Now a team of scientists at Texas' Rice University believes that microbes are accelerating evolution by constantly transporting whole chunks of DNA that may represent new and beneficial functions�like resistance to disease.

This process�called horizontal gene transfer (HGT)�may allow life-forms to evolve more quickly than they would by occasional, random mutations alone, the scientists say.

"We know that the majority of the DNA in the genomes of some animal and plant species�including humans, mice, wheat and corn�came from HGT insertions," said Michael Deem, a genetic engineer at Rice, in a press statement.

"For example, we can trace the development of the adaptive immune system in humans and other jointed vertebrates to an HGT insertion about 400 million years ago."

"Once [viruses and bacteria] find a useful protein or gene, it can be transmitted to more complex species by [this process]," Deem told National Geographic News.

"I think this is the main mechanism by which dramatically new function evolves."

Evolution in Overdrive?

Evolution, as most scientists understand the process, has been getting faster and more complex over time.

Fossil records indicate that single-celled organisms appeared on Earth some 3.5 billion years ago. It took a further 2.5 billion years for the first multicellular life-forms to evolve.

But over the next billion years, those first multi-cellular organisms evolved into the staggering diversity of plant and animal life found on modern Earth.