In March I wrote about two studies that raised the tantalizing possibility that the tree of life, which till now has appeared to have three main branches, turns out to have a fourth.

Some of the evidence for the fourth branch (or “domain,” as taxonomists would call it) came from a newly discovered and very strange group of viruses. They’re known as giant viruses, because they’re about a hundred times bigger than typical viruses and can have over a thousand genes. If there was indeed a fourth domain , it meant that giant viruses were part of one of the oldest lineages on Earth. By studying them we might learn about the earliest stages in life’s evolution.

Since then, there have been a couple developments that merit a follow-up. In April, Didier Raoult of Mediterranean University in Marseille and his colleagues published a new study on another species of giant virus. Their previous studies on the fourth domain involved giant viruses that were first discovered in the water in air conditioners, infecting amoebae called Acanthamoeba. But now scientists are finding giant viruses all over the world, in lots of ...