There may be many Earth-like planets, but few with intelligent beings
The odds of intelligent life arising on another Earth-like planet are low, a British scientist has calculated.
He argues that humans evolved via a series of four "critical steps" - the likelihood of all of which occurring on one planet is less than 0.01%.
Discoveries of new planets outside the Solar System has boosted the search for extra-terrestrial intelligence.
Professor Andrew Watson has published his findings in the academic journal Astrobiology.
"Complex life may be a rare phenomenon, observers rarer still," he wrote.
We may have to discover tens of thousands of Earth-like planets before we find one which harbours sophisticated organisms, according to Professor Andrew Watson, from the University of East Anglia.
The reason is that the "habitable lifespan" of an Earth-like planet - estimated at five billion years - will rarely be long enough for complex life to evolve.
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"We now believe that we evolved late in the Earth's 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," he says.
"This has implications for our understanding of the likelihood of complex life and intelligence arising on any given planet."
'Billion years left'
Models of future global temperature suggest that, due to the increasing solar luminosity, the future life span of Earth will be "only" about another billion years - a short time compared to the four billion years since life first appeared on the planet.
Previous models are founded on the rationale that intelligent life on Earth emerged from a sequence of unlikely "critical steps".
Prof Watson identifies four - the emergence of single-celled bacteria; complex cells; specialised cells allowing complex life forms; intelligent life with an established language.
He estimates that the probability of each of these "critical steps" occurring in relation to the lifespan of Earth is no more than 10%.
Thus, the chances of intelligent life on any given Earth-like planet is tiny - less than 0.01% over four billion years.
So is there any hope for ET? Optimists point out that with 100,000,000,000 stars in our galaxy alone, there may be many thousands of Earth-like planets - enough to make these 0.01% odds look quite promising.
But even then, we must be cautious, says Prof Watson. "The view that evolution involves a predictable progression, such that the emergence of intelligence is inevitable, is today generally considered to be overly anthropocentric.
"Any directionality to evolution; and, in general, the kind of outcome seen on Earth may be vanishingly unlikely.
"On the other hand, the rapid establishment of life on Earth after its formation may indicate that simple microbial life is relatively common."
Prof Watson completed his PhD under the supervision of James Lovelock, author of the Gaia hypothesis, "whose view of the earth as a whole system has influenced me ever since".
His model has echoes of the Drake equation, a formula for predicting the number of civilizations in our galaxy with which we might hope to be able to communicate.
The answer depends on the fraction of stars in our galaxy which have planets that can support life, the time it would take for them to release detectable signals into space, and other variables.
Based on the values used by Frank Drake and his colleagues in 1961, the number of detectable civilisations was ten, in the Milky Way alone.