Even the most conservative scientists would have told you they thought
Mars had at least some primitive plant life on it. Less conservative
ones opted for animals. A few even held out for at least the ruins of
an ancient civilization on the Red Planet if not actual Martians themselves.

Others thought Venus was covered in an ocean of liquid water (the
Soviets built their first Venera landers with the ability to float in that
perceived alien sea, that is how serious the idea was taken in some
quarters) and a few still held on to the idea of Venus as one big swamp.

The US seriously considered exploding an atomic bomb on the lunar
surface for several purposes. One of them included digging up lunar
microorganisms from deep underground or at least their remains.

The early Space Age practioners thought it would be relatively easy to
find alien life on those worlds. That's one reason why Viking was
equipped with a scoop instead of a drill. And it still boggles my mind
that they did not provide those landers with better ways to determine
the makeup of the planet's surface to help differentiate between the
organic and inorganic materials. This is why we have had three
decades of puzzlement and fights over exactly what Viking found.

Regarding SETI, once it was acceptable to look for aliens via radio
signals, the scientists who first participated in this endeavor thought
it would be relatively easy to pick up ETI transmissions. They thought
there was a rather large galactic society out there constantly signalling
to each other (the bad guys had destroyed themselves before they
could leave their planets and wreck havoc with the more peaceful and
sophisticated members of the Milky Way) and we just needed to tune
in to the right frequencies to listen in and eventually become club members.

It is not that the Universe has changed in five decades, it is our
perceptions about what is out there.

I also note a fair amount of disappointment that alien life signs are not
more obvious, to say nothing of the fact that now we are aware of
the concept of others in the space and have at least the basic
technology to listen and talk to them, that they haven't been waving
giant red flags to get our attention. And those darn alien spaceships
that supposedly keep buzzing our planet - not one has landed on the
White House lawn to say Hello, or at least threaten us with destruction
if we don't clean up our acts real soon.

The point is, it's still early in the game. Most of our SETI programs
have been sporadic until recently and it has only been about a decade
now that government threats and acts of budgt cutting to those
programs were finally circumvented. In addition, the mainstream SETI
community began getting serious about looking for signals other
than radio only about ten years ago.

As for looking for life on worlds in this system, we haven't done
anything serious in that regard since Viking (the rovers looked for
water, not life) and MSL has been pushed out a few more years.
I don't expect to see a lander to any of the potentially interesting
Jovian moons for at least several decades. So as I said, we've made
a start but we have a lot to do and a long way to go before saying
we're the only ones in town.