Rather than the official and generally accepted theory of aquatic forms to land forms that is part of ‘evolution’ model, I’d personally see some of us (creatures from Earth) as children of the soil (ashes to ashes) and more of us as from the water (less ‘physical’/visible to those from the soil, more malleable-form changing/moving with other qualities & influence of water).
I remembered this sketch today; it’s a drawing I did for a skirt I started making for mum a while back. I didn’t finish it as the mermaid was composed of sequins and as sewing them on the fabric was time consuming and delicate other things got in the way and I didn’t finish it. Sadly I don’t have the fabric or sequins anymore but it was coming along nicely at the time. Ah well it’s the thought that counts (ha I could of finished it with more effort). Everything takes such a long time.
In ‘real life’ I’d surmise that merfolk look more like those portrayed in Harry Potter, different to the current norm but still beautiful.