It just seems like it would be a really cool thing to have gills and be able to populate the oceans in the same way we populate the land. We could have houses and shops and vehicles, andgo on walks/swims and just kind of live underwater.
Start a whole new second species of human here on earth maybe, Who knows?
So there isn’t a way to “add” a feature on top of the existing organism’s physical system? We have to lose ribs to get gills? That kinda sucks.
In a way, your jaw is a gill arch, just built in a different way with some interesting diversions. After a couple of 100 million years, the changes do add up.
If you really had to add in a gill, i have a plan, but I need to talk about one important evolutionary trick: duplication and divergence.
A fairly common DNA copying error causes a section of a chromosome to be duplicated in the offpring. In most cases this is fatal or prevents children, but some duplications work out just fine.
For instance mammals lost colour vision in the time of the dinosaurs - mammals were probably nocturnal. The loss was caused by losing genes for the yellow colour receptors in the eye. This is why dogs and cats see in something akin to black and white (they do see red and blue and all the yellows and greens are just shades of red and blue).
But apes were lucky. An accident duplicated the existant red receptor and, over time, because there are now two genes, one gene was gradually selected for a higher and higher light frequency. This has become our green receptor and all apes see in red-green-blue colour.
Duplication is not necessarily fatal because it just codes for something we already have. But once there are 2 genes, evolution can select away for different capabilities and we end up with something new.
Ok, with that out the way let’s plan!
Step 1 might be possible today. Step 2 might be within current reach (but it would take incredible work to disentangle all the connected system in development and the working body. Step 3 is beyond current tech (as I understand).