Viruses have a huge impact on our lives, and we're making great strides into understanding how to protect ourselves from the flu and HIV. But one thing that scientists have struggled to agree on is whether or not viruses are alive. After all, they can't survive or replicate without a host cell, and due to their rapidly changing genes, scientists have never been able to work out how or when they evolved.

Now a study by researchers in the US has managed to complete the first viral tree of life, and it suggests that not only are viruses alive, they're also really, really old, and they share a long evolutionary history with cells. "Viruses now merit a place in the tree of life," lead researcher Gustavo Caetano-Anollés said in a press release. "Obviously, there is much more to viruses than we once thought."

The confusion about viruses is a result of their unique life cycle. Specifically the fact that they can't metabolise nutrients, and they don't contain the proteins needed to copy their own DNA and RNA - instead they invade other animals' cells and use their equipment to do it for them. This has led some scientists to argue that viruses are merely non-living strands of DNA and RNA taken from other cells, enclosed in a neat little protein envelope.

To make things even more complicated, some viruses have incredibly low numbers of genes, including Ebola, which does all its deadly damage with just seven genes. While others, such as the recently discovered giant viruses, have more genes than bacteria.

Attempts have been made to map out how all these very different types of viruses evolved, but because they replicate so many times within each host, their genes end up mutating rapidly and often get mixed in with their host's genes, so it's a pretty impossible task.

The new study gave up on that idea, and instead looked at something called protein 'folds', which are the structures that give proteins their complex, 3D shapes. These folds are far less likely to change than viral genes, because they maintain their structure even if the genetic sequences that code for them begin to change.

After analysing the folds in 5,080 organisms and 3,460 viruses, the researchers found that viruses and modern cells share 442 protein folds, and only 66 are virus-specific. But those 66 are unlike anything seen in cells, which contradicts the hypothesis that viruses simply took all their genetic materials from cells.

This information allowed them to build a rough tree of life, which showed that viruses share a common ancestor with modern cells, but are more ancient.

"Viruses originated from multiple ancient cells … and co-existed with the ancestors of modern cells," the researchers write in Science Advances. 

Of course, this doesn't mean that viruses suddenly fit neatly into our definition of life. But the researchers suggest that the evidence is strong enough that we may simply need to expand our understanding of what it means to be 'alive'.

"Viruses are living," Caetano'-Anollés' graduate student on the project told Jennifer Viegas from Discovery News. "They simply have an atypical mode of living that is slightly different from ours. They are not fully independent. Instead, they move in and out of our bodies, stealing the resources and producing their offspring. In short, we need to broaden how we define life and its associated activities."