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?