Deoxyribonucleic acid (DNA) functions as both the blueprint for life and a detailed historical ledger. This molecule holds the complete instructional set for building and operating every living organism, containing accumulated errors and changes passed down through generations. By comparing the genetic sequences of different species, scientists can trace the lineage of life backward through time. These molecular archives reveal species relationships, showing that all life on Earth shares a common ancestor. The closer the relationship between two species, the more recently they diverged, and the more similar their genetic instructions remain.
DNA Variation and the Mechanism of Change
Evolutionary change begins with genetic variation generated primarily through mutation, the source of all new genetic material. Mutations are spontaneous, random alterations in the DNA nucleotide sequence, ranging from single base-pair changes to larger insertions or deletions. These changes are not directed by any specific need of the organism.
Many mutations are neutral or harmful, but some are beneficial. This random variation provides the raw material for natural selection. Natural selection is the environmental filter that favors traits enhancing survival and reproductive success. Organisms with advantageous mutations pass those genes to the next generation, increasing the trait’s frequency in the population.
The accumulation of these selected changes causes populations to gradually diverge, leading to new species formation. This divergence is the molecular basis of evolution. Small genetic differences are magnified as populations adapt to distinct environmental pressures. This process explains how a single ancestral species gave rise to the diversity of life observed today.
Evidence from Shared Genetic Structures
Comparing DNA structure and sequence across different organisms provides strong evidence for a shared evolutionary history. The genetic code is nearly universal, using the same four nucleotide bases and three-base codons to direct protein production in almost all life forms, from bacteria to humans. This shared molecular machinery points directly to a single common ancestor. The degree of sequence identity between two species’ genes reflects the time elapsed since they last shared an ancestor.
Closely related species exhibit a high degree of genomic overlap. For example, humans and chimpanzees share roughly 98% identical DNA sequences. This similarity is a direct consequence of their recent divergence from a common primate ancestor, estimated six to eight million years ago. The differences in the remaining 2% of the genome account for the anatomical and behavioral distinctions between the two species.
Some genes are highly conserved, meaning they have remained unchanged across vast stretches of evolutionary time because they are fundamental to life. Genes for basic cellular functions, such as those coding for ribosomal RNA or proteins involved in cell respiration, can be identical in organisms as diverse as yeast and humans. For instance, the homeobox (Hox) genes control body plan development in nearly all animal species, from insects to mammals. This demonstrates a shared architectural toolkit inherited from an ancient common ancestor.
Measuring Evolutionary Time with Molecular Clocks
Molecular data estimates the timeline of evolutionary divergence using the molecular clock concept. This technique relies on neutral mutations—those not subject to natural selection—accumulating in DNA at a relatively consistent rate. These neutral changes record the passage of time since two lineages separated.
Scientists calculate a gene’s mutation rate by comparing its sequence in two species with a known divergence time, often established through fossil evidence. This known date provides a calibration point. The number of accumulated genetic differences between any two species is then divided by that rate to estimate when their common ancestor lived. Different genes and DNA regions evolve at different speeds, requiring careful selection of the molecular clock.
Mitochondrial DNA mutates faster than nuclear DNA, making it useful for dating recent evolutionary events, such as the divergence of human populations. Conversely, slowly evolving nuclear genes are suited for estimating deep divergence times between major groups like mammals and reptiles. Although the clock’s pace varies, modern methods use complex statistical models to account for rate variations, providing robust estimates of evolutionary chronology.
Evidence from Genetic Fossils
The genome contains non-functional remnants of genes and viral insertions that serve as molecular fossils of evolutionary history. These genetic fossils are DNA segments that were once functional in an ancestor but were inactivated by mutation, remaining as silent markers. An example is the non-functional gene for L-gulonolactone oxidase (GULO), required for Vitamin C synthesis in most mammals.
Humans, chimpanzees, and other higher primates cannot produce Vitamin C because the GULO gene is inactive, turning it into a pseudogene. All these primate species share the exact same inactivating mutation—a single-nucleotide deletion in exon 10. This shared defect indicates the mutation occurred in a common primate ancestor millions of years ago and was inherited by all descendants. If the loss of function happened independently, the inactivating mutation would likely be different in each species.
Another genetic fossil is the presence of endogenous retroviruses (ERVs), remnants of ancient viral infections that integrated into an ancestor’s germline DNA. If an ERV insertion occurred in the past, all its descendants carry the viral sequence in the exact same genomic location. Finding an identical ERV insertion at the same chromosomal spot in humans and chimpanzees proves the insertion event predates the divergence of these two lineages. These shared genetic markers offer a historical record frozen in time.