Humans contain ‘alien’ genes not passed on from our ancestors, researchers have discovered.The say we acquired essential ‘foreign’ genes from microorganisms co-habiting their environment in ancient times.

According to the study, humans—and a wide variety of other animals—possess tens, if not hundreds of “foreign” genes that have been passed on from single-celled organisms, such as bacteria. What’s more, these genes play active roles in the body, such as contributing to metabolism, and this process of gene acquisition could still be occurring, at least in some lineages.

‘This is the first study to show how widely horizontal gene transfer (HGT) occurs in animals, including humans, giving rise to tens or hundreds of active ‘foreign’ genes,‘ said lead author Alastair Crisp from the University of Cambridge.However, the idea that HGT occurs in more complex animals, such as humans, rather than them solely gaining genes directly from ancestors, has been widely debated and contested.

The majority of HGT in primates was found to be ancient, occurring sometime between the common ancestor of Chordata and the common ancestor of the primates.“Surprisingly,” says lead author Alastair Crisp, “far from being a rare occurrence, it appears that HGT has contributed to the evolution of many, perhaps all, animals and that the process is ongoing, meaning that we may need to re-evaluate how we think about evolution.”

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