As someone with a modicum of biological knowledge, I am not aware of any single cell that is critical to human survival in the way you describe. As for your an

As someone with a modicum of biological knowledge, I am not aware of any single cell that is critical to human survival in the way you describe. As for your analogy, it is true that microorganisms can perform wonderful feats, but it is a bit misleading to equate that to general intelligence. It is like saying a compiler is smarter than a programmer, because its design incorporates tricks the human may not know. (The bacteria being "designed" by the trial-and-error process of natural selection, rather than an intelligent entity, of course.)