Evolution :).
Descent with modification helps to explain anatomical similarities among certain species. The bones of the forelimbs of humans, cats, and bats (you can add in penguins, alligators, and more) are used in different ways but have similar structures.
The best explanation is that these animals have a common ancestor. With new generations, new species emerged by natural selection from different populations, and with this, the bones become better for different tasks.
The forelimbs and other such structures with anatomical similarities due to a common ancestor are called homologous structures.
