The theory of evolution was and continues to be a highly controversial topic because it challenges so many spiritual beliefs. We will review the definition of evolution, then discuss different lines of evidence supporting the theory of evolution. We will also discuss what fossils and archaeological evidence tell us about human evolution.
Recap: What is evolution?
Evolution is a gradual and cumulative change in the heritable traits of a population of organisms.
It is mainly driven by natural selection, a process where individuals with traits that help them survive in their environment are able to reproduce more because of those traits. This change takes place over the course of many generations.
Examples of evidence of evolution
The theory of evolution is supported by a wide range of evidence including fossils, homology, molecular biology, biogeography, and direct observations. In this section, we will discuss each line of evidence and provide examples.
Fossils as evidence of evolution
Fossils are preserved remnants or traces of organisms from a past geologic age. Fossils provide evidence that life forms have changed over time and resulted in the diversity in life forms that we see today. Fossils show how organisms evolved, how new groups of organisms emerged, and how some species became extinct (Fig. 1).
Figure 1. This diagram shows how the great white shark evolved using transitional fossils. Source: The diagram is the own work of the uploader. Derived image authors from left to right: SaberrexStrongheart (CC-BY-SA) Meghunter99 (Public domain) Meghunter99 (Public domain) Meghunter99 (Public domain) Meghunter99 (Public domain), CC BY-SA, via Wikimedia Commons.
Fossils show patterns in the changes caused by evolution in different groups of organisms. For example, the pelvic bone in fossil stickleback fish consistently became smaller over time. The consistent shrinking of the pelvic bone is a pattern that suggests natural selection was the driving force behind the change.
Fossils also show how new groups of organisms emerge. For example, fossils provide evidence that cetaceans (an order of marine mammals that includes whales, dolphins, and porpoises) evolved from terrestrial mammals like hippopotamuses, pigs, and cows (Figs. 2-3). This is because fossils show that the pelvis and hind limb bones of extinct cetacean ancestors became smaller over time, eventually disappearing completely and developing into flukes and flippers.
Figures 2-3. Fossils show that the hippopotamus (left) is the closest living relative of the whale (right). Source (left): Diego Delso, CC BY-SA 4.0, via Wikimedia Commons. Source (right): Gabriel Barathieu, CC BY-SA 2.0, via Wikimedia Commons.
Homology as evidence of evolution
Similarities in the structure or development of different life forms (known as homology) also provide evidence of evolution because these show:
How different species with similar structures can be traced back to common ancestors (divergent evolution)
How different species with similar structures but no recent common ancestors were subjected to common selection pressures (convergent evolution)
Divergent evolution is the type of evolution where a species changes over time and branches off into new distinct species.
Species that share a recent common ancestor can have homologous structures: similar structures shared by closely related species. Homologous structures appear similar but serve different functions (Fig. 4).
For example, vertebrates like pigs, birds, and whales have forelimbs with the same basic composition that came from a common ancestor. Their forelimbs changed over time to serve different purposes that suit their present environment.
Figure 4. This diagram shows the relationship of bones in the forelimbs of humans, dogs, birds, and whales. Source: Волков Владислав Петрович, CC BY-SA 4.0, via Wikimedia Commons.
On the other hand, species that are not closely related have also evolved with similar physical characteristics due to common selection pressures. This process is called convergent evolution. For example, birds, bats, and pterodactyls all have wings that can be used for flight even though they are not closely related (Fig. 5).
Figure 5. Birds, bats, and pterodactyls have similar structures with the same function--they have wings that they use to fly--but are not closely related. Source: John Romanes (1848-1894), Public domain, via Wikimedia Commons.
Molecular biology as evidence of evolution
All life forms share the same genetic material. From bacteria to humans, all life forms have DNA, as well as its mechanism for replication and expression (Figs. 6-7). This suggests that all species came from a very distant common ancestor.
Figure 6-7. Bacteria (left) and humans (right) have DNA as their genetic material. Source (left): Mark Amend - NOAA Photo Library, Public domain, via Wikimedia Commons. Source (right): Reinhold Möller, CC BY-SA 4.0, via Wikimedia Commons.
Biogeography as evidence of evolution
We can also observe patterns in the geographic distribution of life forms on Earth (a branch of biology called biogeography). These patterns can be explained by evolution alongside the movement of tectonic plates.
For example, members of the plant family Proteaceae are found in Australia, southern Africa, and South America, which are all distant from each other. The presence of Proteaceae in these areas can be explained by their descent from a common ancestor that existed in the supercontinent Gondwana before it broke up into different landmasses (Fig. 8).
Figure 8. An illustration depicting continents Laurasia-Gondwana dating back to 200 million years ago. Source: Lennart Kudling, CC BY 3.0, via Wikimedia Commons.
Evolution can also explain why islands tend to have many endemic plant and animal species. Endemic species are native to a specific geographical area and do not occur naturally elsewhere. Darwin proposed that species from the nearest mainland colonized islands and eventually evolved into new species as they adapt to their environments. The finches on the Galapagos Islands are an example of this.
Direct observations as evidence of evolution
Evidence of evolution can also be observed directly in species with fast reproductive cycles, such as bacteria.
For example, when bacteria are exposed to antibiotics, individuals with no resistance quickly die off. Individuals with resistance to the antibiotic are able to survive and reproduce. Then, resistant traits are passed on to more individuals in the population. Eventually, the population becomes more resistant to antibiotic treatment.
Evolution by natural selection can also be observed in species that evolve as a response to introduced species in their environment. An example of this is the different beak lengths of the soapberry bug which evolved according to the available food source. In Southern Florida, soapberry bugs feed on the seeds of the native balloon vine. In Central Florida, balloon vines (Fig. 9) have become rare, so soapberry bugs shifted to the seeds of the golden rain tree (Fig. 10)–an introduced species—as their food source.
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Figures 9-10. The seeds of the balloon vine fruit are farther from the surface than the seeds of the golden rain tree fruit (right). Source (left): H. Zell, CC BY-SA 3.0, via Wikimedia Commons. Source (right): Art Davis, CC BY-SA 4.0, via Wikimedia Commons.
Soapberry bugs can feed more effectively when their beak is long enough to reach the seeds within a fruit. Because seeds of the golden rain tree fruit are closer to the surface than the seeds of the balloon vine, soapberry bugs that feed on the seeds of the golden rain tree have shorter beaks. In Louisiana, Oklahoma, and Australia, soapberry bug populations feed on introduced plants that have fruits larger than those of the balloon vine. In these areas, soapberry bugs evolved to have longer beaks.
Is there evidence of human evolution?
Humans are Homo sapiens, a primate species that walks upright and has a large, complex brain with a capacity for the use of tools, language, symbolic expression, and culture. Fossilized bones revealed the physical appearance of early humans and how they changed over time. On the other hand, tools, pottery, jewelry, and other archaeological evidence show the activities of early humans.
Evidence shows that humans first emerged in Africa. Bipedalism in humans evolved over 4 million years ago, while other traits like the use of tools and symbolic expression emerged only tens of thousands of years ago.
Homo sapiens is the last living species of the zoological tribe Hominini. Fossils show that our species once existed alongside another species of Homo, the Neanderthals (Fig. 11). Fossils and genetics also show that we and other species of Homo are closely related to and share a common ancestor with other Great Apes, like chimpanzees and gorillas.
Figure 11. Using fossils and archaeological evidence, we are able to reconstruct what a Neanderthal man could have looked like. Source: Jakub Hałun, CC BY-SA 4.0, via Wikimedia Commons.
Evidence of Evolution - Key takeaways
- The theory of evolution is supported by a wide range of evidence including fossils, homology, molecular biology, biogeography, and direct observations.
- Fossils show how organisms evolved, how new groups of organisms emerged, and how some species became extinct.
- Homology shows how different species with similar structures can be traced back to common ancestors and how different species with similar structures but no recent common ancestors were subjected to common selection pressures.
- All life forms have DNA which suggests that all species came from a very distant common ancestor.
- Some patterns in the geographic distribution of life forms can be explained by evolution alongside the movement of tectonic plates.
- Evidence of evolution can be observed directly in species with fast reproductive cycles and in species that evolve as a response to introduced species in their environment.
- Fossilized bones and archaeological evidence give us insight into human evolution.
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