Story highlights Microbes in our bodies greatly outnumber our human cells

In millions of years, microbe DNA has slipped into cells, diversifying our evolution, scientists say

(CNN) Evolutionary diagrams usually connect humans and monkeys with common primate ancestors, but now, scientists say there's a missing link that deserves a spot on that family tree -- our bacteria, fungi, and viruses.

Though most of our genes come from primate ancestors, many of them slipped into our DNA from microbes living in our bodies, says British researcher Alastair Crisp.

It's called horizontal gene transfer. Scientists have known of examples of this for a long time: Bacteria slip genes to each other, and it helps them evolve. And scientists have seen insects pick up bacterial genes that allow them to digest certain foods.

An artist's rendering shows a DNA double helix.

Some researchers have disputed that microbes have swapped genes with the cells of complex animals, such as humans. But a new study at the University of Cambridge indicates it has probably happened a lot.

Humans may have as many as hundreds of so-called foreign genes they picked up from microbes.

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