Galileo (17th century) -
"... we deduce with the highest degree of probability that Jupiter is inhabited." 

Percival Lowell (early 20th century) -
"That Mars is inhabited by beings of some sort or other we may consider as certain as it is uncertain what those beings may be."

Orson Welles War of the Worlds (30 Oct. 1938)

Search for origin of life on Earth is difficult because geological processes have erased the evidence.