If dolphins came out of the water again and evolved into something approaching human intelligence, how would that look, taking into account their physical characteristics and what happened evolutionarily when they moved into the water? They would have to be able to walk again, so how would this work?
Well, some might argue that dolphins already have something approaching human intelligence...
We can take it that the three necessary factors for a species to develop technology are:
- Intelligence - to conceptualise what technology they want to develop
- Language - to communicate complex instructions and ideas with other individuals
- Manipulation - to be able to use tools and materials to realise their ideas
You can think of these as a virtuous cycle, if you have all three they reinforce each other and eventually you end up with a technological society like we have today. Dolphins arguably have the first two, but they lack the ability to manipulate their environment: they can think about building tools, they might even be able to discuss building tools, but they'll never be able to actually build them because instead of arms and hands they have useless fingerless flippers.
So back to your question, we can assume that dolphins, being perfectly adapted for marine life, are probably not going to leave the sea willingly. So let's assume a push factor: something is gradually making the oceans uninhabitable and the only course for survival to adapt to the land.
There are several significant obstacles for dolphins surviving on land. Beached dolphins tend to die relatively quickly of either thirst, overheating, or asphyxiation. The first is due to the fact dolphins don't actually drink; in the sea they gain all their hydration from the fish they eat, a dolphin that doesn't eat will die of thirst long before it dies of hunger.
Overheating and asphyxiation occur because of the sheer weight of the dolphin's body fat, which, while essential for maintaining warmth, buoyancy and internal pressure while the dolphin is in the water, will quickly kill a dolphin outside of the water by trapping in too much body heat or just weighing down the pumping of its lungs.
So, to survive out of the water, a dolphin is going to have to become a lot more like a seal. It will have to stick to very cold environments, it will have to reduce its internal body fat content, and get used to eating less frequently. While on land it will probably move like a seal does: by somewhat gracelessly bouncing around on its belly.
Walking is probably out of the picture. As you can see from the below, a dolphin's hind legs are completely atrophied, and only a tiny vestigial pelvis remains. Once evolution atrophies a feature to this extent, it is very unlikely for it to return.
The unhappy conclusion though is that intelligence is expensive. It requires a lot of resources diverted from elsewhere and a constant supply of high protein food to fuel the large brain. In lean times, and when facing having to adapt to a new environment, the massive brain is probably going to be the first thing the dolphin will sacrifice.
If the same thing that makes the sea inhospitable for the dolphins also makes it inhospitable for the fish however, then the dolphin will also have to eventually adapt away from the semi-aquatic seal lifestyle too, into something completely terrestrial. Below is my idea for what the 'final product' would be:
This is an animal with a much smaller brain than present-day dolphins, and a greatly truncated body size (probably about waist-high against a human). It has an ill-adapted sense of hearing and smell, so it hunts mainly by sight, and mainly for prey like small rodents. The hind-legs are still gone, so its flippers have adapted back into legs, allowing it to walk somewhat like a flightless bird. Dolphins are born with some vestigial fur, so I've been kind and allowed it to regain a full pelt.
The big question is this: Will it become human like, then come out of the water, or will it come out of the water, then become more human like?
For a dolphin to come out of the water, it will have to sacrifice a lot of what already makes it human like as Tom O'Daighre's answer already attests to. Coming straight onto land is only an evolutionary pressure for the dolphin as is, if the sea has become very inhospitable at which point a lot of a dolphin's intelligence will likely be sacrificed first.
Instead consider that dolphins first become tool users. It could start as a simple discovering they can use thier flippers to pick up rocks for something useful like building houses where they can safely hide from sharks or other predators. Granted, dolphins are already normally apex or near apex predators in most areas that they live, but a population boom in killer whales, or one of the larger shark species could force them onto the defense. Or perhaps they are making "hiding spots" to attract smaller fish that they could then feed off of, does not matter why they are stacking rocks as long as they convey some advantage. As the generations go on, the dolphins who have longer more dexterous flippers can make better structures more quickly improving thier odds of survival. Eventually some lucky dolphin will be born with stubby little fingers, this is a huge advantage because now they can hold smaller rocks or even sticks adding all sorts of construction options for these builder dolphins, like being able to make a door to keep out predator who could otherwise follow them into thier homes or something to snap shut trapping prey.
Now this is where going back to land becomes a thing. Fresh water has a lot more rocks and sticks to work with; so, dolphins who go inland into gulfs, rivers, and lakes will have a lot more resources to work with. Once thier fins become more arms like they can even start figuring out spears and tool making which will make them the undisputed apex predator of the world's bodies of fresh water. Living in warmer fresh water, they now don't need all that blubber; so, those who are lighter can start to drag themselves up on to land to take advantage of all the edible plants and wildlife that likes to live near the water. So now you have something that is more or less a freshwater mermaid.
Now, thier tails will continue to prefer water for a while, but land now offers something they will never get in the water, and that is fire. Being able to cook will open up a whole plethora of new food sources for them and help them fight food born illnesses; so, this will lead to features that prefer the ground to thier rivers. Re-evolving legs is a long path to go for the dolphin; so, they may more likely go with slithering like a snake.
In the end your dolphins would end up looking more or less like this:
Well, as you already know, dolphins have already lived on land before. They're descendants of terrestrial mammals.
Going back? They've the lungs and the unique construction of their flippers (former forelimbs) to work with. That's a plus. Their lungs presumably keep them in shallower waters, so they're already that close to land. Not to mention the fact that they already breathe air. Their flippers seem to have the complexity that our ancient aquatic ancestors had before making the jump, so that's another plus. And they probably have some vestigial remains of former hind limbs. All in all I'd say that's a fair start.
I think the simplest way to approach this question is as if you were reversing the process of their evolution a bit. By means of absurd chance and natural selection they somehow become tetrapods again. They take up life on land and ever so slowly continue to adapt.
Eventually they'd find themselves in a survival-based scenario that promotes the evolution of walking on their hind limbs. Probably gathering food faster than the competition or something along those lines.
As for what exactly that would leave them looking like... I don't want to sound dismissive, but a lot of that will be up to you.
It's just the nature of evolution. It's all circumstance. Even with my limited knowledge of biology, I think it's fair to say that it's impossible for anyone to be sure.
But that can be a good thing. Plenty of room to be creative. Play around with the jaw, the dorsal fin, the positioning of their eyes. Do they continue to give live birth? Does their skin remain rubbery, soften, or become something we've never seen before? Maybe emphasize the characteristics they have now.
I'd look for some artists' depictions of dolphins' ancestors to start.
Hope this helps.
Given the need to walk again, I imagine that they would have to go through a stage akin to that of mudskippers, though with possibly less overall change required to resituate themselves for living on land again.
Beyond that, the rest would depend too strongly upon random variables and chaos theory for me to hazard any guesses.
My answer will incorporate some non-dolphin get them out of the toxicity of the changing water. Since the water is likely becoming toxic due to humans, I will allow a group of humans to help out some dolphins by creating a "landed dolphin refuge". Since dolphins are intelligent, humans chose to help them out in hopes of them one day being evolved enough to function in modern society. They would likely start coming out of water near the poles as Tom O'Daighre suggested in his answer because their fat layers would insulate them heavily, so the first dolphin refuge will be somewhere in Antarctica.
since dolphins can't really move on land the humans will have to help them out here. My first thought is that they lay on a skateboard roll around using their fins to push them. Of course a human manufacturer will have to train them at first and then provide the skateboards.
I'm not sure what language capabilities dolphins have, but I will assume they currently have a vocabulary much less than humans, and also much lesser sentence structure. Quickly their speech will become somewhat similar to humans (likely still as patterns of clicks and whistles); language is learned by humans, so I think this will evolve as quickly as dolphins can invent new words and structures. There will likely also be a human effort to learn dolphin language, and communicate with them, further boosting their linguistic abilities. Within 3 or 4 generations I would expect them to communicate on a level similar to humans. their language transmission and recieving capability over air will likely also improve.
Humans could introduce more advanced evolutionary tactics such as selective breeding or eugenics, to develop better on land characteristics. I will also prohibit direct gene manipulation for ethical and possibly technological reasons.
over 25 or 30 generations of dolphin-imposed selective breeding, their fins would begin to lengthen into arms, allowing them to scoot better on land while still swimming, and catch things with their arms, likely an early development will be throwing/kicking dirt (or snow, as it is Antarctica) at prey. In a few more generations they may also begin to re-develop fingers to use tools like spears or axes to hunt or fell trees, dig holes and develop primitive structures.
Likely they will be able to hunt on land but decide to continue hunting/fishing underwater.
With human level communications skills, I'd be surprised if they did not catch the whole capitalism thing. Drilling companies could employ them to inspect underwater equipment, with time they could possibly repair or perform preventative maintenance underwater. The navy may employ them in submarine detection, and possibly covert insertions. they could also be employed as underwater tour guides, or underwater archaeological specialists (or marine biologists, etc). As amphibious beings dolphins could hold many positions we have not thought of yet.