We tend to think that viruses are bad things that can only cause us harm. But new evidence suggests that in our ancient past, infections of some viruses may have been the spark that led to the splitting of bacteria from eukaryotes.

It turns out that a family of giant viruses called Marseilleviridae share an intriguing set of similar genes with eukaryotes, the group of organisms that contains us. Analyzing the genomes of the viruses revealed that they code for a set of proteins known as core histones. These are a type of protein around which DNA winds, not unlike how thread is wound around a spool.

“When I saw this, it was wild,” said Albert Erives, who carried out the research, published in Epigenetics & Chromatin. “No one has ever seen a virus with histones.”

Histones play a key role in the packaging of DNA into the cell, but while all eukaryotes (and a few Archaea) have histones, they are not found in bacteria. All eukaryotes studied so far have the same core genes that code for these proteins, with very little variation. The fact that they are so highly conserved even between such distantly related organisms as fungi and chimpanzees hints at the crucial role that they play.   

This means that somewhere between eukaryotes and bacteria on the evolutionary tree some 2 billion years ago, early life evolved histones to make the packing of information in cells more efficient. This might be where the giant viruses come in.

“It's exciting and significant to find a living family of giant viruses with eukaryote-specific genes in a form that predates the latest common ancestor of all eukaryotes,” explained Erives, an associate professor at the University of Iowa. “These viruses are like time machines that tell us more about how life on our planet came to be.”

This suggests that the Marseilleviridae fit in somewhere along our evolutionary pathway. While the genes found code for the core histones known as H2B-H2A and H3-4, the ones that are found in all eukaryotes, there are some slight differences, mainly in that the viral versions are fused as dimer proteins.

There are two possible ways in which these giant viruses have ended up with the genetic blueprint for these essential proteins stamped into their genetic code. The first is that perhaps while infecting some proto-eukaryote over 2 billion years ago, the viruses picked up the early genes for histones.

But the other, more intriguing possibility is that maybe these viruses are the origin of the histones in the first place, and that after sharing them with their host, one early lineage was given the tools to branch off and become eukaryotes.