Because (for example) when a new breed of plant is created, there is one type of that plant, hypothetically let's say it's a white flower plant. So when that plant starts evolving, mutations in its genotypes from its surroundings will eventually cause different types of that flower, like yellow flowers or pink flowers, so scientists believe that one organism was created and slowly over a period of time our dna was mutated slightly to create different "branches" or "types" of organisms that stem from the first type of life
