>be cartilaginous fish
>need to be more resistant to predators
>first strategy is to have armored skin
>but it's too heavy and not maneuverable
>second strategy is to reinforce skeleton with calcium
>now be bony fish
>but skeleton is now heavier
>now need stabilizing organ
>introducing swim bladder
>gulp air and store it in a sack so buoyancy and gravity stabilize you underwater and allow you to stay close to the surface with less effort
>evolve capacity to absorb O2 through swim bladder
>can now survive in stagnant, O2-depleted water
>evolve actual limbs supporting fins to resist current better
>eventually limbs are strong enough to crawl outside
>start eating dead animals on the shore
>it's safer because predators are in water
>get better at it
>evolve pelvis, stronger limbs, internal choana
>need to stay in water less and less
>lose gills because useless now
>evolve thicker skin to retain water better
>can now venture far from water
>evolve dicks so you can cum inside babes instead of boring external fertilization
>can now fuck outside water
>evolve egg shells that retain water
>no need to stay near water even when making babies
All than from 425 M to 315 M BCE.
