 Sterain molecular fossils with side-chain methylations have not been reported in the bacterial domain. One such sterain, 24-isopropylcholestane, has been attributed to ancient sponges, potentially representing the earliest evidence for animals on Earth. This study demonstrates that symbiotic bacteria are capable of producing the 24-isopropyl sterile side-chain, suggesting that bacteria should not be dismissed as sources of these biomarkers in the rock record. This article was authored by Mallory O. Brown, Barbatund O. Olegundu, Jose Luis Jinnah, and others.