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Gills help fish breathe underwater. Gills are made up of many tiny filaments (like little fingers) that absorb oxygen from the water and help the fish get rid of carbon dioxide. The reason why fish can breathe underwater is that water contains dissolved gases, including oxygen, that the gills can absorb.
When fish are out of water, they effectively suffocate (or drown). All the filaments in their gills clump together and can’t absorb oxygen from the air.
Gills don’t work out of water, so humans and other land-dwelling animals have lost their gills during the evolutionary process. Funny enough, embryos of humans, other mammals, birds and reptiles have gills at an early developmental stage, but these seal up and disappear before babies are born.
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Fish have gills because they need them to survive underwater. Gills absorb oxygen from the water just like we absorb oxygen from the air. At some point way back in time, the ancestors of fish needed to be able to breathe underwater, and those that evolved gills were able to. Therefore these fish survived, and had more fish babies with gills. Fish today still have gills because they still need them.
Humans don’t have gills because we moved onto the land and no longer needed to breathe underwater. Through evolution our gills were lost, or changed into something else inside our body, because we didn’t need them to survive. It’s also possible that having gills on the land was a negative trait (made you more likely to die) so a human ancestor without gills was more likely to survive, having more children without gills.
People used to think that as a human embryo developed it went through previous stages of evolution, resembling a worm, then fish, then amphibian (frog), before looking like a human. It was thought that in the ‘fish stage’ these embryos had gills just like a fish. It’s now known that this wasn’t entirely correct. During development a human embryo does have four ridges and grooves in the neck which look a bit like gills. They’re not made from the same type of cells as fish gills though, and they are not involved in breathing. These ridges develop into muscles of the face and bones of the ear. It’s thought that the gills were internalised (moved inside our head) and developed new functions, such as an involvement in hearing.
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