The human blood classification system is based on the presence or absence of specific antigens on the surface of red blood cells. The ABO system categorizes blood into four main groups: A, B, AB, and O, while the Rh factor determines if the blood is positive or negative. These blood types are inherited genetic markers that provide anthropologists with clues about human migration and population history. While the blood types of Indigenous populations in the Americas have often been simplified, the actual distribution is complex and reflects thousands of years of history and population movement.
The Historical Dominance of Type O
The genetic baseline for nearly all pre-Columbian Indigenous populations across the Americas was overwhelmingly Type O. This high prevalence is a direct consequence of the unique population dynamics that occurred during the initial settlement of the continent. The widely accepted theory suggests that the earliest human groups migrating from Asia across the Bering land bridge carried a limited gene pool.
This event created a severe genetic bottleneck, where only a small number of people contributed their genes to the subsequent generations. The resulting phenomenon, known as the founder effect, meant that the genetic traits present in that small founding group became amplified in the much larger descendant population. Since Type O was dominant or fixed in this small initial group, it became the defining blood type across the newly settled continents.
Explaining the Presence of Blood Type A
The presence of Type A blood in Native American populations is attributed to two distinct historical mechanisms. One theory posits an ancient, separate migration event that occurred after the initial Type O founders settled the continent. This later wave, possibly arriving thousands of years after the first settlers, may have carried the Type A gene from a different, isolated region of Asia.
This ancient Type A introduction is thought to be most prevalent in certain populations in the northern parts of North America, suggesting a different point of entry or ancestral population. However, the more widespread explanation for Type A in the Americas is post-Columbian admixture with European populations. Following the arrival of Europeans in the 15th century, intermarriage and genetic exchange introduced the Type A allele into the Indigenous gene pool, particularly in regions with heavy colonial contact. European populations generally have high frequencies of Type A blood, making this a common source of the gene in contemporary tribes.
The “Positive” factor refers to the presence of the Rh antigen on the red blood cells. Since the Rh-positive trait is extremely common globally, including in Asia and Europe, the A Positive designation is unsurprising and not specific to any particular ancient migration route.
Blood Types as Markers of Human Migration
Anthropologists utilize blood group distributions to model ancient human movements. The geographical patterns of A and O frequencies help researchers hypothesize about the sequence and timing of settlement waves into the Americas. For instance, the highest concentrations of Type O are typically found in South America, supporting the idea that the earliest Type O groups migrated rapidly southward, establishing strong founder effects before subsequent migration waves arrived. In contrast, areas with higher Type A frequencies are often found closer to the proposed northern entry points.
A particularly informative marker is the near-total absence of the Type B allele in most pre-Columbian populations. Since Type B is common in certain parts of Asia, its rarity in the Americas strongly suggests that the specific populations that crossed the Bering Strait were derived from Asian groups where the B allele was also rare or absent. This data provides a strong constraint on migration models, helping scientists narrow down the specific source populations in Siberia or Northeast Asia that contributed to the peopling of the New World.
Current Distribution and Tribal Variation
While historical dominance of Type O is a foundational genetic truth for the Americas, current blood type statistics reveal significant variation across different tribal nations. This modern distribution reflects both deep ancestral history and the subsequent effects of gene flow and population shifts. Some groups, particularly in the Pacific Northwest and the Blackfoot Confederacy, show notably higher frequencies of the Type A gene than many other Indigenous populations.
This pattern aligns geographically with the theory of a possible second, ancient migration wave carrying Type A that settled in the northern regions. Conversely, many tribes in Central and South America retain a nearly 100 percent Type O frequency, underscoring their relative isolation from later migration waves and minimizing the impact of post-Columbian admixture.