We have talked a lot about how long do different races live, but i have a thought and wonder what do you guys think about it.

I was wondering, if all the presently known elves are descendents of the aldmer, meaning they all belonged to the same race, that suggests at some point they all had the same length of lifespawn, didn't they? Considering that the altmer are believed to be the closest to them, we can assume that lifespawn was much longer than how long the races live now. It could be around a thousand years maybe.
How come that there is such a huge difference in the length of life of an altmer and a bosmer or a dunmer then?
They may have changed due to their different lifestyles, but even the chimer lived longer then the dunmer do now, as the history of the First Council is about 400 years long. Of course we could say that they were nobles and nobles live longer among the dunmer too.

My point is, that I believe the sifespawn of the elves is lessening rapidly as time passes. From the Real Barenziah we know that the dunmer have often interbred with humans, especially the nords, and even if the child of a dunmer woman and a hman man would be an elf, it would still keep some human traits - like it was believed that Symmachos was tall because he had nord blood, and dunmer women will became fertile around those who have human ancestors. Now I know that is not the most trustable books in the lore, but I also haven't seen anything that would contradict these things in it.

What if altmers live longer because they kept their bloodline mostly clear of humans? What if, all the interbreeding with humans, had brought an even higher amount of mortality, and pushed the elves even farther from their godly forms?

Could this shortening be a tendency, that could continue on until there won't be any real difference between the human and elven length of lives?

given how much stuff goes on in tamriel i dont think there a good way to judge. plus the mages would use magic to help them stay alive longer. how many of these elves lives weren;t allready extended beyond their mortal times. divyth fyr seems to have made it a good ways with out magic for life extension. breeding sounds like a good factor though. i want to cite eragon for a theory in that series elves had eternal life from the magical pact they made with dragons and had magic imbued into their species. maby the magically inclined elves of tes are similar in some respect. in that the natural talent with magic extends their mortal times to begin with.

That is a popular thought but there is no evidence to that there would be any connection between ones ability to cast spells and their lifespawn, beyond the fact that mages often lengthen their lives through magic. If the simple ability to cast spells would cause for lengthened lives, then the Bretons should live many hundred years long too, as they are one of the "basic mage races". And yet they don't.

Also, I don't understand why do you think Divathy Fyr didn't lengthen his life with magic. He is a Telvanni Lord, and a master of the magical art.

Eragon shouldn't be compare to TES, because there is little similarity between the two worlds beyond the fantasy setting. In the Elder Scrolls the elves began as immortal spirits that were tied to Mundus after the creation, and this is in a nutshell that makes the base of the Thalmor phylosophy about rising to heaven through destroying the 8 towers that keep the world together. They want to regain their elven importality, what they see as the elven birthright - so as all elves came from the same breed (aldmers) it is the birthright of all elves, only the thalmor are raccsit and consider bosmers and dunmers "impure" for cetrain reasons. Their immortality has no connection to any kind of pact - if anything, their mortality has!
Also every dunmer born as magic users - that was at least very strongly suggested in Skyrim and it was a fact in Morrowind, and therefore i am not certain if we can talk about elves who are not magically inclined. Weather they use their abilities and do became spellcasters in the end or not, is another matter.

Bear in mind that Divayth was born a Chimer. That would probably support the idea. Yes it's not a new idea, the Altmer believe it and nobody is trying to refute them.