V1 (IMAGE) Arizona State University Caption Mysteries abound in the viral world. Scientists still aren't quite sure where they came from. The illustration describes three leading theories. According to the virus-first hypothesis, RNA molecules capable of enzymatic activity and self-replication preceded cellular forms at the dawn of life. According to the reduction hypothesis, viruses came from small primordial cells that lost their cellular elements in the course of evolution, while retaining their genetic material and the machinery required for replication. According to the escape hypothesis, viruses arose from cellular RNA or/and DNA fragments such as plasmids and transpozons. During cell fission, a smaller cell-like entity may have formed, engulfing a self replicating RNA fragment and a coat encoding RNA segment, forming a virus. Credit Shireen Dooling Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.