Plausibility of only two cultures/peoples existing on one planet?

Question

On Earth, it's virtually impossible to enumerate all the different cultures and ethnicities that exist, but if you hypothetically categorized every people as occurring with a language, that's roughly 7000 different cultures/peoples on Earth. (If you were to not do that, there are seven widely accepted cultural regions on Earth.)

With this knowledge, on Planet X, a Mars-sized body with an evolutionary timeline for its intelligent life much like our timeline (with tens of billions of years of adaptation), there are two warring peoples with their own two distinct cultures. Just two.

People Y live in cold, barren ice deserts that expand from the planet's poles. People Z live in lush, overgrown marshy forest that is wrapped like a belt around the planet's equator. Ocean separates the two regions from each other and split parts of the regions up.

Given the fact that there are only two distinct climates on the planet, would the development of only two cultures be plausible? Should I include sub-cultures inside the two main ones to account for separation distance and island isolation?

Answer 1

The main reasion of cultural derversity is low to absent communication between peoples.

In your asumptions environment is very communication unfrendly. Both ice deserts and lush forests are known for the great number of different small cultures living there. So it is not plausable.

To have only 2 different cultures you need to have 2 "australias" - small, relativly flat, mostly deserted continets with single mountain ridge along the one of the coasts and unhabitatable desert in a center. That's make people to live in relativly small area near mountains and along the coast - that would create a lot of reasons (farmes-fishers trade) and ways (sea/rivers travels) to communicate and form almost single-culture society (there would be some small wild tribes in desert)

So it would be more like Japan-Australia world.

Answer 2

Short answer: Yes, its possible. By including details you can make it more plausible.

It's especially possible if you define culture really broadly. Define culture as a (generally) shared law, language, and set of valued principles and you can claim countries of hundreds of millions of people spanning whole continents/islands IRL are one culture (e.g., the US, Russia, or Japan). And these are cultures that often started out as more than one culture. With some lamp-shading you could make this even more extreme and provide a reason they were just two cultures to begin with.

Plausible beginning

If you're willing to have let there been other cultures in the distant past, the same things that effect a lack of genetic diversity can apply to explain a lack of cultural diversity (see bottleneck and founder effects). Some kind of unifying natural disaster or mass migration to an uninhabited area can cover a large territory with one culture. Make it recent enough and no other explanation is needed. However, if you want any diversity to be long and forgotten explaining how they stayed one culture over a long time is harder. Cultures tend to fragment as people adjust to their own environment, specialize, or simply are separated for a long time.

Here are some factors to increase probability of preserving one (or two) broad cultures.

• Extreme unifying environment. You already have this. There's only so many ways to scrape a living out of the tundra. And honestly, the marsh isn't much better. Consider, the survival-related cultural aspects that would have to stay the same between one of your cultures no matter how spread-out they become. The tundra people will have to prepare for periods of long night, the arrival of migrating prey/fish, and collecting resources that can only be found as the ice recedes in summer. The swap people will have monsoons, harvests, fish-spawning, and increased alligator attacks. These steady, recurring, life-altering events force a way of living that will be shared regardless of distance or time. IRL things like this turn into religions and holidays (think pagan solstices, or harvest festivals) that further cement the ties between people.

• A unifying enemy. The two cultures serve as this for each other. Feel free to emphasize it to make your world more plausible. A swap-man may not really know or agree with what his swap-neighbors think on the other side of the continent, but they're swamp-people and that's important. It means they're not Ice-people. Swamp-man will always identify as a swamp-man when asked about world-politics because the Ice-people are out there, serving as an example of not swamp people.

• Communication. Probably the most important thing that isn't already in your question. If these cultures communicate well within themselves, then the plausibility of staying just one culture is vastly improved. IRL the written word and literacy played huge roles in forming the large one-to-few culture countries of today. If you want to avoid such modernism, consider the role of traveling bards, heralds, and messengers. People who share a language, a past (history), and a present (news, messages, pop-culture) will plausibly stay united.

• Hierarchy and loyalty. Even if the ruling body is distant and feudal, people tend to stay united in a culture if they know their place in the culture. The people obey the baron and are repaid with peace and order. The baron keeps his position by playing nice with the others and obeying the Marquis. Then there's the Count, Duke, Archduke, and King. Switch it up with the kind of government you want. Include oaths and pilgrimages to swear them. There can also be supporting hierarchies. A church, multiple branches of government (maybe the military is separate, or there are judges and enforcers), and even companies/guilds can all play a role in tying everyone together through survival, loyalty and distribution of information/resources.

It is strange to see only two cultures, but that can part of the mystery of your world. As details are added in and core problems addressed, it can be downright plausible.

Answer 3

Here are some fun tools to reduce the number of cultures on a planet and keep it small:

1) assimilation via conquest (or extermination). Self-explanatory

2) assimilation through immigration. Tribal culture meets sophisticated urban culture. Tribal members decide they prefer the nice comfy life of urban culture, and start leaving the tribe. Tribe eventually disappears.

3) (peaceful) merging/unification. Many cultures live together in a city. Over a long period of time, they fuse together into a distinct culture for that city. Or nation. Or region.

4) hostile environments. People spending all their energy trying to find food and stay alive won't have much time to develop any sophisticated "culture". Different groups of people in the same hostile region will tend to be quite similar (and uncivilized) for this reason.

5) continuous contact. A group of people under one culture spread out over a large region will tend to fragment and develop their local independent cultures if isolated. Counter this with active networks of trade, migration, and communication.

6) natural disasters (i.e. really hostile environments). Can't get one annoying group to fit in? Blow them up with a volcano [1]

Remember that culture is a completely arbitrary thing. You can take French people and subdivide them into regional groups, which you can subdivide even further into towns if you like. How one decides the boundary of a "culture" is arbitrary. Although language is often one convenient boundary.

Use enough of these methods, and there's no reason why you can't be left with only two cultures. Another approach is to not have too many human beings to start with, and keeping them together in specific regions to develop together.

[1] may result in unforseen consequences