Viruses are living entities that share a long evolutionary history with cells, says a new study that traces viral evolution back to a time when neither viruses nor cells existed in the forms recognised today.
Viruses have been difficult to classify, till now. Part of the confusion stems from the abundance and diversity of viruses.
Less than 4,900 viruses have been identified and sequenced so far, even though scientists estimate there are more than a million viral species.
The new study focused on the vast repertoire of protein structures, called "folds" that are encoded in the genomes of all cells and viruses.
Folds are the structural building blocks of proteins, giving them their complex, three-dimensional shapes.
Using advanced bioinformatics methods, scientists identified 442 protein folds that are shared between cells and viruses, and 66 that are unique to viruses.
"Viruses also have unique components besides those that are shared with cells," said study leader professor Gustavo Caetano-Anolles from the University of Illinois at Urbana-Champaign.
In fact, the analysis revealed genetic sequences in viruses that are unlike anything seen in cells, Caetano-Anolles said.
The data also suggested that at some point in their evolutionary history, most viruses gained the ability to encapsulate themselves in protein coats that protected their genetic payloads, enabling them to spend part of their life-cycle outside of host cells and spread, the researchers said.
Some scientists have argued that viruses are non-living entities, bits of DNA and RNA shed by cellular life.
They point to the fact that viruses are not able to replicate (reproduce) outside of host cells, and rely on cells' protein-building machinery to function.
But much evidence supports the idea that viruses are not that different from other living entities, Caetano-Anolles said.
"Many organisms require other organisms to live, including bacteria that live inside cells, and fungi that engage in obligate parasitic relationships -- they rely on their hosts to complete their lifecycle," he said.
"And this is what viruses do," he said.
The findings appeared in the journal Science Advances.
