No mermaids.

In very broad strokes, some fish started going on land, these were the ancestors of amphibians. Some amphibians developed scales and eggs that didn't need to be in water, these were the ancestors of reptiles. Some reptiles became warm blooded and maybe developed fur instead of scales, proto-mammals. Some proto-mammals started producing a nutritious secretion from pores on their stomachs, monotremes. Some of these monotremes developed nipples, pouches and started giving birth to live, but underdeveloped young instead of laying eggs, marsupials. Some marsupials developed more stable placentas that allowed more mature young and longer pregnancies, they also lost the pouches; These were the placental mammals.

From there, you had little rodent like things that started living in trees became more like early primates, then became early monkeys, then old world monkeys, then apes then something chimp-like, then australopithecus, then other human ancestors, then humans and our cousins the Neanderthals and Denisovans and probably others, and after everyone either interbred with us or died off, we were basically modern humans. At no point were mermaids involved.

Do you want to go back to being an amphibian? Not likely, but more realistic than trying to be a merperson.