gene sequences hidden within samples are so unusual they seem to have come from organisms that are only distantly related to cellular life as we know

So distantly related, in fact, that they may belong to an organism that sits in an entirely new domain.

Until the 1990s, life had just two branches:

one for eukaryotes – animals, plants, fungi and some other strange forms, including the slime moulds –

and one for everything else.

Then, gene analysis revealed that the "everything else" branch could be divided into two domains: bacteria and archaea.

Biology's 'dark matter' hints at fourth domain of life - life - 18 March 2011 - New Scientist