have occurred on Venus (water evaporates, concentrates in the atmosphere, traps the sun's rays, and the temperature eventually becomes an oven). Too far, and it will freeze into an ice planet. The only way to prevent the water from freezing would be to increase atmospheric carbon dioxide to trap the sun's heat, but too much CO2 would mean not enough oxygen necessary to sustain life as we know it.

Our sun also has to be exactly the right size. Too small, and it would be a red dwarf, emitting far less light, and most of it in the red end of the spectrum. This would greatly impede photosynthesis, as plants require both sufficient sunlight, and both blue and red spectrum light as well. Impaired photosynthesis means not enough oxygen. A smaller star would also have a much closer circumstellar habitable zone; the problem is, a much closer orbit to a star would dramatically increase the tides on the planet, too. This would cause the planet to become tidally locked, like Pluto and its moon, Charon. This means one side would always face the star, while the other would always face away, causing dramatic temperature variations.

If the sun were larger, its light would be more toward the blue end of the spectrum, which would allow for oxygen production, but would leave us susceptible to intense ultraviolet radiation.

Jupiter and Saturn act as guards for earth: their immense gravitational pull tends to protect earth from stray comets that might otherwise collide with us and cause mass extinction.

Our moon's size and proximity stabilizes Earth's precise tilt of 23.56 degrees, which is necessary to keep our seasons mild. It also is responsible for 60% of the tides, which, among other things, drive the ocean current and thus help to distribute heat throughout the planet.

Earth also has to be placed exactly where it is within the galaxy. There is a 'habitable zone' within galaxies too, such that we have access to heavier elements from the larger stars, but we're still far enough from the spiral arms of the galaxy where supernovae occur from the most massive stars.

These are just a few examples of cosmic fine-tuning. Many scientists recognize the improbability of these parameters being just so. A rather circular non-explanation is called the Weak Anthropic Principle by Brandon Carter, which states, "We must be prepared to take account of the fact that our location in the universe is necessarily privileged to the extent of being compatible with our existence as observers." In other words, things are the way they are because if they weren't, we wouldn't be here to ask the questions of why they are they way they are. The corresponding Strong Anthropic Principle states, "[T]he Universe (and hence the fundamental parameters on which it depends) must be such as to admit within it the creation of observers within it at some stage." The classic logical objection to this argument is that of a criminal expecting to die by firing squad, who nevertheless faces the squad and lives. Would it not beg the question for him to conclude that the firing squad missed him simply because if they hadn't, he wouldn't be alive to ask why he was still alive?

Those who do not believe in a designer generally get around this objection via the multiverse interpretation of Quantum Mechanics: the idea that every possible quantum event does actually occur in some universe or another. Therefore every possibility, no matter how unlikely, must occur somewhere, at least once... and in the universe where it does, humans will  evolve to ask questions such as "why is everything so perfectly fine-tuned for life?"

Such an interpretation certainly seems to me to violate Occam's Razor: the simplest explanation is usually the correct one. (Not to mention, it begs the question--how one universe began in the first place now becomes a far more complicated problem of how multiple universes might be continuously generated with every Quantum Mechanical "choice.")

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