I read this story in the US in the early- or mid-80s. It was probably written in the mid-late 70s, I'd guess.
Earth's colonies have not been in contact with Earth for a very long time. One of them sends a spaceship back to Earth. It finds Earth covered with a biological 'soup' of bacteria and viruses. Decades or centuries of biological warfare caused this. The descendants of humanity look like large amoebas. (I remember picturing them as looking like the parasites from the Star Trek (TOS) episode, "Operation: Annihilate!"). They communicated by telepathy, I think, were the descendants of Americans, and had names like "Washington" and "Jefferson".