The Wiki article Life on Mars states the following:
The primary mission of the Viking probes of the mid-1970s was to carry out experiments designed to detect microorganisms in Martian soil because the favorable conditions for the evolution of multicellular organisms ceased some four billion years ago on Mars.
Is it generally believed that multicellular life could exist within the confines of the Solar System (outside of Earth, of course)? Or is there consensus that any life that could possibly exist would be unicellular, similar to extremophiles on our own planet?