The ending would have fit a little more nicely if Adama ordered all tech destroyed and they arrived on earth just a few thousand years ago. This would have played into the "it all happens" again theme.
Basically, a sentient life form gains enough knowledge to create AI and eventually wipes itself out only to have the same thing happen to the next generation of sentient life forms. Is this what you mean by Gen 1, Gen 2, etc?
The gods (generation one) made man (generation 2) who then made the cylons (generation 3) who then made the skin jobs (generation 4) who then interbred with generation 2 to make the Hera, Nicki and the Teranians (generation 5) who 2 hundred and 50 thousand years later are on the verge of creating NeoCylons (generation 6.).
The primates running about on Earth quarter of a million years ago might have raped their way through the Galacticans once their technology started failing, but I doubt the galacticans would have chosen to interbreed with monkeys... So it's a loaded question to ask about the missing link if the true men native of Earth died out and were replaced by aliens, or only MOSTLY replaced by aliens if there were a small number of sexual cross overs resulting in child birth and some of their monkey dna got through to modern times.
Of course just because the Galacticans wouldn't boff the monkeys it doesn't mean that the Cylons wouldn't. More so that the Cylons owed the refugees of the twelve colonies a new home world on which they could frakk, and quite frankly a hundred years later withe the grandchildren or great grandchildren of sdo many Cylon/monkey orgies continuously filtering and upgrading Monkey DNA through the reproductive facilities of the cylons, these grandshildren might actually be frakkable to the Galactican's lilly sentiments, so that after all that waiting for a good lay, it might actually be possible to have enough genetic diversity to maintain a growing culture without massive mutation and deformity from inbreeding. It's then that you have to ask yourself isn't it amazing how the Cylons can breed so eaily with the Terran monkeys but fail continuously to make babies with Galacticans, and themselves each other, as if it has always been monkeys they were supposed to frakk and never humans becuase this has always been Gods plan for them?
It's really a question of if the gods of kobol evolved on Kobol, or if they were a machine intellect created by another species that had run through it's life expectancy earlier. I see that the three prime reasons to invent skin job technology are... Immortality (any damage/disease to the body is covered by immortality), vanity and/or interplanetary travel. If it's 3 hundred years between stars, no form of stasis or cold storage is going to keep "people" fresh for revivification... Which means certainly that if the Gods of Kobol did not evolve on Kobol, if Kobol is not their home planet, and they did not have FTL technology then the first Gods on Kobol had to have been Cylons riving from extended boxing/storage from after a vast interstellar flight.
There's a Generation Zero that we know nothing about.