Spheroidal features, 50 to 200 nm in size and found in sedimentary rocks, have been described as nannobacteria. The idea that they are minute fossilized life forms—and especially the discovery of such features in Martian meteorite ALH84001—sparked a lively debate with regard to identification of ancient microbial life. Because biologists consider 200–300 nm to be the lower viable size limit for microorganisms, an alternative explanation is needed for features that have been described as nannobacteria by geologists. We report here on tissue-decay experiments that produced abundant proteinaceous spheroids in the size range of nannobacteria (described as nannoballs in the remainder of this paper). Experimental conditions were comparable to those found in Earth's surface sediments, and diagenetic mineralization of these spheroids may be a common process for preservation of nannoballs that are observed in the rock record.