On Intraterrestrials – microbes surviving deep below the Earth's surface without sun and oxygen but instead via thermodynamics, and the ability to respire most elements of the periodic table…
We know they must be refreshing their cellular biomass. But they are doing it immensely slowly — gradually replacing their parts, lipid by lipid, nucleotide by nucleotide. It takes roughly half a century for them to replace all their molecules.
These single-celled organisms can live for hundreds of thousands, perhaps millions, of years. (By contrast, the oldest multicellular organisms are trees, some of which can live to be thousands of years old, although none of the cells that compose them live for that long.)
Theoretically, therefore, when I scoop deep sediment samples, I could be touching microbes that have been living and breathing continuously since well before humans were even a species.
—Karen Lloyd (link)