I was disturbed by the rate of human evolution in Waterworld- maybe 500 years to evolve humans with gills*, and about a 1000 years to evolve into (back into?) something that is stronger than a chimpanzee and faster than an Olympic sprinter.
How long should it take to evolve these sort of characteristics?
The short answer is that evolution couldn't create gills in humans. It doesn't work that way unless we're talking millions of years. However, if the humans in Waterworld had genetic engineering, it's perfectly reasonable to believe that they could create a human with gills in 50 years. The long explanation is below, along with a pretty detailed description of how evolution works.
I think one of the issues in discussing evolution is that we are conflating two different processes.
The first one is mutations. These pop up in populations at some rate -- which might be different for different species and for different populations. A population which is exposed to high levels of radiation might have a high level of mutation. However, the mutation is random. And many of the mutations will be detrimental -- even to the point of causing large numbers of failed pregnancies. (High rates of miscarriage, or of death during or shortly after pregnancy -- or of unviable eggs in an egg-laying species.)
Even currently in humans, there is actually a fairly high rate of failed pregnancies. 15% of known pregnancies result in miscarriage -- and there are probably even more that we don't know about because they happen before the woman is even aware she's pregnant. Many times, these miscarriages are because the fertilized egg doesn't develop properly, and you don't ever move from the blastocyst to a fetus. Instead, you get junk which the woman's body disposes of in a miscarriage.
The second process is selection. Mutations that are not detrimental enough to cause death will result in new beings. But the ultimate question in terms of evolution is: do these beings reproduce? And will those offspring also reproduce? In natural selection, the environment can determine whether an individual survives long enough to reproduce. Some mutations may affect this process. If a mutation makes an individual fertile for a longer period, they may have more offspring. Similarly, if a mutation ensures that the individual survives longer, that may lead to more offspring. Or if a mutation makes the individual a more attractive parent (long and colorful tails in peacocks, for example), they're more likely to mate.
However, with humans, there's a mitigating factor. We have tools, which can lead to survival of beings that might not otherwise survive. Take the Waterworld example: we have boats, and the means to build them. Therefore, in the short term, there wouldn't be any survival pressure to cause any mutation to be beneficial enough to cause a change in the population. In the longer term, however, we'd start running out of ways to fix the boats. It's a lot harder to cut down trees or mine ore if there's no dry land. At this point, the survival of those who can stay off the boats for longer may in fact start to affect the population.
There's also a question of "unnatural" selection or selective breeding. We can see that in dogs. Wolves look pretty much the same all over the world. But when humans started domesticating canines, we started choosing which males to breed with which females. Therefore, if we wanted particularly small dogs, we breed the smallest males to the smallest females, and in each generation, we continue the process. Eventually, we'll end up with toy dogs, because we're selecting for that feature. (A fascinating version of this process was used in the 1800s to create sheep and cattle with more meat.)
I think one of the most confusing aspects of evolution, however, is the idea that we can evolve a completely new organ (gills) out of nowhere. What mutations do is that they change the blueprint for what is already there. For example, they might enable a slightly higher lung capacity, which would enable people to hold their breath for longer. Over time, if this is beneficial enough, it would spread and perhaps even increase itself even more. But evolution is random. But evolution can't start from scratch. All that can happen is that the current blueprints can be adjusted. So we can start playing with lung capacity, but our genes can't randomly create something that's fully functional right from scratch. For a new organ to evolve would take FAR longer than for a minor change like being able to hold your breath longer to spread.
The only thing that matters in natural evolution is whether there is a greater likelihood of producing offspring from that mutation. If yes, then the mutation is selected for. If no, it's selected against. But most mutations have no effect. (A mutation might cause, for example, dimples to appear not just on your cheeks, but also on your elbows.) This is a completely useless mutation in terms of survival. But it might happen to get passed on simply by chance. (Our elbow-dimpled person might also happen to have lots of children.) This is evolution as it normally happens. If a mutation isn't harmful, it is just as likely to get passed on as not.
This is why in many of the more extreme animal breeds, there are other factors that tend to get passed on along with the trait that has been selected for. (For example, Thoroughbred horses, which were bred for speed, tend to have a number of problems including small heart size and bleeding from the lungs.)
The last factor to consider is a staple of science fiction -- genetic engineering. This is where we take the DNA sequence to grow X feature and place it into creature Y's DNA. Or where we go into the DNA of an organism and snip out bits that we don't like. This is currently being done in a number of areas. The most common is work being done on crops such as tomatoes or tobacco. (Just do a search for genetically modified food to see a sampling of the range of discussion on the topic.)
Depending on when Waterworld is supposed to take place, that would get my vote for the most likely origin of the gills of Kevin Costner's character. That could theoretically happen in one generation, although there would likely be a number of tries where the genes don't get expressed properly, or they don't work right, or they cause problems for some other system. (I believe the gills were in his neck, which is already chock full of other stuff like the blood vessels leading to the brain -- if the gills interfered with those, the modified baby wouldn't survive.)
Evolution is a funny thing. You can't ask of it magic. The real answer is that human lungs would probably never turn into gills. After all, look at every sea mammal. Not one of them has lost their lungs in favor of gills. Lungs are better.
What you would see is increased lung capacity and the ability for the blood to carry more oxygen. And this kind of change could happen very quickly if properly selected for. However, it would have been the sea-bound folks that would have gained this ability. Not a group of people who were lucky enough to find the one land mass left on earth.
Changes like I describe above could easily make humans aquatic in 500 years. This is roughly 25 generations. That is long enough for webbed feet and hands for sure, and long enough for at least a doubled lung capacity, and probably for wider blood vessels and an increase in the amount of blood, if not an increase in the storage capacity of that blood.
Provided that these are really survival features for humans. This would basically require that they live in the water full time, and it would probably come with lots of other dolphin like adaptations.
I agree with @DampeS8N. Think about cetaceans (whales, dolphins, stuff) -- 50 million years of evolution towards aquatic life and they still breathe atmospheric air using normal lungs.
Cetaceans needed about 10-20 My to be pretty good aquatics; of course the speed of evolution is a very variant thing, especially looking on phenotype only, but this is quite reasonable approximation for aquatic humans.
I think Ringworld got it about right, somewhere around 250 thousand years for some moderate changes, but nothing like gills for a long while, so...
I don't think we have a solid theory of mammal speciation. I'd like some pointers from people more knowledgeable in the field.
As for sci-fi, I like the Greg Bear's Darwin's Radio and Darwin's children which are indeed hard sci-fi about human evolution and more specifically speciation. In those books speciation is sudden, following an evolutionary jump theory.
If you believe in evolution then gills would not be a new trait but a recessive trait that re-emerges due to random mutation or some external force.
I may be mistaken, I am not a doctor or geneticist, but I believe I remember reading that at some point in the human gestation process we (humans) do in fact have gills that convert/evolve into lungs as the pregnancy progresses.
We could also go a little off topic and discuss Humans breathing liquid. I am sure we have all seen the Abyss, In that film Ed Harris wears a dive suit that uses oxygen saturated liquid to allow him to descend to great depths. I am not aware of any commercially available suits at this time but I would not be surprised if they exist. And while there may not be any dive suits, this technology is tested with burn victims and premature baby’s whose lungs are not yet able to process oxygen from the atmosphere.
If the growth of gills is merely something switched on or off by certain control genes, called hox genes, might allow gills to grow as the human foetus has the right clefts to develop into gills rather than a chin. Though it might mean that the human was deformed, because they no longer have whatever that hox gene would switch on.
But it's pretty unlikely that gills would re-evolve by divergent evolution, some other mechanism would be more likely I'd imagine, considering they haven't in dolphins yet. As such I'd say it is a pretty safe bet this wouldn't happen.
So in short, depending on what characteristic and which genes are involved mutations can take place over a 'handful of generations' or hundreds of thousands of generations!
I remember reading a New Scientist article some years ago, and their was an article that discussed how anthropologiest and biologists, thought it was likely that eyes evolved in under 100k generations, and that was scientific research. A generation is 30 years in science land.
So if you use 300,000 years as a metric, for eyes, then any evolution that was designed to enable a species to grow gills to get better food and become better hunters, I think would take longer, since gills are on your neck. As it is standalone evolutionary advantage and less complex than eyes, but still a new organ on the neck, it would take either a similar, slightly less, as gills are much less complex as eyes, or slightly longer because people would have spend a lot of time in the water, to make the evolution happen.
So using that metric it would take anywhere between 250,000 years and 500,000 years and no more than that.
@mbq, as regards the 50 million and cetaceans not growning gills, I think the reasons that cetaceans never developed some kind of gill system, is do with the amout of Oxygen then need. They need vast amount, even a dophin is 10+ feet, hugely bigger than us, and related to the fact they warm blooded. They need vast amount of oxygen, that would never be retreivable with the biggest gills, from seawater, hence surface breathing. I don't know if it's plausible, but we are about 1/4 the size of a dolphin....
