The pathogen which caused SARS was identified as the coronavirus but where had the pathogen come from.Scientists suspected that the virus had been in a different species and then had crossed into humans. Versions of the virus were found in horseshoe bats and civet cats. The genomes of the virus found in bats, civet cats and humans were sequenced.Viruses evolve rapidly. By looking at how different the genome is in the different animals we can create an evolutionary tree (phylogeny) which shows how the virus evolved over time.

The tree in the diagram shows that species F and G share a recent common ancestor (they evolved to be different species from the same species) . They also share a recent common ancestor with species E but that was further back in time. Even further back in time we can see that species C,D,E,F and G all share a common ancestor