The last remnants of the ancient realm of Numenor who came to Middle-earth after Numenor's destruction.
The Dunedain were descendants of Elrond's
brother Elros, who chose to be counted amond
Men, rather than Elves since he and Elrond are half man/half elf. Since Elros was half Elf, he lived three times longer than normal
men, and died at the age of 500.
Since the Dunedain were descendants of he, they had the same
long life. But after comming to Middle-Earth and mating with the humans already there, their long life diminished. However some still retained the gift of long life. Aragorn was 87 at the
time of Lord of the Rings, and died at the age of 210.