Rayleigh Scattering Links Blue Skies, Oceans, and Ancestry of Blue Eyes

The blue coloration of the Earth's sky is scientifically attributed to Rayleigh scattering, a phenomenon where shorter wavelengths of light are dispersed more effectively than longer ones as solar radiation interacts with minute atmospheric particles, primarily nitrogen and oxygen. This mechanism, recently discussed by science communicator Andrea Danta on the Cadena SER program 'Serendipias' in a broadcast updated in December 2025, explains why blue light scatters intensely across the visual field. This identical physical process is also responsible for the blue appearance of the world's oceans and, critically, the coloration of human irises, refuting the notion that blue eyes result from surface reflection.

Blue eyes are structurally colored, lacking any actual blue pigment in the iris's outermost layer, the stroma, which remains translucent. Light entering this stroma undergoes scattering, and due to the Rayleigh effect, the shorter blue wavelengths are reflected outward, creating the characteristic visual effect. Scientific consensus, reinforced by research reiterated in 2025, confirms that the ancestral human eye color was universally brown thousands of years ago. The transition to blue eyes is traced to a singular genetic event: a mutation within the OCA2 gene, which regulates melanin production.

Research spearheaded by Professor Hans Eiberg of the University of Copenhagen firmly links all blue-eyed individuals to one common progenitor who lived approximately 6,000 to 10,000 years in the past, with the likely origin situated north of the Black Sea region. This specific genetic alteration, located in the adjacent HERC2 gene, functioned as a molecular switch, curtailing melanin production in the iris stroma and diluting the brown base color to blue without causing complete albinism. This specific genetic switch has propagated across successive generations to the current human population.

The HERC2 gene, situated near OCA2 on chromosome 15, plays a pivotal regulatory role, with a specific polymorphism in its intron 86 region attenuating the expression of the OCA2 promoter, consequently reducing the functional P protein required for melanosome maturation. An examination of mitochondrial DNA across 800 blue-eyed individuals spanning countries from Denmark to Jordan identified a shared haplotype linked to this singular mutation, strongly suggesting a common founder event. While initial hypotheses placed this mutation during the Neolithic period, more recent ancient DNA analysis suggests the OCA2 allele responsible for blue eyes may date back to the initial out-of-Africa migration around 50,000 years ago, entering Europe from Western Asia.

The prevalence of blue eyes today is estimated to be between 8% and 10% of the global population, a relatively small fraction compared to the dominance of brown eyes worldwide. The emergence of this trait represents a neutral evolutionary change, one of several mutations affecting pigmentation that do not inherently confer a survival advantage or disadvantage. The study of this single genetic switch provides a compelling case study in human population genetics and the profound impact of minor genomic variations on observable phenotypic traits across millennia.