So this is my view. The immortal organism would not evolve. Imagine you and I lived 100K years from now. Other humans would evolve somewhat during that time bu

So this is my view. The immortal organism would not evolve. Imagine you and I lived 100K years from now. Other humans would evolve somewhat during that time but we would be fixed. Climate change might happen, new humans would adapt to the changing environment via evolution, but we would be stuck suffering the new environment. The humanity example takes a long time to be noticeable. But for early multi-cellular organisms this would kick in much faster. Thing about micro organism fighting against toxins. The ones that do not evolve and adapt would die off sooner. And while they are still alive they are competing for resources with the newer versions of themselves. So for the colony it is advantageous for the older ones to die quicker since we already know they will be unnecessary sooner or later. Now we evolved from these primitive creatures. And the mechanisms for things like aging, circadian rhythm, sexual reproduction, etc - we inherited from the early creatures. So regardless if the advantage of dying sooner is beneficial or not to the humanity now - the mechanism itself is far more ancient than humanity or even mammals.