Page last updated at 09:57 GMT, Thursday, 17 April 2008 10:57 UK

Odds on finding aliens 'too low'

Roswell incident
The possibility of alien life has provoked excitement for centuries

Scientists have counted out the chance of finding advanced alien life during the life-time of the Earth.

Prof Andrew Watson at the University of East Anglia has produced a mathematical model suggesting the odds of finding new life on other planets are too low.

Findings are based on the time it has taken for humans to evolve compared with the remaining life span of Earth.

Intelligent life evolved late on Earth through several difficult evolutionary steps, the professor suggests.

According to Professor Watson, from the School of Environmental Sciences, solar models predict that the brightness of the sun is increasing.

Temperature models also suggest the future life span of Earth will be about another billion years which is too short a time compared to the four billion years since life first appeared on the planet.

"The Earth's biosphere is now in its old age and this has implications for our understanding of the likelihood of complex life and intelligence arising on any given planet," said Prof Watson.

"At present, Earth is the only example we have of a planet with life.

Unlikely steps

"If we learned the planet would be habitable for a set period and we had evolved early in this period, then we'd suspect evolution from simple to complex and intelligent life was quite likely to occur.

"We now believe we evolved late in the habitable period, and this suggests that our evolution is rather unlikely. In fact, the timing of events is consistent with it being very rare indeed."

Prof Watson suggests four numbers of evolutionary steps are needed to create intelligent life in the case of humans.

"These probably include the emergence of single-celled bacteria, complex cells, specialized cells allowing complex life forms and 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," said Prof Watson.

His model, published in the journal Astrobiology, suggests an upper limit for the probability of each step occurring is 10% or less so the chances of intelligent life emerging is low - less than 0.01 per cent over four billion years.

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