A bat's wing, a whale's flipper and your arm look nothing alike on the outside, yet inside they are built from the same set of bones in the same order. Why would a flying animal, a swimming animal and a tool-using animal share one skeletal blueprint? Evolution answers that question: species change over many generations, and species that share a common ancestor inherit a shared body plan that has been modified for different ways of life. In this topic you will see where the idea came from and, more importantly, the evidence that makes evolution one of the best-supported explanations in all of science.
Ideas that shaped Darwin's thinking
- Charles Lyell (geologist) argued in Principles of Geology that Earth's features were shaped by slow processes we can still watch today, such as erosion and sediment deposition, acting over enormous spans of time. This idea is called uniformitarianism. If Earth is very old, there has been enough time for small changes in living things to add up to large ones. Darwin read Lyell's work during his voyage.
- Thomas Malthus (economist) wrote in 1798 that human populations tend to grow faster than their food supply, so many individuals must face shortages, disease and competition. Darwin applied this to every species: far more offspring are produced than can survive, so there is a constant struggle for existence.
- Jean-Baptiste Lamarck (naturalist) was one of the first to propose that species change over time. His mechanism, however, was the inheritance of acquired characteristics: body parts that are used a lot grow stronger, unused parts shrink, and those changes are passed to offspring. His idea that species change was right; his mechanism was wrong.
- Georges Cuvier (anatomist) showed from fossil bones that some animals, such as mammoths, no longer exist anywhere on Earth. That made extinction an accepted fact, and it meant the living world had not always looked the way it does now.
- Ancestral giraffes stretched their necks to reach high leaves.
- Stretching made each giraffe's own neck longer during its lifetime.
- Parents passed their stretched necks on to their young.
- Problem: changes to body cells during life do not change the DNA in eggs and sperm, so they are not inherited.
- Ancestral giraffes already varied in neck length because of inherited differences.
- When food was scarce, longer-necked individuals reached more leaves and survived better.
- Survivors had more offspring, who inherited the genes for longer necks.
- Over many generations, the average neck length of the population increased.
Charles Darwin sailed around the world aboard HMS Beagle from 1831 to 1836, collecting fossils and living specimens, including from the Galápagos Islands. Over the following two decades he built the case for evolution by natural selection. Alfred Russel Wallace reached the same idea independently, and their work was presented together in 1858. Darwin published On the Origin of Species in 1859. The central claims are that all living things are related through descent with modification from common ancestors, and that natural selection is a major cause of that modification.
Evidence 1: the fossil record
A fossil is the preserved remains or trace of an organism from the past. In undisturbed sedimentary rock, lower layers were deposited first, so they are older. When palaeontologists compare layers, they find simpler, single-celled life in the oldest rocks and more complex organisms appearing later, and they find species that no longer exist. Transitional fossils show a mix of features from an ancestral group and a descendant group. Tiktaalik, found on Ellesmere Island in Nunavut, is about 375 million years old and has fish features (scales, gills, fins) together with features of four-limbed animals (a flat head, a neck, and fins containing wrist-like bones). Archaeopteryx, about 150 million years old, has feathers and wings like a bird but also teeth, clawed fingers and a long bony tail like small dinosaurs.
Rock layers give relative ages (older or younger). Radiometric dating gives absolute ages in years. A radioactive isotope decays at a fixed rate, measured by its half-life, the time for half of it to decay. Carbon-14 has a half-life of about 5730 years and is used for once-living material up to roughly 50 000 years old; isotopes with half-lives of millions or billions of years are used to date rock. These methods place Earth's age at about 4.6 billion years, giving evolution the long time span that Lyell's geology suggested.
Evidence 2: biogeography
Biogeography is the study of where species live. If species had simply been placed where conditions suit them, islands with similar climates around the world would have similar species. Instead, island species most closely resemble species on the nearest mainland. The plants and animals of the Galápagos, for example, resemble those of mainland South America, which is what you would expect if ancestors arrived from that mainland and then changed. Likewise, Australia, isolated for a very long time, is dominated by marsupial mammals such as kangaroos and koalas that are found almost nowhere else.
Evidence 3: anatomy, embryology and DNA
| Type of structure | Meaning | Example | What it tells us |
|---|---|---|---|
| Homologous | Same underlying structure and origin, often different functions | Human arm, cat leg, whale flipper, bat wing | The species share a common ancestor |
| Analogous | Same function, but different structure and origin | Bird wing and butterfly wing | Similar environments can select for similar solutions (convergent evolution); not evidence of close relationship |
| Vestigial | Reduced structure that has lost most or all of its original function | Small pelvic bones in whales; reduced eyes in blind cave fish; the human tailbone | The species descended from ancestors in which the structure was fully functional |
Embryology adds more evidence. Early embryos of fish, reptiles, birds and mammals all have pharyngeal arches in the neck region and a tail extending past the anus. In fish the arches develop into gill supports; in mammals they develop into parts of the jaw, ear and throat. Shared early development points to shared ancestry. Finally, molecular evidence is the strongest of all. Nearly every organism uses the same genetic code, and the more closely two species are related by anatomy and fossils, the more similar their DNA and protein sequences are. Humans and chimpanzees, for instance, have DNA sequences that are about 98 to 99 percent identical in regions that can be directly compared. Independent lines of evidence (rocks, geography, bones, embryos and genes) all point to the same family tree.