Common descent

The idea of common descent predates Darwin, but it was Charles Darwin who first marshaled compelling evidence and proposed a mechanism. In On the Origin of Species (1859), Darwin argued that all organisms are united in a single branching tree of life, a view that was controversial at the time.² Earlier thinkers, such as Jean-Baptiste Lamarck, had suggested transformation of species, but Darwin's synthesis of evidence from biogeography, paleontology, and comparative anatomy made common descent a testable scientific hypothesis.

Darwin's contemporary, Thomas Henry Huxley, was an early and vocal advocate, famously debating Bishop Samuel Wilberforce in 1860. The acceptance of common descent grew as paleontologists discovered transitional fossils, such as Archaeopteryx, which bridged reptiles and birds.

The discovery of DNA structure and the genetic code provided the most powerful evidence for common descent. All known life uses the same genetic code, with only minor variations, and shares core metabolic pathways, indicating a single origin. Molecular phylogenetics reconstructs evolutionary relationships by comparing DNA sequences; the resulting trees are largely congruent with those based on morphology.

Comparative genomics has revealed that humans share about 98.8% of their DNA with chimpanzees and about 60% with chickens, reflecting degrees of relatedness. The existence of pseudogenes — nonfunctional remnants of genes — also supports common ancestry, as they are shared among species that inherited them from a common ancestor.

The last universal common ancestor (LUCA) is the hypothetical population from which all life on Earth descends. It is not the first life form, but the most recent ancestor of all extant organisms. LUCA likely lived around 3.5–4 billion years ago, in a high-temperature, anaerobic environment, possibly near hydrothermal vents.³

Genomic studies have inferred LUCA's gene set, which includes genes for transcription, translation, and basic metabolism. However, LUCA was not a simple organism; it likely had a complex machinery and may have been part of a community of proto-cells that exchanged genes via horizontal gene transfer.

Common descent has surprising implications. For example, the human appendix, once considered vestigial, is now known to have immune functions, but its evolutionary history still reflects descent from herbivorous ancestors.⁴ The concept also extends to non-coding DNA: some transposons, or 'jumping genes', are shared across species, indicating ancient insertions.

Another edge case is the possibility of a second origin of life on Earth, which would challenge common descent. Some scientists have proposed that life may have started multiple times, but all known life shares the same biochemistry, suggesting either a single origin or that other lineages went extinct. The search for alien life, such as on Mars or Europa, could reveal a separate origin, which would be a profound discovery.

LUCA

Last Universal Common Ancestor; the most recent population from which all extant organisms descend.

Molecular phylogenetics

The use of molecular data (DNA, RNA, protein sequences) to infer evolutionary relationships.

Pseudogene

A nonfunctional copy of a gene, often with mutations that prevent expression.

Horizontal gene transfer

The movement of genetic material between organisms other than parent-to-offspring transmission.