Reading the Story of Life
The theory of evolution by natural selection is one of the most well-supported theories in all of science. It is backed by a massive and diverse body of evidence collected over 150 years. Let's look at four of the strongest lines of evidence.
1. The Fossil Record
Fossils are the preserved remains or traces of ancient organisms. The fossil record is the total collection of fossils that have been found all over the world.
What it shows: The fossil record provides a timeline of life on Earth. It shows that life has changed dramatically over time. We find fossils of organisms that are no longer alive today (like dinosaurs). We can also see the gradual changes in species over millions of years. For example, the fossil record for the horse shows a clear transition from a small, dog-sized creature with multiple toes to the large, single-hoofed animal we know today.
Transitional Fossils: Fossils like Archaeopteryx, which has features of both reptiles (teeth, bony tail) and birds (feathers), show the evolutionary link between different groups of organisms.
2. Comparative Anatomy
By comparing the body structures, or anatomy, of different living things, we can see signs of their shared ancestry.
Homologous Structures: These are structures that are similar in different species because they were inherited from a common ancestor. They may have been modified to perform different functions in different species.
Example: The forelimbs of a human, a cat, a whale, and a bat. They all have the same basic bone structure (one upper arm bone, two forearm bones, wrist bones, and finger bones). This structure was inherited from a common mammalian ancestor. In humans, it's used for grasping; in cats, for walking; in whales, for swimming; and in bats, for flying. The similar structure points to a shared origin.
Analogous Structures: These are structures that have a similar function but evolved independently and do not share a common ancestral origin.
Example: The wings of a bird and the wings of an insect. Both are used for flying, but they have completely different structures and developmental origins.
Vestigial Structures: These are leftover, reduced-size structures that had a function in an ancestor but no longer have a clear function in the modern organism.
Example: The tiny leg bones found in some whales and snakes are remnants from their four-legged ancestors.
3. Embryology
Embryology is the study of how organisms develop before they are born. The embryos of different vertebrate animals look remarkably similar in their early stages.
Example: Early on, the embryos of fish, reptiles, birds, and humans all have gill slits and a tail. In fish, the gill slits develop into gills. In humans, they develop into parts of the ears and throat. This similarity in early development suggests that all these animals inherited the same basic developmental plan from a common ancestor.
4. Molecular Biology (DNA)
This is one of the strongest modern lines of evidence for evolution. All living things use DNA as their genetic material. The genetic code is universal.
DNA Sequencing: We can compare the sequence of DNA bases in the genes of different species. The more similar the DNA sequences are, the more closely related the species are.
Example: The DNA of humans and chimpanzees is about 98-99% identical, indicating a very recent common ancestor. The DNA of humans and bananas is about 50% identical, indicating a much more distant common ancestor. This molecular evidence strongly supports the evolutionary relationships first suggested by fossils and anatomy.