(Now we understand that these seasonal variations are due to changing patterns of dust storms, polar cap changes, etc -- not life. Oh well...)

In 1877, Giovanni Schiaparelli observed Mars at opposition and observed streaks and linear marks which he called canali ("channels").

Percival Lowell (1890s) took this one step further and proclaimed them canals -- artificial constructs of alien civilizations.
 

(Lowell was good at finding canals -- he even found them here.)

Now, there are lots of problems with this idea, even in Lowell's time. But  like all of human history, people remember the interesting bad science, and ignore the boring good science -- life on Mars became part of the popular culture.