Why did the humans leave their original planet to found the colonies?

Okay, I think I finally have this:

1. Humans evolve on Kobol
2. Human-Cylons colonize the 13th Colony, "Earth" and begin to procreate naturally
3. 2,000 years later, after some unknown calamity, the Great Exodus to the 12 colonies occurs
4. Resurrection technology falls away over time on the 13th Colony
5. Eventually, knowing something bad is about to happen, the Final Five build a new resurrection ship around Earth and bring back the resurrection tech
6. The mechanical cylons attack Earth, and the Final Five are resurrected
7. They try to warn the other 12 colonies, but because of the Theory of Relativity, thousands of years have passed and the colonies are embroiled in wars with mechanical cylons
8. A peace accord is struck with mechanical Cylons
9. 40 years later, the Cylons, led by human-cylon hybrids attack and destroy the 12 colonies
10. After years of searching the survivors from the 12 colonies find the 13th colony, it's still a nuclear wasteland
11. The survivors then find and colonize our Earth, 150,000 years in our past.
12. A child born to a human and a cylon, Hera becomes mitochondrial eve on our Earth.

If I've got this time line wrong, please leave a comment and I'll edit so it's up to date.

The cause of the Great Exodus is unnamed. The thirteenth tribe left 2000 years before the great exodus, according to Battlestar Wiki. They were humanoid at the time of their exit. But they had evolved from mechanical cylons created by the humans of Kobol. They were eventually destroyed by their próprio mechanical cylons (centurions). It's unclear whether it's a reinvention of the cylon or centurions who never evolved. I always figured it was reinvention.

I'd also always thought the cause of the Great Exodus was a cylon - human war. You know the whole "this has all happened before and it will all happen again". Battlestar's cycle is:

1. humans invent cylons
2. cylons rebel
3. cylons become human
4. humans invent cylons
5. rinse, repeat

Great Exodus

Thirteenth Tribe

There seems to be a lot of confusion & mix-ups about the general plot / timeline of the universe so I am going to try my best to explain this:

1. On Kobol : Humans Naturally evolve (as far as we know) and 12 "tribes" of humans exist, who create their own Cylons (becoming the 13th tribe)

2. This is where "Man & Gods" lived in Paradise

3. The Gods mentioned here are actually the first naturally evolved humans (as we are told) & the "Man" is referring to the humanoid Cylons... Because the entirety of the Colonial religion is based on Cylon (13th tribe) documentation. (Figured out after Ellen returns to BSG - Also mentioned by Athena : "We know more about your religion than you do..." while searching for Tomb of Athena on Kobol) OR These Gods, could have been Humans, who claimed to be Gods upon the creation of Cylons, and through time even became "False Gods" to the humans themselves, namely Athena, Apollo, Aphrodite etc... (Which COULD explain their despair over the exodus of the 12 tribes & their utter failure) again this is up for debate...

4. At this point on Kobol, an unknown event (most likely a disagreement between cylon & man) causes the "13th" tribe (of cylons) to be exiled from Kobol.

5. The 13th tribe reaches a distant planet and calls it "Earth" (Cylon Earth)

6. The 12 tribes are "punished" by the "One True God" for their sins & claims of being "Gods" as "God turned his back on Kobol" (mentioned by Head 6 to Baltar while on Kobol) and experience an apocalyptic disaster (As explained by Mind 6 to Baltar) & are forced into exile them selves.
   This could be due to the humans sin & refusal to accept Cylons as a sentient being (with souls)* (This part is open to suggestions as it is never clearly explained in the show and we are only told that god punished man on Kobol)

7. By that time, the 13th tribe of Cylons (who have now become humanoid & procreate naturally) have created their own mechanical servants, who rebel against them

8. Prior to this rebellion the "final 5" (Anders, Tory, Tigh, Ellen & Tyrol) are warned. (By what Sam claims to be angels that only they could see). ("Ellen, you saw a man, I saw a woman...") This is most likely a reference to Head 6 & Head Baltar that appear to Baltar & Six throughout the show)

9. The 5 "re-create" resurrection (as mentioned by Anders again - This means that the 13th tribe were in fact humanoid for a long time but could not procreate naturally on Kobol, and could be the reason for their exile as Humans were either envious or thought it blasphemous because procreation is one of Gods commandments)

10. Upon their resurrection they intend to return to their lost brothers & sisters of the 12 tribes to warn them that they MUST treat their mechanical creations with kindness & fairness in order to avoid a holocaust they experienced on "Cylon Earth"

11. Because they traveled at Sub-Light speed, it took them thousands of years to reach the 12 colonies, and upon their arrival they realize that they are too late & their brothers & sisters are already & war with the robotic Cylons (created in Caprica series).

12. During which the robotic Cylons of the 12 colonies attempt to create humanoid models, but have only achieved creating Hybrids (This too is mentioned by Anders)

13. Therefore, in order to stop the war the Five decide to give the (12, 13-1) humanoid models to the Cylons (1 for each tribe & give them resurrection as they too were unable to procreate naturally (lacking sympathy & love as their Kobolian ancestors once did)

14. It is at this point that they introduce these humanoid models with the idea of a singular loving God (Possibly in an attempt to enable them to procreate naturally but certainly to avoid having the 13 models deify their creators as their own ancestors once did the humans(?) mentioned by Cavel to Ellen upon her resurrection after New Caprica) (Also because if they had in fact communicated with "angels" (whether in the form of Baltar & 6 or not) then they truly DID believe in the same God that Baltar comes to accept).

15. John/Cavel, refusing to accept the "human nature" embedded within him, is affected by THE most human attribute of jealousy & kills Daniels entire line; traps his creators & suffocates them, planting them with false "human" memories and introduces them on the 12 colonies as individual humans in order to prove to them that humans ARE indeed cruel and innately evil. (He may have also did the same with the other 11 models in order to remove their memory of the truth to carry out his "plan".

16. The show begins...

So as you can see, Cylons could have initially created resurrection first on Kobol (assumption), and it caused them to be exiled. On Cylon Earth they stopped using Resurrection until the 5 were informed by "angels" to recreate it in order to save them selves and reach the 12 colonies.

Also we know that Tomb of Athena on Kobol IS real, and it housed the 12 bodies of the 12 tribe leaders who possibly knew about the location of Cylon Earth, and therefore remained on Kobol in order to avoid the 12 colonies from ever knowing the real truth & returning to Earth for vengeance against their mechanical slaves...

These little things are all up for debate. There is no real answer to many of the little things.

However the 15 points mentioned above are mostly discussed and mentioned one way or another within the show. (I've rerun the show about 10 times and actually noted stuff down... Yes I am obsessed)

It's never made clear in the series itself. Kobol was clearly abandoned, but a reason wasn't included in the narrative, or if it was, it was too subtle for me to catch.

See the article on Kobol at Battlestar Wiki - The great exodus has no known reason I can find. There are some hints in the sacred scrolls, but no form reason is given, and the scrolls are treated as unreliable in the series.

According to the original BSG there was a natural disaster that forced the 13 tribes to leave Kobol. 12 of the tribes were human & left for the 12 colonies, whilst simultaneously the 13th tribe left for earth. The 13th tribe were created by humans as living robots. There was a Nuclear war on Cylon Occupied earth of which the final five recreated resurrection. They left for the 12 colonies to warn them. By the time the final five got to the 12 colonies the first cylon war broke out.

It's from here that I don't understand the finer details. 1. Why was there a nuclear war on the Earth of the 13th tribe? Was it a civil war? 2. Why do the final five need to travel to the 12 colonies? What are they warning them? Are they warning them about the cylons? That makes no scents considering they the humans created them on Kobol to begin with. 2. If the humans created the cylons on Kobol did they create resurrection? And did they purposefully forgot how to create cylons?

In the scriptures they never mentioned the 13th tribe as cylons. It's unfortunate Caprica ended, it was starting to answer some interesting questions.

Here is my take ...

Humans on Kobol invent Humanoid Cylons. There is a Cylon and Humans conflict. 12 tribes leave for Caprica one tribe leaves for Earth (humanoid Cylons). The tribe on Earth creates mechanized Cylons. The Mechanized Cylons destroy Earth. The final five recreate Resurrection and travel to Caprica. It takes them a freaking long time because the Space Jump system was more rudimentary. Finally the 5 get to Caprica and witness the Cylon and Human War. They upgrade the Cylons with Humanoid technology in return for Peace. Humanoid and Mechanic Cylons attack Caprica 40 years later and brainwash the 5.

The reason Kobol was destroyed was due to the fact that it had become decadent and technology run amok, gluttony.

When the 13 tribes left kobol, 12 tribes settled on twelve planets to become the 12 colonies of kobol. The 12 tribes were human... The 13th were human created cylons. Direct memory transfer (resurrection) originated from Kobol.

When the 13th tribe settled on Earth, they began to procreate biologically and direct memory transfer fell out of use. Then the final five cylons on earth were warned by "Angels" that a calamity was coming and therefore instructed them to recreate direct memory transfer (ressurection).

When the holocaust happened on Earth, the final five ressurected on a ship they had in orbit. They then headed back to the 12 colonies to warn them to treat their mechanical cylons well to avert catastrophe. But they didn't have jump technology, so traveling at sub luminal speeds they got to the colonies during the war with the humans and centurions. They agreed to help the centurions make flesh bodies if they would stop the war. There was peace for 40 years until the fall of the colonies.

Because the home planet can be boring? For example, why did the British leave England for America? The fact is that human beings (homo sapiens) left Earth for Caprica and Gemenon because there were more opportunities there than at Earth. As well, Earth had become overpopulated and was too polluted to live on! Also, they thought they were the only ones to colonize Caprica and Gemenon.