 In the early 1970s, Crick and Orgel further speculated about the possibility that the production of living systems from molecules may have been a very rare event in the universe, but once it had developed it could be spread by intelligent life forms using space travel technology, a process they called "directed panspermia".In a retrospective article, Crick and Orgel noted that they had been overly pessimistic about the chances of abiogenesis on Earth when they had assumed that some kind of self-replicating protein system was the molecular origin of life.In 1976 Crick addressed the origin of protein synthesis in a paper with Sydney Brenner, Aaron Klug, and George Pieczenik.