This pessimism is founded on the fact that life emerged on Earth relatively late in the age of the biosphere.

“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 Professor Andrew Watson of the School of Environmental Sciences at the University of East Anglia.

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

"If we learned the planet would be habitable for a set period and that we had evolved early in this period, 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, and this suggests that our evolution is rather unlikely."

"In fact, the timing of events is consistent with it being very rare indeed.”

Professor Watson has identified four key stages in the development of life on earth: single-celled bacteria; more complex cells; specialised cells allowing complex life forms; intelligent life using language.

He estimates that the probability of each of these stages occurring during the four billion year life-span of the Earth is no more than 10%.

Therefore the probability of intelligent life emerging is low -- less than 0.01% over four billion years.

“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.”