Excellent question Leah. And that type of thought is exactly what we scientists do! Though we now think that land-based animals with backbones (“chordata”) originally came from fish who evolved the ability to walk on land. This may have led to amphibians and reptiles, which then led to us mammals (including monkeys).
So, scientists believe that fish evolved over a long period of time from things without true backbones or true bones at all in fact. Something along the lines of modern day eels called lancelets (“amphiouxus”). In order to swim forwards they needed something rigid for the muscles to pull against, which is why the backbone evolved, as those who could move forward were slightly better at catching food than those who couldn’t. Before lancelets, these were believed to come from something similar to modern day jellyfish, so not round.
So there you have it, complicated eh!? But it took sooooo long and there are still so many missing links in our knowledge that we might just find a different story soon!