Biological evolution describes the process by which populations of organisms change over generations through mechanisms such as natural selection and genetic drift. The theory of common descent is the overarching framework that organizes this understanding of life. This concept posits that all living things on Earth—from the smallest bacteria to the largest blue whale—are related through a single ancestry. Common descent provides the explanatory power for the immense diversity of life and is the foundational principle of modern biology.
The Core Concept of Shared Ancestry
The theoretical basis for common descent rests on the idea of a single, ancient population that gave rise to all subsequent life forms through gradual modification. This ancestral population is often referred to as the Last Universal Common Ancestor, or LUCA. LUCA represents the point from which the three major domains of life—Bacteria, Archaea, and Eukarya—began to diverge.
LUCA is a complex, hypothetical entity whose traits are inferred by identifying features shared across all modern life. It is believed to have been a single-celled, prokaryote-like organism that existed between 3.5 and 4.3 billion years ago. This ancestor already possessed sophisticated cellular machinery, including the ability to store genetic information and produce proteins. Descent with modification explains how slight genetic changes passed down over vast geological timescales resulted in the variety of species seen today.
The Evidence from DNA and Molecular Biology
The strongest support for common descent comes from the molecular level, where relationships between organisms are preserved in their biochemistry. A striking piece of evidence is the universality of the genetic code. Nearly all cellular life uses the same four nucleotide bases (Adenine, Thymine, Cytosine, and Guanine) to form DNA and RNA, and uses a three-base sequence, or codon, to specify one of the twenty common amino acids. This shared coding system indicates that all extant life inherited its fundamental information processing mechanism from a single source.
Further evidence is found in conserved genes, which are sequences that have remained largely unchanged across diverse species because they code for necessary functions. For instance, genes involved in cellular respiration are highly similar between yeast, plants, and humans. The presence of nearly identical genes in distantly related organisms supports a shared origin rather than multiple independent developments.
Genetic sequencing allows scientists to trace lineage by comparing the DNA sequences of different species. The more time that has passed since two species shared an ancestor, the more differences will have accumulated in their genomes due to random mutations. By measuring the rate at which mutations accumulate, scientists use these conserved sequences as a “molecular clock” to estimate divergence times. This technique confirms dates derived from the fossil record, such as estimating the common ancestor of humans and chimpanzees lived approximately 6 to 7 million years ago.
Physical Evidence in Anatomy and Fossils
The physical structures of organisms also provide evidence for shared ancestry through comparative anatomy. Homologous structures are features that share a common ancestral origin but have evolved to serve different functions in descendant species. A classic example is the pentadactyl limb, the five-fingered skeletal structure found in the forelimbs of all mammals, including the human hand, the flipper of a whale, and the wing of a bat. The underlying bone arrangement is nearly identical, demonstrating a shared structural blueprint inherited from a common ancestor.
Another line of evidence comes from vestigial structures, which are reduced or non-functional remnants of features that were functional in an ancestral organism. For example, some snakes, like the boa constrictor, possess tiny pelvic bones and hind limbs embedded within their bodies that serve no clear purpose. These structures are remnants from their tetrapod ancestors, reflecting a loss of function over time.
The fossil record offers a chronological sequence of life, documenting the appearance and extinction of species over geological time. The fossil record contains transitional fossils that exhibit features intermediate between different groups of organisms. The sequence charting the evolution of whales, for instance, shows forms with hind legs that gradually became smaller and more specialized for aquatic life. This series provides a direct history of descent with modification, illustrating the evolutionary steps between terrestrial mammals and modern whales.
Understanding the Tree of Life
The theory of common descent is visualized through a diagrammatic framework known as the Tree of Life, or a phylogenetic tree. This model represents the evolutionary history and relationships among groups of organisms. The base of the tree is the root, which represents the Last Universal Common Ancestor of all species included in the diagram.
The lines extending from the root are branches, representing evolutionary lineages. The points where branches split are called nodes, and each node signifies a divergence event where an ancestral species split into two or more descendant species. This branching pattern demonstrates how all life is interconnected, with more closely related species sharing a more recent common node.
Groups that include an ancestor and all of its descendants are called clades. These nested groups reflect the hierarchical nature of evolution. The Tree of Life model is constantly refined and tested by evidence from molecular biology and physical anatomy. By mapping genetic sequences and structural similarities onto this branching diagram, scientists build a detailed map of the vast biodiversity resulting from a single common origin.