McKay and his collaborators build the case for life on four lines of evidence. These multilayered formations, called carbonate rosettes, tend to have cores rich in manganese, surrounded by a layer of iron carbonate and then by an iron sulfide rind. Bacteria in ponds can produce similar rosettes as they metabolize minerals. The second line of evidence centers on the discovery of organic compounds called polycyclic aromatic hydrocarbons, or PAHs, in and around the carbonate. Richard N. Zare, a Stanford University chemist and co-author of the Science paper, reports that the rock contains an unusual mixture of certain lightweight PAHs.
Critics suggest other possible explanations, however.
Hundreds of organic compounds have been identified in it, including amino acids and compounds closer to the things organisms actually use. The authors note that certain bacteria manufacture broadly similar magnetite and iron sulfide crystals. Joseph Kirschvink, a biomineralogist at the California Institute of Technology, agrees that the mineral formations are intriguing. I don't know of anything else that can make crystals like that. Shock remains unconvinced.