My guess is that they were predominantly haploid (like sperm/eggs), and just had a diploid (like most other human cells) phase for reproduction. Somewhere along the way this switched (which is thought to have happened in plants also, mosses are primarily haploid while flowering plants are primarily diploid).
...which is totally bonkers. It's like if sperm/eggs went around making all of the important decisions and only bothered to spin up a human briefly for reproductive purposes. But then the sex got a bit out of control and got a job and an apartment and now it's the sperm who look like they're just for reproduction.
Disclaimer: I took Bio 101 two semesters back, so I'm pretty much an expert.