Does life's rapid appearance imply a Martian origin?
  The hypothesis that the rapid appearance of life on Earth justifies the belief that life is widespread in the universe has been investigated statistically by Lineweaver and Davis. However, a rapid appearance could also be interpreted as evidence for a nonterrestrial origin. I attempt to quantify the relative probabilities for a nonindigenous versus an indigenous origin of life on Earth, on the assumption that biogenesis involved one or more highly improbable steps, using a generalization of the well known observer selection argument of Carter. The analysis is specifically applied to a Martian origin of life, with subsequent transfer to Earth within impact ejecta. 
P.C.W. Davies