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From Matt Palmer

You say viruses are “as old as life itself, if not older” (8 May, p 16).

I thought that the definition of a virus was essentially that its genetic material relies on other organisms to reproduce. So how did these early viruses manage to pre-date life itself, before there was a convenient organism to hijack in order to reproduce?

The editor writes:

• It all depends on what you mean by “life”. Viruses could have piggybacked on even the incomplete metabolisms of pre-cellular, er, life.

Richmond, Surrey, UK