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