Life arose more than once on Earth. In fact, ever since life begun, it never stopped. No matter what happened, life has proven its unnerving resilience. Evidence suggests that life on Earth has existed for about 3.7 billion years ( "History of life through time". University of California Museum of Paleontology). Humans however, have only been around for 40,000 years.
The universe is 12 to 15 billion years old. Therefore, between the "birth" of humans and the birth of universe, there is almost a 12 to 15 billion years of difference. To put this into perspective; 12.000.000.000.000 to 15.000.000.000.000 years of difference. Given such a vast time frame, it is highly unlikely that nothing else evolved in this universe.
Speaking of universe, based on observations from the Hubble Space Telescope, there are at least 125 billion galaxies in the universe. It is estimated that at least ten percent of all sun-like stars have a system of planets (Marcy, G.; Butler, R.; Fischer, D.; et al. (2005). "Observed Properties of Exoplanets: Masses, Orbits and Metallicities"), thus there are 6.25×1018 stars with planets orbiting them in the universe. If even a billionth of these stars have planets supporting life, there are some 6.25 billion life-supporting planetary systems in the universe.
Given such astonishing vastness, it is just somewhat illogical to assume that planet Earth is the only planet amongst the other 6.250.000.000.000 planets capable of supporting life forms. Further, allow me to point an equation proposed by University of California, Santa Cruz astronomer and astrophysicist Dr. Frank Drake dubbed the Drake equation. Drake used the equation to estimate that there are approximately 10,000 planets in the Milky Way galaxy containing intelligent life with the possible capability of communicating with Earth. (Boyd, Padi. "The Drake Equation". http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970924.html. Retrieved 2010-20-04).
With the resilient characteristics of life, and given evidences of how life on Earth itself has evolved and prospered in conditions unimaginable, it is highly probable that life forms may have flourished elsewhere. It may not be life as we know it is, but still, a life form.
Therefore, the question is not whether there are other life forms outside planet Earth, rather, when will we encounter those life forms. It may take years if not more for it to happen though. After all, we are practically the new kids on the block, highly unattractive to meddle with. Too primitive perhaps.
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