Could a species of marine mammal evolve from humans?

Question

I want to write a science fantasy novel about humans evolving into five new species. The five races are: the Groses and Grosettes (they are my Ogres) (their name comes from the french word gros/grosse that means fat, big, large, overweight or obese) (I am a francophone from Quebec), the Dwarves, the Giants and Giantesses, the Skeletons (they are my Elves) and the Mundanes (they are the "standard" humans).

The Mundanes live everywhere, the Dwarves live beneath the ground like moles, the Skeletons are arboreal like spider-monkeys, the Giants and Giantesses live in freshwater like goldfish, and the Groses and Grosettes live in saltwater like whales.

The Groses and Grosettes have evolved from Austronesians (Polynesians, Micronesians, Filipinos, Indonesians, Malaysians, aboriginal people from Taiwan and aboriginal people from Madagascar). The Skeletons evolved from aboriginal people of South America. The Giants and Giantesses evolved from Central Asian and South Asian people. The Dwarves evolved from Europeans. The Mundanes come from all origins.

I want my Groses and Grosettes to be obligate carnivores like penguins (their favourite food is decapods) (decapods are lobsters, crabs and shrimps). I want my Dwarves to be facultative carnivores like dogs (their favourite food is insects) (insects are flies, moths, ants, bees and termites). I want my Giants and Giantesses to be facultative herbivores like oxen (their favourite food is plants of the Malvales order) (Malvales are cocoa, durians and mallows). I want my Skeletons to be obligate herbivores like sloth (their favourite food is plants of the Rosales order) (Rosales are apples, pears, roses, strawberries, cherries, almonds, elms and hemp). I want my Mundanes to be true omnivores.

I heard somewhere that humans cannot be obligate carnivores because of the lack of vitamin C synthesis. But I know four types of obligate carnivores that need to consume vitamin C: pikes, salmonids, swallows and tarsiers. I think a species that lack vitamin C synthesis could be an obligate herbivore if it eats fruits and not only seeds.

So, is it realistic for humans to evolve into such creatures?

Answer 1

Yes

It's already started to happen already.

The Bajau population of Indonesia have lived on houseboats for the last 1000 years and have evolved genetically enlarged spleens which allows them to use oxygen more efficiently so they can stay underwater for longer.

See https://www.telegraph.co.uk/science/2018/04/19/nomadic-divers-evolve-larger-spleens-stay-underwater-13-minutes/

As time continues more and more changes will occur assuming the same evolutionary pressures.

The problem is these evolutionary traits will be lost if these people lose their lifestyle or breed with other groups. A larger spleen doesn't help you work in a factory.

Humans will evolve if they have the right environmental pressures but it's harder because our large brain allows tool use which bypasses the need for evolution to survive.

For it to happen you really need a more primitive society in an aquatic environment and a lot of time.

Answer 2

It's hard to make humans evolve

The idea of humans evolving into marine mammals is explored extensively in Kurt Vonnegut's Galapagos (1985). In his novel, a group of people are shipwrecked on a fictional island in the Pacific. Simultaneously, a disease renders all mainland humans infertile, leaving the stranded island population as the last vestige of the human race.

Over a million years and countless generations, they evolve into aquatic mammals because of the evolutionary pressures of island life.

What's important about Vonnegut's setup is that he uses his plot to achieve reproductive isolation. With no mainland humans to rescue the protagonists or breed with them, his humans face consistent evolutionary pressures that eventually drive them to become aquatic. Plus, their genes aren't diluted by a global population, so evolutionary changes manifest faster.

While there are some advantages to humans diverging into different species (like accessing certain types of food) it doesn't make sense evolutionarily without isolating each population. Considering that early humans managed to populate every continent except Antarctica with rudimentary technology, you're gonna need some pretty strong physical barriers - or a cataclysmic, population-reducing event - to drive the divergent evolution you're looking for.

Answer 3

No, not realistic at all

Humans use technology, and adapt to their environments by using that technology. Your marine based humans won't start growing fins because they'll have boats and scuba gear. Even if they don't have those things now, they will in a few tens or hundreds of generations, a time span far too short for any significant evolution. You might get some localized evolutionary adaptions, but once technology kicks in, it removes the stressing factor that leads to evolution (and likely, instead, creates its own evolutionary influence).

Answer 4

There is some speculation that human evolution already went through an aquatic (or at least littoral) (waterside) phase; while the hypothesis is regarded as disproven (or even "junk science") in scientific circles, it has captured a bit of the collective imagination, and may have generated some discussion and fragments of suggestive evidence you'd find helpful.

Modern versions date to Alister Hardy in year 1960, but in particular, Elaine Morgan's The Descent of Woman (1972) and The Aquatic Ape (1982), etc. developed a picture of an evolutionary chain driven by women foraging in the water versus men hunting on the savannah; adding a feminist (or at least female-centric) perspective to evolution was noteworthy.

Wikipedia has an overview; apparently books are still being published on it, and articles are still being written, e.g. Why is it junk science? vs Why are biologists refusing to accept it?