In a recent edition of Discover, an article, "Aliens Among Us," asks the question "do we share our planet with alternative forms of life?" (62). Mathematical models indicate that there is a 95% chance that life originated two or more times on the Earth. There is no reason, from natural laws, that life didn't have more than one origin. DNA based life dominated, but that doesn't mean that niches are filled with RNA based life (DNA based life uses RNA, and evolved from RNA based life). At present, most of our methods for detecting life do not look for RNA, they look for DNA instead.
In early Earth history, the Earth would've been bombarded by large planetoids, comets and meteors - some of which may have wiped out early life. Life would've begun again. Why? Why can chemical reactions be predicted if the type and quantity of the chemicals involved are known as well as the environment (temperature, pressure, etc)? Why can large (and thus Newtonian) physical systems be predicted given known values (mass, density, magnetic fields, etc)? Because laws are universal. Given certain range of conditions, life will rise.
Life has great variety on Earth "but at the molecular level they are staggeringly uniform" (62). This doesn't mean that alien life will share the same uniformity as Earth life. But it will probably have its own uniformity. And it does mean that when looking for life, we need to broaden the tool set we use for looking for life. But it also means that here on Earth we need to study all the forms of life that exist here to help us gain an idea of the universal biology laws.
You see, forces all work to find an equilibrium. A star burns because gravity compresses it down, igniting a nuclear furnace. The radiation pressure from that furnace pushes back against the forces of gravity. While they balance, the star burns in an overall steady state. Depending upon mass, the star lives a long life, or a short life, and ends in a supernova or a blackhole (OK, this is a bit oversimplified). This happens over and over again throughout the universe because the forces are everywhere - gravity, light, and electromagnetism, for instance, are common and they react each according to their natures; and because their natures are not random natures, they do not interact randomly: gravity doesn't randomly become light and then magnetism, for instance, and it doesn't randomly change the way it interacts with the universe.
True, constants may actually change over time (there are debates about just how constant constants are), but overall forces have a nature to them, a pattern to them. And patterns interacting with patterns, and you get great variety of results, but it isn't chaos. Same with biology. There will be universal biological laws. And given the right range of conditions, and life will arise again and again in the universe, just like stars die and stars are born, they didn't come into existence only once, to die as a one time occurrence. Galaxies weren't created only once. There is an overall evolution to the universe (change may be the only true constant), but it is a slow evolution, and right now life exists, and we are learning more and more just how tenacious life is. It survives terrible calamities, terrible upsets - particular life forms may not survive, but life itself tenaciously persists.
Zimmer, Carl. "Aliens Among Us." Discover. July 2007. 62 - 65. Can also be found at <http://discovermagazine.com/2007/jul/aliens-among-us>.