Earth is teeming with life miles beneath the surface, scientists have discovered, leading to speculation that our distant ancestors may even have evolved deep underground.

Researchers at the Deep Carbon Observatory (DCO) said they had found barely-living ‘zombie’ bacteria and tiny worms, inhabiting entirely new ecosystems more than three miles into the crust.

The lifeforms are so numerous that their mass may be up to 385 times that of all humans.

And some are so odd and striking, living for millions of years without replicating, that scientists may need to rewrite the fundamental concept that all cellular life can be divided into three domains of archaea, bacteria, and eukaryote.

“They are not Christmas ornaments, but the tiny balls and tinsel of deep life look they could decorate a tree as well as Swarovski glass,” said Dr Jesse Ausubel, of The Rockefeller University, a founder of the DCO, which is made up of dozens of international researchers.

“Why would nature make deep life beautiful when there is no light, no mirrors?"