April 21, 2008 -- Earth-like planets have relatively short windows of opportunity for life to evolve, making it highly doubtful intelligent beings exist elsewhere in the universe, according to newly published research based on a mathematical probability model.
Given the amount of time it has taken for human beings to evolve on Earth and the fact that the planet will no longer be habitable in a billion years or so when the sun brightens, Andrew Watson, with the United Kingdom's University of East Anglia in Norwich, says we are probably alone.
Earthlings overcame horrendous odds -- Watson pegs it at less than 0.01 percent over 4 billion years -- to achieve life. The harsh reality is that we don't have much time left.
No need to cash out your 401K or anything like that, but intelligent life appeared relatively late on the scene. Scientists believe the first life forms emerged four billion years ago. Humans have walked the planet for only the last 100,000 years or so.
"If we had evolved early … then even with a sample of one, we'd suspect that evolution from simple to complex and intelligent life was quite likely to occur. By contrast, we now believe that we evolved late in the habitable period," Watson said.
Earth's days are numbered. In another billion years or so, the sun will grow hotter and brighter, toasting our blue world beyond recognition.
"Earth's biosphere is now in its old age," Watson said.
"This has implications for our understanding of the likelihood of complex life and intelligence arising on any given planet," he added. "It suggests that our evolution is rather unlikely -- in fact, the timing of events is consistent with it being very rare indeed."
Watson pieced together his mathematical model by examining four critical stages that needed to occur in a particular order for intelligent life to arise on Earth. He then factored in how long a planet like Earth is suitable for habitation.
The steps include the emergence of single-celled bacteria, complex cells, specialized cells allowing for complex life forms and finally intelligent life with an established language.
"Complex life is separated from the simplest life forms by several very unlikely steps and therefore will be much less common. Intelligence is one step further, so it is much less common still," Watson said.
Based on the model, Watson said each stage of evolution had at most a 10 percent chance of occurring, and each development could manifest only after each proceeding step. Further, the evolutionary stepping-stones are spaced evenly throughout Earth's history, Watson added.
Unfortunately, by the time intelligent life reared, Earth was in its twilight years. Watson believes the same conditions would hold true on other habitable planets as well. His research is published Astrobiology magazine.
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