A crucially important part of the biosphere - the virosphere - is too often overlooked. Inclusion of the virosphere into the global picture of protein structure space reveals that 63 protein domain superfamilies in viruses do not have any structural and evolutionary relatives in modern cellular organisms. More than half of these have functions which are not virus-specific and thus might be a source of new folds and functions for cellular life. The number of viruses on the planet exceeds that of cells by an order of magnitude and viruses evolve up to six orders of magnitude faster. As a result, cellular species are subject to a constitutive 'flow-through' of new viral genetic material. Due to this and the relaxed evolutionary constraints in viruses, the transfer of domains between host-to-virus could be a mechanism for accelerated protein evolution. The virosphere could be an engine for the genesis of protein structures, and may even have been so before the last universal common ancestor of cellular life.
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