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Ignoring the tails humans already have.

Let's say it's the year 2016. Merpeople exist. They live in the ocean or rivers, breathe air, and have to surface occasionally. They also evolved from earlier humans.

They're pretty diverse, maybe as far as having different subspecies of merfolk.

Under intense selective pressure, how long ago would merpeople have to diverge from landpeople to develop tails, from legs, fit for swimming?

Side question: what would they look like compared to fantasy version of mermaids?

Edit: Being in the genus "homo" counts as human. Merpeople do not necessarily need to be in this genus at the "end" of their evolution, but must for the start

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    $\begingroup$ I would strongly doubt that humans ever could evolve into merpeople. Simply put, people are smart and developed tool use and the ability to use things like logs to float with a very long time ago. Rather than being forced into an environment which we literally can't survive in for more than 5 minutes without aid, I can't help but think the people under evolutionary pressure would instead build tools to relieve the pressure or move away from it. It would improve the answer about what people would look like if you could describe what this pressure was and what area of earth it occurred in. $\endgroup$
    – GrinningX
    Sep 7, 2016 at 20:12
  • $\begingroup$ I would think it would be much easier to go the other route: aquatic mammals like seals or sea otters that evolve hands, intelligence, and language. $\endgroup$
    – jamesqf
    Oct 10, 2016 at 17:28
  • $\begingroup$ @jamesqf seems a lot more improbable to me that they'd end up looking like people $\endgroup$
    – One Normal Night
    Oct 10, 2016 at 18:57
  • $\begingroup$ @One Normal Night: Well, they would BE people, just not people descended from humans, or proto-humans, depending on how long ago you think that evolutionary split would be. But then, human-descended merpeople wouldn't be likely to look much like land-based humans, as the evolutionary pressures of aquatic life would be much different. Kind of the opposite of why dolphins and ichthyosaurs look so much alike. $\endgroup$
    – jamesqf
    Oct 11, 2016 at 4:04

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Look at the evolution of Cetacea, which illustrates that adapting to water is rapid.

clade

Where “rapid” means 50 million years.

Now you can imagine starting with apes rather than horses and get the same kind of body radical reshaping. But humans have a very shallow gene pool, having been through bottlenecks. Perhaps that’s why we developed intelligence instead of just adapting to new niches like other animals! So humans in there present form would need more time and large populations to significantly evolve at all.

I don’t know what the rate is for acquiring such diversity. But I would suppose the scale of millions of years would do it.

So I would suggest 60 to 80 million years as a final answer.

Ah, but you asked how long ago for it to be finished now. Well, that’s before a genetic bottleneck so nevermind that complication. But, nothing remotely human was around 50 million years ago! The earliest bipedal hominins were 7 or 8 million years ago. There were barely monkeys that far back.

How would they look? Like seals with useful hands that work as paddles or steering mechanisms as well as having opposable thumbs and some reach. Maybe the front limb folds up like a birds wing, to form a flipper with the elbow at the end and the hand tucked under the armpit.

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Humans are tough work with when it comes to evolution.

The main problem being that as humans we have effectively removed ourselves from natural selection.

For your scenario to happen at all, the world we find ourselves in would have to drastically change.

  • Scenario wise you are looking at a world dominated by water. There is no reason to turn into a mer-person if you aren't going to live in water.

  • Humans would have to regress dramatically. Intelligence is the weapon of humanity. We don't evolve so much as use our brains to keep us out of trouble. For Darwinian selection to get us to the point of turning into dolphin people we would have to lose our ability to survive via our wits.

  • Given all that is true, evolution is still a really really slow process. Additionally for natural selection to be the way we get there the environmental changes would have to take millions of years to come into effect. Natural selection cannot cope well with rapid environmental change

So in short, you are talking about gradual environmental change combined with mental regression (somehow) leading to humans that now have dolphin tails (since we are both mammals). At best I would ballpark things at around 100 million years but that is a very very rough guesstimate.

Ironically if things change this much...these aren't really even humans anymore.

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  • $\begingroup$ You don't need a world dominated by water. Just inescapable water. Like a very slowly sinking island. And I am not necessarily talking about modern humans. My post says 'by 2016' there is a diverse population of merfolk, who may or may not be technically classified as human $\endgroup$
    – One Normal Night
    Sep 7, 2016 at 20:33
  • $\begingroup$ Natural selection can cope quite well with rapid environmental change, but it usually involves a replacement of species rather than an adaptation within a species... $\endgroup$
    – user
    Oct 10, 2016 at 15:45
  • $\begingroup$ Humans really have not removed themselves form natural selection. Natural selection is still working just fine on humans. $\endgroup$
    – John
    Jun 18, 2019 at 10:41

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