Structurally complex life, particularly sentient beings, are very rare

Professor Andrew Watson

"I think we can conclude from this that we are probably effectively alone in the universe."

He said complex organisms took a long time to develop on Earth, not appearing until a good fraction "through the likely lifespan of the planet".

"This is consistent with the notion that structurally complex life, particularly sentient beings, are very rare - and most planets never reach that stage of evolution," said Professor Watson.

"On the rare occasions when sentient life does arise, it will almost always find itself awakening towards the end of the life of the biosphere in which it has arisen."

Bacteria evolve

He said there was more hope of finding bacterial life.

"We know that simple life was established on Earth probably as soon as it became habitable.

"This suggests that bacteria evolve relatively easily from inorganic matter, or that they were 'seeded' from elsewhere.

"In either case, it is reasonable to assume this could also have happened on other planets."

Professor Watson believes planets such as Mars, which once had water on its surface, may well have had some simple life forms such as bacteria.