Asked by: CAROLINE, PLYMOUTHIt’s sometimes claimed that certain species have remained completely unchanged for hundreds of millions of years. That’s untrue. Lungfish, Horseshoe crabs, and Lingula ‘lamp shells’ are famous as ‘living fossils’, but they do look slightly different from their relatives of bygone geological eras, and are classified as different species. It’s only rarely that no visible differences have been found: one possible example is the tadpole shrimp, Triops cancriformis, arguably indistinguishable from 220 million-year-old fossils. However, fossils reveal relatively little about an organism. Ancient Triops aren’t really the same species as modern forms, because invisible genetic differences would stop them interbreeding. In fact, analysing living fossils shows their DNA evolves just as fast as normal. That makes your question difficult to answer, but I’d guess it’s actually some sort of bacterium. Bacterial fossils of uncertain type (but looking much like modern cyanobacteria) existed at least two billion and maybe up to 3.5 billion years ago. They would probably interbreed with modern forms, because bacterial sex is pretty indiscriminate.