By Robert Hazen, George Mason University
Life has evolved and changed over billions of years of Earth history. Today, virtually all scientists accept evolution as a a historical fact, based on independently verifiable observations. Perhaps the first strong evidence, for evolution, came from the exacting discipline of comparative anatomy and from the French naturalist Georges Cuvier.
Georges Cuvier
Comparative anatomy was founded by Georges Cuvier who lived from 1769 to 1832. In the 18th and 19th centuries, naturalists accumulated great collections of plants and of animals, and studied their structures. Usually, these studies focused on differences between species, but Cuvier did something different.
He focused on the similarities of structure between wildly different organisms. In particular, he amassed a huge collection of skeletons of various vertebrates of all types—now visible in the Museum of Comparative Zoology, Paris.
Georges Cuvier’s Method
Cuvier noticed the similarities in structure, for example between the wings of a bat, the flippers of a whale, and the arms of a man. All three had very similar structures. They’re all related forms, even though the functions differ widely—one for flying, one for swimming, one for grasping and manipulating things. One can do the same sort of thing with flowers, with corals, even with single cells. We can see similarities in structure, even though different organisms are vastly different species.
Since it was really too controversial in that time, and the religious climate didn’t permit it, Cuvier didn’t himself embrace the theory of evolution. But subsequent scientists used his methods of comparative anatomy to trace the evolutionary diversity of life, especially by comparing living and fossil life forms.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
Vestigial Organs
A second line of strong evidence for evolution, is provided by vestigial organs. These are portions of anatomy that are no longer used. We find curious anatomical features in many different organisms, and these provide clues to evolutionary processes, because they’re related to ancestral characters.
If one sees perfectly adapted organs, they can’t provide any convincing evidence for evolution. If an organ is perfectly suited to its function, then it just as easily could have been created in a miraculous act, because that’s one possibility. What we really need to look for, thus, are imperfections. One need to look for organs that have no function, or organs that somehow have inefficiencies built into them. That becomes evidence for evolution. Here’s a good example that shows us why. It is that of the human appendix.
Appendix: An Organ with No Function?
The human appendix is unused. If we look at other primates, the appendix has a specific digestive function; in humans, it no longer does. In fact, the appendix can kill us. It can become infected, and therefore, it can lead to death. What’s the role of the appendix? It’s a vestigial organ, which no longer has a function.
In any complex, competitive system, vestigial situations can occur. There are other systems, other vestigial organs of the human body that aren’t dangerous but kind of amusing. For example, we have a vestigial tailbone. We even have vestigial muscles with which to wag that tailbone. We don’t need the tail anymore, but we retain that little vestige of it.
In other species, too, there are many examples of vestigial organs—penguins’ wings, horses’ toes. These traits point to evolutionary processes.
The Structural Unity of all Life
A third line of evidence of evolution is the structural unity of all life. All life shares certain features; things that go much deeper than organs, bones, and other large physical structures. All life is based on cells and uses the same molecular building blocks. Additionally, individual cells from humans can be extracted.
Thus, when a cell from a muscle is extracted and put in a nutrient dish, it will become like a single-celled organism. It’ll start dividing like a single-celled organism and make copies of itself. Hence, there seems to be a direct link between the single-celled organisms—the microbes, the eukaryotes that we find in pond water—and the much more complex structures of our own body. If humans had been created separately from microbes, we wouldn’t necessarily expect to see these identical types of structures in both organisms, as they have such different physical organization.
Similarity and Common Ancestry
There are 100 trillion cells in our body, versus the single cell of a microbe, and yet the similarities are there. When we look at all the molecular building blocks, we see the same proteins used in microbes and in humans; the same lipids; the same carbohydrates; the same metabolism; the same nucleic acids exactly—in microbes, plants, and animals. That unity of life can suggest a common ancestry.
Indeed, the study of DNA, too, provides one of the most compelling pieces of evidence for a common heritage, a common evolution from a single ancestor. We see this shared genetic heritage because all organisms use certain genetic materials, certain proteins that are common to every organism.
Common Questions about Biological Evolution
Georges Cuvier didn’t himself embrace the theory of evolution. But subsequent scientists used his methods of comparative anatomy to trace the evolutionary diversity of life, especially by comparing living and fossil life forms.
In other species, too, there are many examples of vestigial organs—for example, penguins’ wings, horses’ toes. These traits point to evolutionary processes.
There seems to be a direct link between the single-celled organisms—the microbes, the eukaryotes that we find in pond water—and the much more complex structures of our own body. If humans had been created separately from microbes, we wouldn’t necessarily expect to see these identical types of structures in both organisms, as they have such different physical organization.
