by comparing fossils and dna, we know that all life on earth today had a shared ancestor, called the last universal common ancestor (luca). other living things may have been alive at the same time as the luca, but they died out. a study from 2018 suggests that the luca is about 4.5 billion (4,500,000,000) years old, nearly as old as the earth. the oldest fossil evidence of life is about 3.5 billion years old.