The devolution virus didn't do something as simple as "make everyone into primates."
DATA: I have analyzed Commander Riker's DNA structure. A synthetic T-cell has invaded his genetic code. This T-cell has begun to activate his latent introns.
DATA: They are genetic codes which are normally dormant. They are evolutionary holdovers -- sequences of DNA that provided key physical and behavioral characteristics millions of years ago, but are no longer necessary. (beat) For instance, Counselor Troi's gill slits and other amphibious characteristics were derived from introns which still contain amphibious codes.
PICARD: So these... introns are causing her DNA to re-combine in an earlier configuration?
DATA: That is correct. In her case, the DNA is creating an amphibious lifeform which became extinct over fifty million years ago.
They weren't implying that the primate version of Betazoids (or half-betazoids) were themselves amphibious, although you could perhaps interpret it that way, but that a life form fifty million years old was being recapitulated. They go on to make it more clear:
Data: Each of these stages is another link in the evolutionary chain, stretching back to the origins of all lifeforms on Earth. (beat) Because introns can include genetic material from many different species over millions of years of evolution, it is possible that a wide variety of transformations is occurring among the crew.
PICARD: What about crewmembers who are not from Earth?
The last line I include because it makes it clear, they're not just saying "Hey, because we've got a lot of aliens, we can get some pretty weird things", they're saying even among Earthlings, a wide variety of transformations were possible, as long as humanity had some introns from species like them. In Barclay's case, it made him something Spider-like.
Whether humanity actually has a spider-like creature in their evolutionary history (or whether introns can pass laterally between species) is a question in itself, but it seems like it may be somewhat arbitrary to be watching a story about a virus that makes people devolve into random creatures by activating introns to then to stop and complain your disbelief just because spiders themselves aren't in our evolutionary line.