The Bajau (Sea Nomads)
Many human populations have unique physiological adaptations. The Bajau, an island population in Southeast Asia, rely on an extreme ability to dive underwater and hunt for fish. Bajau divers, who are capable of holding their breath for extraordinary lengths of time, are underwater for hours everyday. This lifestyle, which the Bajau have practiced for thousands of years, has led many to refer to them as Sea Nomads.
A team of researchers recently examined how these divers are able to tolerate such extreme oxygen deprivation (Ilardo et al. 2018). They compared Bajau divers with individuals from a nearby island population - the Saluan. The Saluan share recent common ancestry with the Bajau, but are rarely in the water.
The researchers found that Bajau divers had significantly larger spleens than Saluan individuals. The spleen stores oxygenated blood, a function that helps enable long bouts without breathing. In fact, marine mammals tend to have have enlarged spleens. This supports the idea that the aquatic lifestyle of the Bajau selected for individuals with larger spleens.
However, spleen size may not have been under selection; larger spleens may simply be the product of phenotypic plasticity. If lifting weights everyday results in larger muscles, perhaps the frequently holding one's breath results in a larger spleen. The researchers found support that spleen size is a genetic adaptation. First, not all Bajau dive, and the spleens of non-diving Bajau are just as large as divers. Next, the researchers found genetic variants unique to the Bajau that showed signals of selection. Several of these genetic variants are associated with hypoxia tolerance, like blood vessel constriction in the extremities.
The aquatic lifestyle of the Bajau is one of the most fascinating examples of human variation and adaptations. As such, the research on the Bajau is likely to capture students' attention, and can be a powerful example to include in EvMed courses. Beyond beying generally interseting, the research with the Bajau can nurture an appreciation for human variation, and challenges students to think deeply about phenotypic plasticity and evolutionary processes. A potential lesson could pose the question - are the Bajau diving abilities of the Bajau a genetic adaptation? Advanced students could be asked to come up with research designs that could test their hypothesis. Less advanced students may instead be asked to analyze both the study design and results from Ilardo and colleagues as a means to practice thinking like a scientist.
Anatomical traits such as an enlarged spleen, and various physiological responses to diving can potentially be the result of phenotypic plasticity, genetic adaptation, or both. The instructor can prompt students to consider what researchers would expect to observe if the traits are the result of plasticity vs. genetic adaptation, and to then interpret the results of the study accordingly.
Research with the Bajau provides an example of ways to test whether natural selection has taken place. It also provides insight into the timescales at which natural selection can occur in humans. Further, the researchers found that one of the genes under selection in the Bajau may have come from Denisovan introgression. This offers a chance to also discuss gene , and thus arrived in a modern human population via gene flow from a more archaic hominin.
It is impossible to understand the evolutionary history of the Bajau without taking culture into consideration. The Bajau have lived a lifestyle of marine hunting and gathering for over 1,000 years. The genetic adaptations that help the Bajau thrive in this ecology were selected for by cultural practices of diving when hunting for fish.