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.
There are other ways to get those shapes. The final thread of evidence has drawn the sharpest attacks. Many scientists are unconvinced that such organisms ever existed on Earth, let alone elsewhere. There is also a real danger of an observer effect at work. Moreover, the putative Martian bacteria are hardly larger than the mineral crystals they are supposed to have produced.