One of the primary evidences given for evolution is homology--the observation that different species have similarities. For example, the vast majority of animals have two eyes, a nose, a mouth, two ears and four limbs. This supposedly indicates that somewhere, far back in the history of life many hundreds of millions of years ago, there was a common ancestor who had these traits and the descendants eventually split off into a variety of species.
The bone structure of many skeletal reptiles and mammals are similar. Notice for example the picture on the right. This cartoon depiction of an x-ray of an alligator's front limb, a human arm and a bird's wing respectively. You can see that the bone structure of all three share some similarities. All three have two bones in the "forearm" part of their limbs. The radius and ulna, as these bones are called, allow the end of the limb (our hand) to rotate about 200 degrees. This benefits us as well as the alligator and the bird. Other similarities are the five digits on both the alligator and human. In Darwinian evolution, these benefits were selected in the original ancestor because of it's ability to compete and survive over it's less endowed siblings and cousins who did not have such features. And then of course these traits were passed down to their descendants and continued to be preserved even in widely divergent species.
You can see that there has been an observation and then a hypothesis proposed but never tested. Such a test is impossible to make because of the extremely slow pace of the process. There is a test however. If the evolution hypothesis is true, there should be evidence in both the fossil record and in some species of life today of transitional forms. This we do not see. Stephen Jay Gould in a moment of honest reflection admitted:
“The extreme rarity of transitional forms in the fossil record persists as the trade secret of paleontology. The evolutionary trees that adorn our textbooks have data only at the tips and nodes of their branches; the rest is inference, however reasonable, not the evidence of fossils… We fancy ourselves as the only true students of life's history, yet to preserve our favored account of evolution by natural selection we view our data as so bad that we never see the very process we profess to study.”
There is another hypothesis. Perhaps this benefit of having two bones in the forearms of so many creatures is a design feature which was used by a wise Designer who efficiently used the same excellent design on many different kinds.