Goofs

Cat is supposedly descended from Lister's cat 3 million years earlier. There wouldn't be enough genetic diversity in such a scenario, and the increasing levels of inbreeding would have wiped the cats out long before evolving into humanoid form.

As with many space operas, Red Dwarf as well ignores the vast distances in space. The one time they attempt travelling at light speed it results in future echoes. The ship would have to be able to travel faster than light to cover the distances it does, as would Star Bug and Blue Midget. We also hear sounds in the vacuum of space which is inaccurate too.

The Cat is the last (on Red Dwarf after season one) of a species that evolved from Lister's pet cat, Frankenstein. It is sometimes stated that he evolved from Lister's pet cat. This in not inconsistent. Evolution is a process that takes place naturally over millions of years and over generation it become expressed. It is not a process that takes place on or to an individual even though an individual would be the first to contain a mutation.

However, during the show's run it is implies and sometimes stated that evolution can take place on the individual level and a mutation can happen to non-living items whether they be organic or inorganic. These concepts are not support in the evolution process or the mechanism of natural selection.

The series frequently refer to Lister as the last human alive, however, because they are so far away from earth with no contact there is no way to be sure of this fact.

However, this isn't a plot hole. It's reasonable to assume that the human race would be extinct after 3 million years, or at least so far evolved from Lister that it's unrecognisable. But even if it isn't, and Lister isn't the last human alive, this wouldn't have changed anything and wouldn't break the plot. Nothing would have happened differently if there were still other humans alive somewhere.