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Imagine a species (the cause is largely irrelevant - could be alien meddling for all I know) that is Homo sapiens in all except one detail: they have additional organ which is kind of like gills - allowing them to breathe under water.

Note that the species has no other aquatic adaptations:

  • can't survive high pressure (so no deep diving)
  • can't echolocate or see better in water (not quite sure what the major benefit of that would be...)
  • can't swim in colder water than real humans
  • can't drink salt water anymore than humans (so can't settle on sea shore without access to freshwater)
  • can't swim any better than humans (so no intercontinental voyages without sea vessels - they aren't any faster at swimming and this need a supply of fresh water for any swim longer than a day as per prior bullet; BUT they can cross large lakes or small patches of sea space without rafts, by swimming underwater).

What would be the main difference in how such a society would develop compared to humans (on Earth, so same exact flora, fauna and geography and weather) in a time period of late paleolithic to early feudal society?

To import the detail from the comment:

  • there are NO non-gilled humans. They are ALL gilled.
  • any assumption on whether gilled humans would have trouble surviving inland must be referenced with actual information from real Earth biology
  • There may be other non-gilled hominids (Neandertals, Homo erectus), but (unless you prove the inland difficulty theory) they would have as much difficulty avoiding extinction as they had at the hands of non-gilled humans in real history.
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  • $\begingroup$ I don't think it matters, but you can assume that there are NO non-gilled H.S. around to compete/hate/etc with the new species.... Whether other hominids (Neanderthals etc...) are gilled doesn't matter to me, but you can assume they all are gilled if it helps narrow down the answer. $\endgroup$ – user4239 Dec 19 '14 at 23:57
  • $\begingroup$ And it is almost certain that there would be other gilled mammal species to exploit as pets/domesticated animals. Evolution just doesn't drop gills in at the last minute. $\endgroup$ – Oldcat Dec 22 '14 at 21:25
  • $\begingroup$ Humans CAN survive high pressure. If diving slowly enough with adapted atmosphere, we can cope up to 100 atm (1000m deep). But I have no idea how it feels with gills. $\endgroup$ – Madlozoz Feb 25 '16 at 16:13
  • $\begingroup$ Keep in mind that if nothing else changes but the addition of gills humans are not going to be breathing salt water only fresh water. If anything it would make venturing into the ocean more dangerous as we would be continously loosing water the entire time. This could even slow human spreading across the globe by making boats slower to develop. $\endgroup$ – John Dec 1 '16 at 21:39
  • $\begingroup$ Most fish can only breath one or the other, freshwater OR saltwater becasue one over hydrates you and the other dehydrates you. $\endgroup$ – John Dec 1 '16 at 21:42
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There are one or two chronological things which I should like to clear up before proceeding here.

This graphic shows the geographical distribution of members of the genus homo over time:

enter image description here
Image courtesy of Wikipedia user Conquistador under the Creative Commons Attribution 2.5 Generic license.

It shows that homo sapiens never existed during the same period as homo erectus, and was only briefly in the same location as homo neanderthalensis. In fact, the former was active mainly in Asia, while the latter lived in Europe. I think it might be best to leave homo erectus out of it for the time being.

We also have to determine just when homo sapiens first developed gills. This is actually very important. If the gills appeared within the last 500,000 or so years, then

  1. homo neanderthalensis would not have had gills
  2. Other members of the genus homo could have expanded further

You did say that it is only a minor point as to whether or not other members of the genus homo had gills. I disagree only because that would influence just where these people (as I'll call them) could have gone. The Out-of-Africa theory says that homo sapiens (Not *all members of the genus homo) came out of Africa about 200,000 years ago. Other members of the genus homo may have already left. Here's a graphic of homo sapiens's travels:

enter image description here
Image courtesy of Wikipedia user Chronus under the Creative Commons Attribution-Share Alike 3.0 Unported license.

The dashed lines are important because they are hypothetical migrations - in other words, while there's evidence in the form of fossils for the others, there is not yet a lot of evidence for the dotted line migrations. If, say, homo neanderthalensis had gills, Neanderthals could have braved the trek from Scandinavia over the North Pole (not incredibly plausible, I'll admit) and to the Americas, thousands of years before homo sapiens got there via the Bering land bridge. That's quite important.


Okay, I'll get to the relevant part. You don't mind a timeline-esque approach to this, do you? I'll go with this format for now. Let's set a date for the gills of about 600,000 years ago.

600,000 years ago

Homo heidelbergensis is the species that first gets gills.$^1$ At this point, homo heidelbergensis lives in Africa, although it's about to venture outwards into the world. It soon does, and reaches southern Europe and western Asia with ease. In this world, though, it may be easier to cross bodies of water. With gills, homo heidelbergensis maybe able to go to many places before homo sapiens or homo neanderthalensis. It may be possible for them to quickly cross the Red Sea at a narrow point, thereby making the Arab world open. The Persian Gulf is also a little less scary, although I think that both bodies would only be crossable at narrow points, likely close to their origins.

300,000 years ago

Homo heidelbergensis has now ventured throughout eastern and central Europe. Crossing the Dardanelles or the Bosphorus was a lot less challenging, so more people can come through Turkey and into Europe. Around this point, homo neanderthalensis may be evolving from homo heidelbergensis. However, the latter could have pushed beyond the places that homo neanderthalensis may have gone in our world.

150,000 years ago

Homo neanderthalensis is in its prime. All of Europe may be open to it, including the British Isles (forget the Channel Tunnel!). Scandinavia is easy to get to, and islands near the Baltic Sea require no skill whatsoever. I won't go so far as to say that Iceland is a possibility (it isn't), but it's much easier to get through the northern parts of Scandinavia and Russia. At around this point, the North Pole crossing becomes an interesting idea.

Neanderthals would have had to get lucky to get across the North Pole, but they may have had some help. The Earth has entered the Wolstonian Stage, a glacial period stretching from about 350,000 years ago to 130,000 years ago.$^2$ This means that an intrepid group might have been able to make it up north. Any water along the way - which there probably would be - would be a trivial obstacle for the gilled explorers to reach.

100,000 years ago

Homo sapiens begins to arise back in Africa. The Red Sea crossing idea now holds for these people, and they will likely undergo that journey. They're starting a journey into Asia, which homo erectus has likely left (metaphorically, that is - they've died out).

Homo floresiensis arises. This peculiar species - believed by some to be a sub-group of homo sapiens - stands three and a half feet tall, and has a small brain size. There's a large debate going on as to whether or not these people were homo sapiens with a growth disorder or a unique species. Either way, we know that roughly 100,000 years ago, there are people - of one sort or another - in Indonesia.

The cool thing about Indonesia is that it's made up of many, many islands. I'd welcome the input of any and all Indonesians on this part, but - and this may be a justified theory - homo floresiensis may have been able to cross many years earlier. Well, the ancestors of homo floresiensis, at least. Gills permit swimming for extended periods; the extent of the periods depends on other characteristics of the creature. Still, it is fair to say that there may very well have been people on Indonesia before 100,000 years ago. It is thought that homo floresiensis may have reached Indonesia by boats. Gills mean that boats are unnecessary in some parts. Therefore, an earlier crossing is possible.

It may be a stretch, but I think it is certainly possible that these "hobbits" may now make their way into Australia. The outback is still as much a desert as ever, but Australia is by no means a wasteland. Homo floresiensis has quite a head start over everyone else.

For the next few sections, see this important graphic:

enter image description here
Image in the public domain.

50,000 years ago

In the world you and I live in, homo sapiens has by now spread throughout south-east and eastern Asia, and is venturing into Europe. The Neanderthals are doing just fine, and homo floresiensis is presumably surviving.

In this world, things are a bit different.

Homo sapiens will still reach Europe, much of Asia, and presumably Australia. However, they will find people already in Australia. Homo floresiensis may have developed quite the culture, and may put up some resistance to the newcomers. Yet if there are any physical confrontations, the "hobbits" are in trouble. Three and a half feet of a small-brained hominid isn't a lot against a human. Homo floresiensis will die out - possibly a bit later than in our Earth, but at some time, nonetheless.

35,000 years ago

Homo sapiens and homo neanderthalensis have met. Any speculative interbreeding is taking place, and the two cultures - if you can call them that - are interacting just as expected. In southern and central Europe - and northern Europe, to some extent - Neanderthals are starting to die out as they're replaced, even in Douglas Adams's stories.

But Neanderthals up north may have crossed during the Wolstonian glaciation, and made their way into Canada. Sure, homo sapiens is now dominant in Europe, Asia, Africa, and eventually Australia (homo floresiensis may have survived until 10,000-15,000 years ago), but the homo neanderthalensis has escaped.

20,000-25,000 years ago

Sometime around this period, some members of homo sapiens cross over from Siberia to North America via the Bering land bridge. I would suggest that an earlier expedition would have been possible, but that leads to the question of why it didn't happen earlier in our world, given that the land bridge presumably exited for many years before humans crossed it. In this world, the humans may or may not meet Neanderthals. The North Pole migration route would have required the Neanderthals to come over somewhere in central- to -eastern Canada. The ice sheets were still in place 50,000 years ago, and so Hudson Bay would not have been a challenge. Still, perhaps the coasts would have appealed to Neanderthals, so they could have started out on the Eastern end of North America.

It doesn't take long to travel from Alaska to Tierra del Fuego - perhaps only a few thousand years. If that's the case, Neanderthals may have already colonized all of the Americas. That's more than a little bit of a problem for homo sapiens, because, once again, they're the newcomers.

There are various ideas as to why Neanderthals died out. One could postulate that it was the mere appearance of homo sapiens that led to their decline, in which case, our group of humans will be just fine. If the deaths had to do with some geological event that was specific to a certain region, then the events here become uncertain. I'm hesitant to speculate about what will happen in this epic meeting, and so I don't think I'll go there. You can decide for yourself. But I think that the two cultures may coexist, at least for a while. If Neanderthals die out, it won't be for a long time. I suppose you could devote an entire question to the idea of homo sapiens and homo neanderthalensis coexisting.

Summary

Gills mean that homo heidelbergensis can reach much of Europe and western Asia easily, and they cover a lot of ground. Homo neanderthalensis quickly goes even further, going up to the northern reaches of Scandinavia. Homo floresiensis reaches Australia and gets along quite well until homo sapiens comes along. Homo neanderthalensis reaches North America but dies out in Europe, and homo sapiens conquers the globe, though coexisting with homo neanderthalensis in the Americas.


Society

That was most likely not at all what you were looking for, so I'll try to devote a bit to the societal aspects of gills.

Fishing is big.

One of the issues with fishing is that it requires tools. tools didn't come about for some time (although obviously Neanderthals, homo sapiens and possibly homo heidelbergensis had them). If you don't have tools and want to fish, you have to use your hands, which is pretty hard.

Gills mean that you still have to sue your hands, but you can follow fish underwater. Have you ever seen someone (actually, Gollum might be a good example) try to catch a fish with their hands, miss, and stumble around in futility trying to get it? Gills mean you can follow the fish underwater. True, this is of no use in the shallows, but you can go into open water, which beats standing in a river looking like a fool with no fishing rod.

However, these are homo sapiens, and so have tools. Even something as unwieldy as a large stick can be helpful. Really, you just have to spear a fish, and if it's pointy enough, you're good. The problem comes when you want to catch bigger fish. You'll want to use your gills to your advantage, and the way to do that would be to go out into the open ocean - well, perhaps simply a bay - and go for something more like a swordfish. So you'll need something more than a simple spear.

In this case, you're not going to be able to just catch the fist (I've never tried to catch a swordfish, with or without a fishing rod, but it's definitely not easy). the best technique here might be to stab it until it dies. Perhaps you could use a tooth from, say, a shark and repeatedly whack it. Or maybe you just get a few pointy sticks and try to attack it with them. Either way, though, you'll have to realize that you can't catch the fish while it's still alive. The risk will be large, but the reward will be huge!

Escapes are easier.

I'm not a fan of swimming, and while I know that many people are, some animals aren't. Some large animals don't like water, and if those large animals are after you, you might have a place to hide! Let's say you have a creature like a cat (remember, a lot of cool creatures died out over the past tens of thousands of years) that's following you. It can climb trees and run really quickly . . . but it hates water. The way to get it off your tail would be to jump in a river/lake. Normally, you'd drown as it watched and waited. But with gills, you can stay there for a long time, and be just fine.

Bridges

I'm guessing that the majority of the time you see a bridge, it's to carry people over a body of water. That's what the earliest bridges would have been for: To get people from point A to point B without having to swim. If you have gills, this isn't a problem anymore; you can swim for long distances. Well, I suppose "long" is a relative term, but you could probably get a good half mile without any problems whatsoever.

Carrying freight makes things a bit more interesting. Do you really want to carry an object that runs on electricity underwater? So there are two options:

  1. Build a bridge.
  2. Use underwater vehicles.

I think that the former is the better option, because underwater vehicles require storage, and if they're just used for crossing a body of water, they're not very economical.


$^1$ Homo heidelbergensis and homo rhodesiensis may very well be the same, so I suppose we can consider them as the same species, or possibly consider homo rhodesiensis as a continuation of homo heidelbergensis, if you want.

$^2$ I'm discounting the possibility of a crossing by homo heidelbergensis because they would have had to get to northern Europe for this to even be possible, and I doubt they would have had the time. They started dying out (in that form, giving rise to the Neanderthals) about 300,000 years ago.

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    $\begingroup$ Amazing! Couple of points: (1) You still have fishing tools (same as H.S. remember?) BUT, you can now fish different, deep-water fish. You got yourself tuna, salmon, and incidentally, maybe better brain development with extra Omega-3. $\endgroup$ – user4239 Dec 20 '14 at 20:36
  • $\begingroup$ @DVK Honestly, I didn't expect anyone to read the whole thing, so that's pretty amazing. Ahhh, I forgot the tools part! Hm, I could go somewhere with that. $\endgroup$ – HDE 226868 Dec 20 '14 at 20:37
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    $\begingroup$ (2) You don't need shallow water straights necessarily to travel - there's a posibility to travel along the coast in the shallows - that's a LOT cheaper/easier energy wise (IMHO) than overland travel. $\endgroup$ – user4239 Dec 20 '14 at 20:37
  • $\begingroup$ @DVK True, but in some spots it's a lot quicker to cross a strait rather than go around. Oh, shoot I forgot to mention a section on bridges! Hold on a sec. $\endgroup$ – HDE 226868 Dec 20 '14 at 20:43
  • $\begingroup$ Do gills really make travel easier, though? Homo sapiens have been building boats for hundreds of thousands of years, and they're great for crossing bodies of water. Gills let you move under the surface of the water for long periods of time, but this doesn't seem inherently superior to swimming on the surface, boating, or walking. $\endgroup$ – ckersch Dec 20 '14 at 22:55
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One thing worth remembering is that humans with gills have an additional vector for being infected, require a bit more food, are a bit heavier, and have another weakness in combat with regards to land people. These all come from having additional openings in your body that are lined with blood vessels (the gills), which have a weight and metabolic requirements that non-gilled people wouldn't have.

These additional requirements would be very non-trivial. For warm-blooded humans, gills would need to be huge in order to supply sufficient oxygen. To meet their oxygen requirements, a standard human would need to take in 51 gallons of water per minute. Gills would likely need to be an external, fringed apparatus, like the gills of an axolotl, but larger in proportion to body size. Alternately, gilled humans could have lower temperature metabolisms. Fish survive with their non-exorbitantly sized gills because they're cold blooded. Humans that were evolved for life in water could use a similar approach to reduce the size of their gills, with lower internal body temperatures and lower needs for oxygen than gill-less humans.

Given the time period of 'paleolithic until now', groups of people that didn't regularly use their gills would probably start to lose them. Why? Because, if you're not using them, gills are a disadvantage for all of the reasons listed above. If I live in a desert, for example, having gills offers little to no benefit. Being cold-blooded out of the water is also a major disadvantage. Food requirements are lower, but so are brain activity and endurance. Cold-blooded, gilled humans would be worse in combat in the best conditions, and extremely vulnerable to cold weather. In an above-ground scenario, victory for gill-less humans would be as simple as attacking at night.

For early humans, the groups that would benefit from having gills are those that live near a significant food source that's easier for gilled people to exploit. These groups would rapidly begin to develop other adaptations for aquatic life, such as larger, webbed hands and feet, eyes better suited for underwater vision, and insulating fat or fur. All of these things would make the humans that had them better suited for life in the water, and less suited for life on land, and for people who rely on swimming to catch food, there would be a huge evolutionary pressure driving the evolution of those traits.

In short, the presence of gills would lead to speciation into aquatic and terrestrial humans. Alternately, if the terrestrial humans weren't as well suited to life on land as the neanderthals were, humans would stick to coastal regions and neanderthals would be the dominant terrestrial hominid.

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  • $\begingroup$ very true. I love bringing up evolution here. probably not what the OP had in mind, but still quite accurate. $\endgroup$ – dsollen Dec 20 '14 at 20:20
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    $\begingroup$ Is there enough time for that much of speciation to occur? $\endgroup$ – user4239 Dec 20 '14 at 20:39
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    $\begingroup$ If we start from the beginning of the Paleolothic: yes, definitely. Hominids have speciated many times since then. $\endgroup$ – ckersch Dec 20 '14 at 22:57
  • $\begingroup$ But the advantage for humans would be at the margin of the land and water, where they could culturally adapt to use the best qualities of both. So there's no reason to exclusively live in water or land and speciate on those those lines. Fast swimming need not be a huge need for hunting any more than primitive humans needed to evolve running faster than gazelles that they hunted. $\endgroup$ – Oldcat Dec 22 '14 at 21:33
  • $\begingroup$ If most or all of the humans live on the coast, there would be a very strong reason for living exclusively inland: far fewer humans to compete with for resources. $\endgroup$ – ckersch Dec 22 '14 at 22:23
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Game is played by guns and steel.

Gilled humans would be able to dominate coastal warfare, (underwater shock troops sinking enemy's boats) but would have little advantage inland. Presumably, gilled humans would be more easily damaged by dehydration and dry air.

I think it would very slowly took over coastal areas. Inland empires would have hard time to protect any forts surrounded by water, but thy would adapt by making dry defenses.

This arrangement would slow down development of classic empires, but would also slow gilled humans, because it would be harder for them to access metal and related technologies deeper inland. World would be separated to two races, warring constantly.

Gilled have good chance to take over, but very slowly, and with slower technological advancement.

Depending of emergence:

Mongol Empire would not attempt to take over Japan, and would not lose hundreds of thousands soldiers in attempt. So Europe would be speaking Chinese, and preparing for war to death with the coalition of Pacific Empire and Greek Empire (both gilled).

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    $\begingroup$ This answer assumes that there are non-gilled humans competig with gilled ones, which I specifically noted in a comment is a false assumption. All humans are gilled (and as far as I'm aware, gills aren't a reason to be maladapted to land though I'm uncertain of what a biologist would say). $\endgroup$ – user4239 Dec 20 '14 at 0:31
  • $\begingroup$ Ok no difference in human history - no reason for it beyond slight difference in military tactics. $\endgroup$ – Peter M. Dec 20 '14 at 0:34
  • $\begingroup$ I can think of at least 2 major possible differences right off the bat. $\endgroup$ – user4239 Dec 20 '14 at 0:36
  • $\begingroup$ OK feel free to answer your question $\endgroup$ – Peter M. Dec 20 '14 at 0:38
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Just a small thought, but if all humans had gills and reliably anticipating needing to swim on any given day, fabric technologies may have evolved slightly differently.

If you carry any/all of your possessions with you, you'll probably want to keep them dry - waterproofing might be more widespread, or else people could have inflatable knapsacks or containers that would float on the surface of the water while attached to the swimmer by a long cord.

Also, if hypothermia becomes a risk, warm clothing would be a necessity, as would clothing that dried faster. Perhaps frequently-travelled bodies of water would offer opportunities for a new kind of trade - settlements nearby selling warm fires and dry clothes (perhaps in exchange for wet clothes, that they can sell on?) This may be a little too modern for your question, but could work on a smaller scale pre-feudal era.

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