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Evolution Getting Faster Thanks to Germs, Viruses, Study Says

Brian Handwerk
for National Geographic News
March 5, 2007
 
Viruses and bacteria have sped up the process of evolution by rapidly transferring DNA from one species to another, a new study suggests.

Gene-mapping projects over the past decade have already shown that genes can move between species via tiny microorganisms.

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.

(See a National Geographic magazine feature on Darwin's theories of evolution.)

Why did the pace of evolution increase so dramatically?

Writing in the January 29 issue of the journal Physical Review Letters, Deem's team says that HGT may have played a significant role in allowing early multicellular organisms to develop into the living things we know today.

"The simple organisms are like the building blocks, the Legos," Deem explained.

"Once you have enough Lego pieces, you can make a more interesting structure fairly easily. It may have taken three billion years to make the pieces, but once you have the pieces, then you can put them together in many different ways."

The modular structure of genes has made them ideally suited for HGT to transfer desirable traits between species, he added.

Not everyone agrees how prominent a part this gene-swapping process has played in the evolution of larger organisms like plants and animals.

But HGT is likely responsible for the rapid evolution of microscopic organisms, said Otto Berg, a professor of molecular evolution at Sweden's Uppsala University who was not affiliated with the study.

"The conclusion that recombination [of genes] and HGT can speed up evolution, at least for microbes, will probably not be contested," Berg said.

HGT may become more prominent when species are put under heavy survival pressures, he added.

For example, when bacteria are exposed to antibiotics, they must become resistant or die. Any traits that bestow resistance to the drugs may therefore become widely transferred among species.

"When a population meets a new environmental challenge or a new ecological niche to exploit, [genetic] selection for new functions can be very strong, and HGT can play a crucial role," Berg said.

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