How “old” is a single-celled organism? I don’t know the precise details for Cyanobacteria, but it could be a relatively equal split (unlike budding, a perfectly

How “old” is a single-celled organism? I don’t know the precise details for Cyanobacteria, but it could be a relatively equal split (unlike budding, a perfectly reasonable alternative). In that case, are both offspring of a cell division the age of the “parent”? If so, why stop at the time since one generation? “But wait”, you may complain, “they are constantly exchanging molecules with their environment!” So is the “age” of the organism the point at which none (or equivalently half or any other fraction you choose) of the atoms inside the organism were inside the organism previously? That implies you could watch a single, discrete cell not dividing, and it’s “age” would be constantly fluctuating, not simply monotonically increasing. It’s entirely possible that age is not a meaningful, well-defined concept here. But less philosophically, when people refer to “living fossils”, they generally mean that the organisms haven’t changed much in form or physiology in a long time. There’s no reason to believe that they aren’t subject to the same pressure to adapt to changing conditions via natural selection, but those adaptations aren’t readily visible.