There is a bewildering variety of different forms of life. From bacteria and archaea, to fungi, protists, plants and animals, the diversity is overwhelming.
The different species exhibit all sorts of specialised adaptations to their environments, including complex dependencies on other species.
It gets even more bewildering when you look at the fossil record and see what lived in prehistoric times. There are animals, such as the non-avian dinosaurs, that no longer roam the Earth. Indeed the non-avian dinosaurs went extinct 65 million years ago.
Moreover, the species that exist now cannot be found arbitrarily far back in the past; for example there are no human fossils older than about 400,000 years. In rocks older than 600 million years ago there is no complex life at all.
What can explain all these different facts? Where does the diversity of life come from? And why are there so many similarities and patterns in organisms today?
The answers is that species evolve over time. Minute changes from generation to generation build up over time to alter the characteristics of organisms. This is evolution.
The environment doesn't determines the selection of species. Even in similar environments the selection of species can be different. Cf. marsupials where many animals occupy similar niches to their placental cousins.
