What came first, cells or viruses?
Do humans really mark the pinnacle of evolution, or do viruses? While we’ve evolved along a pathway of ever-increasing complexity, viruses have streamlined, successfully jettisoning all but a handful of essential genes, research published in Science Advances in September [2015] suggests.
Gustavo Caetano-Anolles and his colleagues at the University of Illinois reached this conclusion after pioneering a new way to map the microbial family tree. Viruses did not evolve first, they found. Instead, viruses and bacteria both descended from an ancient cellular life form. But while – like humans – bacteria evolved to become more complex, viruses became simpler.
Today, viruses are so small and simple, they can’t even replicate on their own. Viruses carry only the essential genetic information they need to be able to slip inside a host cell and coax it into making new copies of the virus. The influenza virus, for instance, has a mere 14 protein-coding genes. Because viruses are usually so basic, many biologists didn’t think they could even be classified as a life form.
But just over a decade ago, our view of viruses began to shift. French scientists who were examining a mystery microbe that looked like a bacterium, but was genetically quite different to bacteria, realised they’d discovered a giant virus. They named this bacteria look-alike the “mimicking microbe,” or “mimivirus”.
And the mimivirus wasn’t only physically large. They showed that it carried more than 1,000 genes – a huge genome for a virus, just a few hundred genes smaller than some bacteria. Several giant viruses have been discovered since, with pandoraviruses packing around 1,100 genes.
The genetic complexity of these monster microbes reawakened interest in a longstanding question about viruses – when did they first evolve? Were viruses an evolutionary stepping stone to more complex cellular life? Or did they spring up later? The question is thorny. Being made of a few short strands of DNA or RNA wrapped in a soft protein shell, viruses don’t fossilise. And without a fossil record to study, it has been almost impossible to untangle their lineage.
To try to unpick the question of virus evolution, Caetano-Anolles developed a new way to reconstruct the microbial family tree, and retrace bacteria and viruses back to their origins. [Continue reading…]