Question: how do fish have gills or humans have lungs

Alice Morris answered on 17 Mar 2020:

I’m definitely not an expert on this, but after some reading I hope my explanation below is helpful!

The available evidence suggests that gills were present in the very earliest fish. However, lungs—gas-filled organs that serve the function of respiration—also evolved very early on. The common ancestor of many fish had lungs as well as gills. So what happened to these lungs and gills?

In some fish, lungs stuck around, and their descendants inherited them and use them to obtain oxygen. We bear evidence indicating that we once had gills but that these were lost in the course of our early evolution.

Other fish retained gills, and some of them also retained lungs. But in the fish that were the ancestors of today’s fish, lungs evolved into the swim-bladder — a gas-filled organ that helps a fish control its buoyancy.