Biological evolution is the process by which the heritable characteristics of biological populations change over successive generations. This concept explains the staggering diversity of life on Earth, from single-celled organisms to complex animals and plants. Scientific support for this process is not based on a single piece of evidence but on multiple, distinct lines of inquiry that converge on the same conclusion. These lines of evidence, drawn from fields like paleontology, anatomy, genetics, and biogeography, illustrate a continuous, branching tree of life connected by common ancestry.
The Historical Record: Evidence from Fossils
The fossil record provides an unparalleled historical sequence of life, acting as a timeline for the appearance and disappearance of species over deep time. Fossils are preserved in sedimentary rock layers, which are deposited sequentially. The principle of superposition establishes that in undisturbed layers, the oldest strata and the fossils they contain are at the bottom, and the youngest are at the top.
Scientists determine the age of these layers using two complementary methods: relative and absolute dating. Relative dating places fossils in chronological order based on their position within the rock strata. Absolute dating assigns a specific, numerical age by measuring the decay of radioactive isotopes, such as Potassium-40 to Argon-40, which proceeds at a known, constant rate. This radiometric technique calibrates the entire geologic time scale.
The fossil record reveals a clear pattern of successive change, with simpler life forms found in older layers and more complex forms appearing in younger strata. The record contains numerous examples of transitional fossils, which exhibit traits intermediate between an ancestral and a descendant group. Archaeopteryx, for instance, displays reptilian features (teeth and a bony tail) alongside flight feathers, linking non-avian dinosaurs and modern birds. Another compelling series documents the evolution of whales from four-legged land mammals, showing the gradual transition of the inner ear structure and the reduction of hind limbs over millions of years.
Shared Blueprint: Comparative Anatomy and Development
Anatomical comparisons show that many species share structural similarities inherited from a common ancestral blueprint. Structures similar in underlying form, but evolved to perform different functions, are known as homologous structures. The forelimbs of all mammals illustrate this concept: the human arm, the bat’s wing, the whale’s flipper, and the cat’s leg all share the same arrangement of bones (humerus, radius, ulna, wrist, and finger bones). This shared internal architecture points to a common mammalian ancestor.
In contrast, analogous structures serve a similar function but evolved independently in different lineages due to similar environmental pressures. For example, the wing of a bat and the wing of an insect both allow for flight, but their internal anatomies are entirely dissimilar. These cases of convergent evolution show how different species arrive at similar solutions when faced with comparable ecological challenges.
Further evidence comes from vestigial structures, which are remnants of organs that were functional in an ancestor but have lost their original function. The tiny, non-functional pelvic bones found in modern whales are a classic example, tracing back to the hind limbs of their terrestrial ancestors. Comparative embryology also shows that the embryos of many different vertebrates, including humans, exhibit shared features in their earliest stages, such as pharyngeal arches and a post-anal tail, reflecting a shared history with aquatic ancestors.
Molecular Connections: DNA and Genetic Evidence
The evidence for common descent is found at the molecular level, in the genetic code shared by virtually all life on Earth. Every known organism, from bacteria to blue whales, utilizes the same four nucleotide bases and the same system of three-base codons to encode the twenty amino acids that build proteins. This near-universal genetic language is the strongest indicator that all life arose from a single, ancient common ancestor.
The degree of genetic similarity between species directly correlates with their evolutionary relatedness, allowing scientists to map out the tree of life. For instance, humans and chimpanzees share an astonishingly high proportion of their DNA sequence, often showing a similarity of approximately 98.8% in protein-coding regions. This small genetic difference accounts for the biological distinctions between the two species.
The sequences of specific, universally conserved proteins also track divergence times. The protein cytochrome c, involved in cellular energy production, is found in nearly all eukaryotes. By counting the number of amino acid differences in the cytochrome c protein between species, scientists can estimate how long ago they diverged. The cytochrome c of a human is identical to that of a chimpanzee, differs by only one amino acid from a rhesus monkey, but differs by 44 amino acids from yeast, accurately reflecting the vast evolutionary distance between these groups.
This concept is formalized in the molecular clock, a technique relying on the observation that neutral mutations—genetic changes that do not affect an organism’s survival—accumulate at a roughly constant rate over long periods. By calibrating this rate against known divergence times from the fossil record, researchers use accumulated mutations in DNA or proteins to date evolutionary branching points that have left no fossil evidence.
Species Distribution: Insights from Biogeography
Biogeography, the study of the geographical distribution of organisms, provides a powerful spatial framework for understanding evolution. The patterns observed in where species live are logical only when viewed through the lens of descent with modification and geological change.
A key observation is the existence of endemic species—those found exclusively in a specific geographic region, often isolated islands. The Galapagos finches are a famous example; 13 to 18 distinct species, each with unique beak adaptations for different food sources, are found only on this archipelago. Evidence points to a single colonization event by a mainland South American ancestor. Geographical isolation between the islands, paired with distinct local environments, drove the ancestral population to diversify into the array of endemic species seen today.
On a larger scale, the distribution of entire groups is explained by continental drift. The concentration of marsupial mammals, such as kangaroos and koalas, primarily in Australia and South America, is a classic biogeographical puzzle. Fossil and genetic evidence indicates that marsupials originated in the Americas and spread when the southern continents were joined in the supercontinent Gondwana. As Australia and South America drifted apart, the marsupial populations were isolated, allowing them to diversify into their current forms without competition from placental mammals that dominated other landmasses.