Evidence for Evolution


            Beyond Darwin’s assumptions based on his findings in the Galapagos Islands, how do scientists support the idea of evolution as a valid scientific theory?  There are many pieces of evidence that help scientists to support the theory of evolution.  The first evidence lies in the presence of homologous structures (structures with similar forms, but different functions) within organisms that have very little else in common.  The diagram below shows the forelimbs of seven species of animals from a variety of genii.  Each of the organisms possesses a humerus (upper arm in humans), radius and ulna (both comprising the forearm in humans), carpals and metacarpals (the human wrist bones) and phalanges (human fingers).  The main difference between the organisms are the uses of the appendage and the size and number of certain bones.


Monroe W. Strickberger, Evolution, Third Edition, © 2000, 1996, 1990: Jones and Bartlett Publishers, Sudbury, MA. www.jbpub.com. Reprinted with permission

The human arm is used for a variety of functions while the lizard’s and the cat’s are used for walking an climbing, the whale’s for swimming, the frog’s for support and the bird’s and the bat’s for flying.  This picture is not draw to scale or we could see that the whale’s bones are huge compared to the other organisms’, but we can see a resemblance that may show that we evolved along different paths from a common ancestor.


            Another piece of evidence that scientists have determined shows the probability of evolution is the presence of analogous structures.  Analogous structures are features that share a similar function, but are very different in structure.  The wings on the hummingbird and butterfly (below) show that there was a need for the development of flight.  The bird’s wings are made from special lightweight bones covered with powerful muscles, skin and feathers that are very similar to the scales of reptiles.  The butterfly’s wings are made of a special protein known as chitin and are covered with a fine powder.


Humming Bird in flightMonarch Butterfly

There is evidence to show that the modern bird may have developed from reptiles because their wings are made from the same material that the reptile scale are made from.  Most birds retain some scales on their legs and feet.  With so many different organisms with wings it is obvious that many organisms that eventually developed their own forms of flight survived longer than other, similar organisms that had no features that resembled wings.


            The early stages of development of many organisms provide a great deal of insight into the development of the theory of evolution.  There is an overlap of some of the features of different organisms that shows that they may have had similar evolutionary paths.  The picture below is a diagram of comparative embryology that shows the relationship between humans and fish.  Comparative embryology is the study of organisms as they develop from a fertilized egg to look for similarities.  Humans have been found to have very similar developmental patterns with fish, pigs, birds and the apes.  These similarities show that there may have been a common ancestor that had a mutation during its early development that caused a completely different organism to be born.  Over time, this organism developed even more differences as it evolved to better meet the needs of its environment.


The embryos of the fish and the human have two very similar features, gill slits and a tail.  The gills that develop in the fish develop very differently in humans and become the inner ear.  The tail of the fish allows it to swim while the human tail disappears later in development.


            One final piece of evidence about the evolution of living things comes from the fossil record itself.  In some cases, fossils have been found of animals with bones that are not attached to anything.  Whale fossils have been found that have a pelvis (hipbone) and what appears to be a femur (the thighbone in humans) despite the fact that whales no not have lower appendages and their tail is an extension of their spin.  These structures that appear to have no function are known as vestigial structures.  A more current example is the human appendix.  We do not know the exact function that the appendix once held (though leading theories surround the idea that it was once an enzyme sack similar to the gall bladder), yet it is still found as a part of the human anatomy.


Diagram showing vestigial pelvis and femur of a whale in an Eocene fossil.

The presence of vestigial structures is very strong evidence that organisms probably shared some similar past and developed along different lines to meet the needs of their environment.  These different paths allowed certain traits to become unnecessary and, eventually, most of the members of the species lost them completely.