Directed Panspermia
In the 1960s through the 1980s, Crick speculated on extraterrestrial life and
the possibility that life elsewhere was deliberately placed here ages ago:
"Directed Panspermia†by Francis Crick and Leslie E Orgel in Icarus (1973) Volume 19 pages 341-346.
http://profiles.nlm.nih.gov/SC/B/C/C/P/_/scbccp.pdf
Crick later wrote a book about directed panspermia called Life Itself (Simon & Schuster, 1981) ISBN 0671255622
See also:
http://en.wikipedia.org/wiki/Francis_Crick