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Humans might be the last species to evolve in the universe, a Harvard astrophysicist has claimed. 

Harvard astrophysicist Abraham Loeb said when the universe was 15 million years old, the temperature of the cosmic microwave background (CMB) was nearly same that of a warm summer day on Earth. If rocky planets existed at the epoch, then possibly the CMB kept its surface warm without residing in the habitable zone around parent star. 

"When the universe was 15 million years old, the cosmic microwave background had a temperature of a warm summer day on Earth," Loeb told space.com. "If rocky planets existed at that epoch, then the CMB could have kept their surface warm even if they did not reside in the habitable zone around their parent star."

Loeb added conditions such as huge stars exploding and emitting heavy elements might have also prevailed resulting into various planets existing today. These planets might have enjoyed the warmth of cosmic microwave background radiation. 

He asserted that only in the presence of water, life forms may have existed. 

Other researchers have also extended their support for the proposed theory by Loeb.

"In our field, it has become traditional to adopt a definition of a 'potentially habitable' planet as one that has a solid surface and a surface temperature conducive to liquid water," said astrophysicist Joshua Winn of the Massachusetts Institute of Technology, according to space.com. 

"Many, many papers have been written about the exact conditions under which we might find such planets - what type of interior composition, atmosphere, and stellar radiation field. Avi has taken this point to a logical extreme, by pointing out that if those two conditions are really the only important conditions, then there is another way to achieve them, which is to make use of the cosmic microwave background."