The reason that our skeletons are so similar to those of so many other species is due to evolution. Over the last 2 to 3 billion years, life as we know it has evolved from a universal common ancestor.
Looking back at some of the first vertebrates on the earth, fish, gives us the first clues as to the development of the skeletal system. Some of the first fish did not have jaws or teeth, but they did have a bony shell that protected the head and front of their body. These fish remained small because they could not catch big prey. Millions of years later, these fish evolved and developed jaws that allowed them to catch and eat larger prey, rendering their ancestors extinct. With this new system, fish became larger. Later they developed fins made of jointed bones, which eventually adapted to carry the fish's weight out of the water and turned them into the first amphibians. These amphibians evolved into reptiles who no longer depended on the water and who developed thick protective skin to prevent evaporation of their bodies' water. The first birds also appeared, evolving thin hollow bones, decreased body weight, and feathers to enable them to fly. Mammals also evolved from reptiles, but developed the ability to regulate their own body temperature and give birth to live offspring.
Despite all of these changes, adaptations, and developments, each new species retained a number of features of those that came before it, including many similar bones.
Neil Shubin, a paleontologist, outlines this idea very clearly in his book,
Your Inner Fish
. His discovery of a fish with bony wrists (known as
), provided evidence for the existence of species that created evolutionary links between animals that appear to be quite different. Scientists have used years of research with fossil records and DNA to show that in many ways diverse animals, both alive and those that have been extinct for millions of years, are more alike than different, and that living things can be classified into subgroups based on their similarities. This allows for predictive power when scientists make new discoveries, which can then be fit into the existing record of living things over time.