Type A is the original human blood type, and Type O evolved later through a genetic mutation.

Evolutionary biologists and geneticists from institutions like the University of California, San Diego, have traced the ABO gene back millions of years. Their research indicates that Type A is the ancestral state shared with our primate relatives, including chimpanzees and bonobos. Type O is actually a derived version of the Type A gene, resulting from a specific deletion mutation in the DNA sequence.This mutation involves a single nucleotide deletion in exon 6 of the ABO gene, which causes a frameshift. This shift results in a truncated, non-functional protein that cannot attach sugar molecules to the surface of red blood cells. Without these sugars, the blood is classified as Type O. Scientists estimate that this specific mutation became prominent in the human lineage roughly 1 million to 2.5 million years ago.The prevalence of Type O is widely attributed to natural selection and 'heterozygote advantage.' In regions where Plasmodium falciparum malaria is endemic, individuals with Type O blood often experience less severe symptoms compared to those with Type A or B. This survival advantage allowed the mutation to spread rapidly through ancient populations.Today, Type O is the most frequent blood type globally, appearing in approximately 45% of the population. While Type A was the original blueprint, the evolutionary pressure of infectious diseases reshaped the human genetic landscape. This transition highlights how a simple genetic 'error' can become a critical survival mechanism over hundreds of thousands of years.