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Imagine a tidally locked planet that revolves around the star at just the right distance that a habitable zone can be formed in between the scorching desert side of the planet and an icy desert side.

Lets assume that we have a humanoid species with intelligence of Earth's apes that lives in that habitable zone. Lets also assume that the species is highy adaptable and manages to survive in the hot and icy zones of the planet in spite of the reason that those two areas are deemed "inhabitable".

What occurence would cause the species to rationally leave the habitable zone and inhabit the "inhabitable" zone instead? The migration needs to be permanent and irreversible and not a product of evolution - more product of some reshapement of enviorment.

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  • $\begingroup$ Why do humans move to places like Miami and Phoenix? $\endgroup$ – jamesqf Oct 22 '17 at 17:27
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    $\begingroup$ "Why would species leave" as in rational decision of most of it's members, or as evolutionary drift that made them live there? Or something between, like great human migrations? $\endgroup$ – Mołot Oct 22 '17 at 18:43
  • $\begingroup$ @Mołot, like a rational decision, for instance some sort of natural reshapment of the habitable zone that rendered it unliveable $\endgroup$ – JanT Oct 23 '17 at 9:12
  • $\begingroup$ @JanT please edit your question to provide better explanation, then. There are 4 close votes now. Two "unclear" and two "primarily opinion based", so please try to make it less opinion based, and clearer. $\endgroup$ – Mołot Oct 23 '17 at 9:14
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The easiest - and most likely answer - is if they are displaced by another group (likely by warfare). Or, similarly, if they need to travel but don't want to go through another group. Another option is for trade or easier transport.

The habitable zone will be a thin ring, so the only way any civilization can expand is by moving north or south. Civilizations will naturally seek to expand - there's not many civilizations that are perfectly stable over the long-term. Surplus food allows surplus population, and that allows expansion (especially if the surplus of food disappears, so they need to spread out to get more farming land). So, it's safe to assume that there will be some form of competition or war.

If a group is pushed out of its land, it will likely flee east or west into the uninhabitable zones.

For the trade/transport scenario, we shouldn't assume that the easiest travel is along the habitable zone. Maybe there are mountain ranges or rough seas that cross that zone, so the easiest way to travel is to venture out of that area and into the less-hospitable places. And maybe the planet has oceans or rivers - so liquid water would be present in areas that may otherwise be uninhabitable (and the water will moderate the temperature of the land around it). If the geography is right, in some places there could be a continuous flow of water into the hot side and a continuous flow of storm clouds back to the cold side. These waters would be very difficult to navigate (they would probably be very fast), and the storms could be very dangerous. So it would be safer to route around this area, venturing far into the desert side (which will actually be something of an oasis because of the constant water) or into the cold side (which may be more like the UK due to the constant warm air and frequent rain) to avoid those. A river that crosses perpendicularly from cold side to hot side would create these climate patterns - and the hot side will always have a lower water level because its water is evaporating, so the river will always from from cold to hot.

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Two main reasons I can think of 1. they're refugees or 2. they're after something.

  1. they are on the run either from natural disaster such as plague or famine or from the neighbours.

  2. there are natural resources that they need and they know they can find them in an area that is otherwise inhospitable and is worth the support of the rest of the rest of their group to have them off in the frozen/ baking wilderness acquiring whatever it is. Options include stone for tools, meat animals, wood that isn't available elsewhere, medicinal plants, spices, metals, jewels, etc...

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There's simply too many of them and all the territory in the habitable ring has been claimed by different groups/bands/tribes/families. As the groups get bigger, they need more resources and space. They are forced to fight with the other groups to get it, with the weaker group being driven out.

Since the deserts are the only place left they can go, that's where they go. It's not a good option but they are highly adaptable and can survive, which is still better for them than going back and being killed by the ones that drove them there in the first place.

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They’re subconsciously attuned to the magnetic fields of the planet and the planet’s core suddenly shifts, causing the magnetic poles to shift. As a result, they feel a subconscious need to migrate to what “feels” like home.

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Generations ago, our ancestors established the Diacta when they rejected the gluttony and weakness of those who came before, those who took this paradise for granted. We, who live in comfort and peace in this narrow band of life, earn our right to remain here. Now, each of us undertakes the Diacta as a pilgrimage- to show that we are worthy, and to show that we have strength enough to deserve our heaven. We come of age when we survive the journeys, designed to test our mettle, that send us around the equator of our planet- first, into the freezing cold, and then through the burning desert. We are tempered by fire and ice, and that is what makes us strong and that is what allows us to survive. Pray that you must only undertake the Diacta once, and that you do not fail to live by our laws and are forced to travel once again.

-Elder of the Diactians to those about to come of age before they set off

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Maybe for the natural resources. Ice is useful for storing food, and if they don't know how to create fire it might be useful for them to live on the hot side, go live underground during the day and only come out at night so it never is too cold. Maybe there are special animals/plants there. High thin trees, which make collecting wood very easy. Sheep-like animals but with an even thicker fur, so they can collect wool more easily. Maybe they like baths in hot steaming water, like the snow monkeys in yudanaka, Japan.

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