The majority of scientists working in the relevant fields of biology, genetics, and paleontology maintain that the theory of evolution is supported by an overwhelming amount of evidence. Here we present some of this evidence.
What is Evolution?
Evolution is the change in the genetic makeup of a population over time. Often two types of evolution have been defined: macroevolution and microevolution. The former refers to large-scale changes occurring over extended periods, such as the formation of new species and groups. Microevolution refers to small-scale changes that affect just one or a few genes and happen in populations over shorter timescales. But macroevolution and microevolution are not really two separate processes but the same process occurring on different timescales. In other words, microevolutionary changes occurring over thousands or millions of years can result in large-scale changes that define new species or groups.
Homologous features are strong indicators of evolution. These are structurally similar organs, bones, or skeletal elements of animals that, because of their similarity, suggest a connection to a common ancestor. For example, the forelimbs of dogs, whales, humans, and birds look quite different from the outside, but when their bone structure and pattern is studied they appear to be very similar. This suggests that the basic layout of bones was already present in a common ancestor of dogs, whales, humans, and birds.
Homologous structures can also be seen in embryos. Vertebrate embryos (including humans) have a tail and gill slits during early development, which become different later on (in humans, for instance, the embryonic tail is the tailbone and the gill slits have evolved into the jaw and inner ear). Homologous embryonic structures reflect that the developmental programs of vertebrates are variations on a similar plan that existed in their last common ancestor.
Vestigial organs are organs that no longer serve any function but once possessed a function that evolved out of a necessity for survival. Their function over time became non-existent. The geneticist and evolutionary biologist Theodosius Dobzhansky (1900-1975) once remarked that “There is, indeed, no doubt that vestigial rudimentary organs silently proclaim the fact of evolution.”
There are several such organs in the human body. The appendix is thought to have once been involved in the digesting of tough plant-based foods. But as human diet changed so did the role of the appendix. The appendix became useless but it still did not leave the body. Wisdom teeth are another vestigial organ. These teeth were once useful for grinding down plant tissue and larger jaws were required to help chew down foliage. When humans began began cooking food and consuming processed food, their chewing became much easier, resulting in the jaw becoming smaller. This reduction in jaw size meant that the molars, particularly the third molars (wisdom teeth), became highly prone to impaction.
In the sixth week of gestation, the human embryo possesses a tail, complete with several vertebrae. The tail eventually disappears after the next couple weeks of development and the vertebrae fuse to form the coccyx, or tailbone, in the adult. On rare occasions, a human infant is born with a vestigial tail that, although mostly harmless, can be removed through surgery.
The palmer grasp reflex is thought to be evidence of vestigial behavior (more so than an organ). When an object is placed in the hand of an infant, the fingers will spontaneously curl around the object. The infant will hold on so tightly that he is even able to support his own body weight. The infant, relying on its grasp reflex, can hold his weight for at least ten seconds when hanging by his hands from a horizontal rod. Monkey infants, moreover, have a similar involuntary grasping behavior and can hang from one hand for more than half an hour. It is believed that this reflex was essential in our distant monkey ancestors because it allowed them to cling to the mother’s body fur. Humans no longer require that powerful grasp and infants typically lose the reflex around three months of age.
The auricular muscles are vestigial in humans which is why a few individuals can move their ears voluntarily. These muscles help animals to perk up their ears when they are alerted to danger or in search of prey. These muscles are generally considered to be of little functional significance.
Some snakes have vestigial organs in their underdeveloped legs, which is a clue that snakes descended from lizards. Pythons and boa constrictors, for example, have tiny hind leg bones buried in muscles toward their tail ends.
Blind fish and salamanders that live in caves still have eye structures. The hind leg bones of whales suggest the ancestors of whales once walked on land on four legs. As these animals changed, they evolved to dwell in water. Their front legs became flippers and they lost their back legs and hips, although modern whales all still retain traces of pelvises.
The Fossil Record
There are fossils showing modifications over time that suggest animals can be traced back to common ancestors in the past. Fossils are the preserved remains of a dead organism found in rocks, many of which show intermediate stages between earlier and later forms.
Often these fossils are of hard remains like teeth, bones, and shells that become embedded in rocks. Paleontologists have studied thousands of fossils although the fossil record is incomplete. But the succession of some forms has been reconstructed in detail, such as the horse and the whale, as their transitional forms are well preserved as fossils.
The horse can be traced to an animal the size of a dog, called the dawn horse (genus Hyracotherium), 55 million ago. Fossils found in North America and Europe show this dog-like animal to have had several toes on each foot and teeth for a general browser diet. The modern horse, the Equus, is much larger, has a larger brain, is one-toed, and has teeth appropriate for grazing.
Around 49 million years ago, the oldest known whales looked like land-dwelling mammals with ears similar to those of modern whales, suggesting adaptations for hearing underwater. Ambulocetus shared these ear traits and had feet that were expanded for swimming. Species like Maiacetus and Rodhocetus appear later and had feet and spines adapted for more specialized swimming modes. By about 40 million years ago, whales like Dorudon and Basilosaurus were fully aquatic animals with a powerful tail that moved up and down through the water during swimming (rather than side to side as in fish) and rudimentary (but fully formed) hind limbs that could no longer support the body on land.
Paleontologists view Archaeopteryx as a transitional fossil between dinosaurs and modern birds. Archaeopteryx lived around 150 million years ago and possesses a combination of traits that place it as a transitional form between non-avian dinosaurs and birds. Its similarities to non-avian dinosaurs include a long feathered tail and small teeth. Unlike non-avian dinosaurs, however, Archaeopteryx also has flight feathers and wings, just like a modern bird. The discovery of the furcula, or fused clavicle bone, in Archaeopteryx was a firm confirmation of the relationship between birds and dinosaurs, as they are the only two groups to have this anatomical feature.
DNA and the genetic code reflect the shared ancestry of life and can show how related species are. Every organism shares the same genetic code.
The evidence for genetic similarity between humans and primates is evident in their shared inability to produce vitamin C. This has had grave consequences for humans historically as it led to sickness and even death on long ship voyages. Vitamin C deficiencies led to increased levels of scurvy in humans on long sea voyages whereas other animals, like horses and dogs, did not contract the disease. Scurvy in humans was the result of their inability to synthesize vitamin C the way these other animals could.
Humans cannot produce vitamin C because a specific gene is “broken” in them, preventing the production of one of the enzymes needed for synthesizing vitamin C. Geneticists have also discovered that other primates like chimpanzees, gorillas, orangutans, and monkeys cannot make their own vitamin C either. Geneticists conclude that these primates are related to humans through a common ancestor. We would expect the same gene to be broken in them in the same way, which is what geneticists have discovered: a mutation event occurred in the common ancestor of these species, rendering all of their descendants unable to make vitamin C.
The global distribution of organisms and the unique features of island species reflect evolution and geological change.
The island of Madagascar was originally connected to the massive landmass that would become South America, Africa, and Australia. At that time species were able to freely inhabit it. But the Indian subcontinent (including Madagascar) broke away about 135 million years ago. Madagascar separated from it about 88 million years ago, leaving the island isolated in the Indian Ocean. Species we find there today, like lemurs, are found nowhere else in the world but can be traced to common ancestors on the mainland, dating from a time when the land was close enough for ancient primates to cross the water and then become isolated.
Further, most of Australia’s mammal species are marsupials (carry their young in a pouch), while most mammal species elsewhere in the world are placental (nourish young through a placenta). Australia’s marsupial species are very diverse and fill a wide range of ecological roles. Because Australia was isolated by water for millions of years, these species were able to evolve without competition from (or exchange with) mammal species elsewhere in the world. These marsupials of Australia, as well as Charles Darwin’s finches in the Galápagos, are unique to their island settings but have distant relationships to ancestral species on mainland.
Within the human time scale, it is possible to see small amounts of biological evolution happening by direct observation. For example, some populations, like those of microbes and some insects, evolve over relatively short time periods and can be observed directly. It is possible to directly observe small-scale evolution in organisms with short life cycles (e.g. pesticide-resistant insects).
The mosquito is an example. There have been efforts to eradicate malaria by eliminating its carriers (certain types of mosquitos). The pesticide DDT was sprayed broadly in areas where the mosquitoes lived. Initially, the DDT was very effective at killing the mosquitos but over time became less and less effective, and more and more mosquitoes survived. In this case, the emergence of DDT resistance is an example of evolution by natural selection.
References and Recommended Resources
Biologos. n.d. “What is the evidence for evolution?” Available.
MacFadden, Bruce J. 2005. “Fossil Horses: Evidence for Evolution.” American Association for the Advancement of Science 307 (5716):1728-1730.
Sober, Elliott. 2008. Evidence and Evolution: The Logic Behind the Science. Cambridge: Cambridge University Press.
Scienceoxford. 2015. “Vestigial organs”. Available.
David Wool. 2020. Milestones in the Evolving Theory of Evolution. Boca Raton: CRC Press.