Evolution is a process in which living things change over time to adapt to the characteristics of their environments. Lamarck and Darwin were pioneers in this field. Both of them believed that life forms change, that all life is related, and that more complex organisms such as human beings evolved from simpler organisms such as reptiles. Today, the evidence gathered to support the concept of evolution by natural selection is convincing, and most scientists view this as a fact and not a theory. Evidence for evolution includes the remains of ancient species, fossil layers, parallels between life forms, and the passage of embryos through earlier stages in the evolutionary process. Recent technological advances in genetics have also added large amounts of data that supports evolution.
Scientists have found the bones and other remains of ancient animals in media such as peat bogs, amber, and ice cores. Remains are dated using carbon-14, and research shows that many ancient animals are comparable to the current versions. The course of development can often be seen in this way. For example, Darwin found bones from Megatherium, which was a large sloth. It is now extinct, but Darwin could see close relationships between Megatherium and the sloths found today. These findings support the idea of selection and adaptation over time because the ancient animals had to adapt to changing circumstances to progress to their current forms (NECSI, 2011).
Fossils, particularly those from shells, bones, and teeth, are often found in sedimentary rock layers. The lowest layers typically have fossils from animals that are either extinct or are clearly ancestors of current living things. In fact, Darwin noted in his books that fossils in the bottom layer were often quite different from those in the top layers. Biologists can learn a great deal about evolutionary processes by studying the changes in the animal fossils within each layer. Also, the layers may provide information about the climate and other environmental factors present when the fossilized animals lived. Similarly, remains found in ice cores have layers which represent time periods and contain information about environmental characteristics (NECSI, 2011).
Darwin and other biologists also noted that certain types of animals were similar, with only small differences that helped them adapt to different environments. For example, a duck has webbed feet, while a sparrow does not, because the duck is a water bird, and the sparrow lives on land. As a rule of thumb, the greater the differences between two animals, the farther back in history one must go to find the common ancestor. DNA technology, which has now progressed so that an entire genome can be decoded quickly and inexpensively, is invaluable in determining the amount of similarity between two organisms. For example, comparing mouse and human DNA reveals that their genes are 85% alike on average. The genomes of the chimpanzee, the rat, the puffer fish, the roundworm, the sea squirt, and the fruit fly have all been sequenced. Genomes of other species are in the sequencing process as well. Comparing DNA sequences adds an extra dimension to evolutionary biology (Gilbert, 2000).
Evidence of evolution can also be seen in the process of embryonic development. Human embryos look very similar to embryos of other species at certain points (Gilbert, 2000). At the earliest stages, it is difficult to differentiate the embryo of one species from that of another. Human embryos develop structures very similar to tortoises, chicks, pigs, cats, and rabbits as they grow. For example, human embryos have gill slits just like fish embryos, but they disappear before the person is born. Humans also have yolk sacs, just like chicken and frog embryos. The yolk sacs all have the same purpose, which is to provide nourishment for the embryo at that stage (Gilbert, 2000).
Remains of ancient organisms, genetics, comparative embryology, and fossil layers are four types of evidence for evolution. Further evidence comes from the observation of evolution taking place, testing predictions based on evolution, and the hierarchies of characteristics from ancestor species within an animal. An example of the last category can be found in the human brain. The brainstem and the cerebellum are the oldest structures in the brain; today these structures are considered reptilian. As mammals appeared, the midbrain or limbic brain developed on top of the reptilian structures. Finally, the neocortex developed in primates, leading to language, consciousness, abstract thought, and human culture. In essence, humans have three brains inside their skulls, all due to evolution (McGill University, 2015).
- Gilbert SF. (2000). Developmental Biology. 6th edition. Sunderland (MA): Sinauer Associates; Comparative Embryology. Available from: http://www.ncbi.nlm.nih.gov /books/NBK9974/
- McGill University. (2015). The brain from top to bottom. Retrieved from http://thebrain.mcgill.ca/index.php.
- New England Complex Systems Institute (NECSI). (2011). Evidence for Evolution. Retrieved from http://necsi.edu/projects/evolution/cover/evolution_cover.html.