According to Professor Brian Cox, Earth could be "the sole spot" in the cosmos containing intelligent life forms capable of forming civilisation.
While microbial life forms may be abundant, the physicist argues that "something like us" are extremely rare.
When asked about US Congressman Andre Carson's remarks earlier this week that UFOs are a "possible national security concern" that are "real" and "need to be studied," he said, "
Prof. Cox described the discovery of intelligent alien life forms on Earth as "one of the big mysteries."
'If you forced me to speculate, I'd say there may be bacteria all over the place, which is why we're looking for life on Mars,' he said on BBC One's Sunday Morning.
But, in terms of intelligence, consider the origin of life on Earth. It appears that we have excellent evidence that life existed 3.8 billion years ago and that the first civilisation appeared around now.
'It took nearly four billion years from the beginning of life on Earth to the emergence of civilisation.'