The Bajau Sea Nomads
Many human populations have unique physiological adaptations. Some of these adaptations are particularly memorable, like the ability to hold one’s breath for an extraordinary amount of time. The Bajau are a seafaring population in Southeast Asia who have this adaptation. They can hold their breath for over 5 minutes, while highly trained divers from other populations can only hold it for 3 or 4. Bajau divers use this extreme diving ability to spend hours each day hunting underwater for fish. 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 (1). 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.
Types of Analyses: Proximate vs. Ultimate
Explaining how it is that the Bajau are able to hold their breath for so long while diving requires both proximate and ultimate explanations. The researchers found that Bajau divers had significantly larger spleens than Saluan individuals (Ilardo et al. 2018). The spleen stores oxygenated blood, which can enable long bouts of not breathing. In fact, marine mammals tend to have enlarged spleens. This additional storage of oxygenated blood provides a proximate explanation for how Bajau divers can hold their breath for so long. This proximate explanation then leads questions about ultimate causes - is an enlarged spleen an evolved adaptation, or is it the result of phenotypic plasticity? And if it is an evolved adaptation, how did it evolve?
First, we must ask: is an enlarged spleen the result of a genetic adaptation produced by natural selection? It is possible that spleen size was not under natural selection in the Bajau. Instead, larger spleens could simply be the product of phenotypic plasticity. If lifting weights every day results in larger muscles, it may be that frequently holding one's breath results in a larger spleen. The researchers tested this hypothesis by comparing the spleen sizes of Bajau divers to Bajau people who do not engage in much diving. They found that non-divers within the population have spleens that are just as large as that of the divers (1).
The researchers’ finding that spleen size among the Bajau is equally large in divers and non-divers suggests that spleen size is a genetic adaptation, and not the product of phenotypic plasticity. Their next step, therefore, was to search for genetic evidence of natural selection. Ilardo et al. used DNA samples from the Bajau, the Saluans, and more distant populations to perform a selection scan to search for genes that have been under positive natural selection in the Bajau. The researchers did find a promising gene: PDEA10, which is associated with spleen size. These findings serve as further evidence that the breath-holding ability of the Bajau is a genetic adaptation caused by natural selection. Overall, this is a good example of how scientists can use empirical evidence to test hypotheses about whether a given trait is an evolved adaptation, without depending on “just-so” stories about human evolution.
Cultural practices: Gene/culture coevolution
Furthermore, the case of the Bajau brings up the idea of the context of natural selection. The Bajau have lived a nomadic lifestyle of marine hunting and gathering for over 1,000 years. The traits that enable hypoxia tolerance in this population appear to be an evolutionary adaptation to this lifestyle. Therefore, this is an example of a recent population-level adaptation that evolved in response to a cultural practice, rather than in response to a largely unavoidable environmental feature such as high altitude or disease risk.
The aquatic lifestyle of the Bajau is one of the most fascinating examples of human variation and adaptation. As such, research on the Bajau is likely to capture students' attention, and can be a powerful example to include in EvMed courses. Beyond being generally interesting, the research with the Bajau can nurture an appreciation for human variation, and challenges students to think deeply about evolutionary processes and phenotypic plasticity.
The testing of spleen size among Bajau divers, Bajau non-divers, and Saluan individuals can be mimicked in a class setting to teach phenotypic plasticity and experimental design. After establishing a proximate explanation for the Bajau diving ability, the instructor can present students with the task of identifying how researchers might determine whether an enlarged spleen can be explained by phenotypic plasticity. Emphasis may be placed on students figuring out what features to measure, which groups to compare, and what results they would expect to see if a large spleen is caused by phenotypic plasticity, vs. what they would expect to see if it is caused by a genetic adaptation. This would be a chance for students to apply the principle of phenotypic plasticity while also practicing the skill of experimental design.
Students are often quick to assume that many traits are adaptations. However, it is important to point out that empirical evidence is required to understand whether or not a trait is adaptive. For instance, instructors can initiate a class discussion about whether evidence against phenotypic plasticity qualifies as sufficient evidence in favor of a trait being a genetic adaptation. Here the instructor may bring in the idea of genetic selection scans, how they work, and how genetic information can be used as further evidence about whether a trait is the product of natural selection.
The Bajau example is a chance for students to grapple with questions about the strength of natural selection and the role of gene flow between populations. Students could be asked to consider: What can we hypothesize about the relative strength of natural selection if these diving adaptations arose in response to the Bajau lifestyle? What does the presence and strength of selection suggest about the historic relationship between diving ability and evolutionary fitness in this population? How would gene flow between the Bajau and neighboring populations affect the evolutionary rate of these adaptations, and what might we hypothesize about the extent of gene flow? These questions are just a few ways to use this example to explore how natural selection and other evolutionary forces shape human traits. This is also a good time to highlight the fact that biological adaptations don’t arise simply because they would be useful - the lack of an advantageous trait must have regularly lead to death and/or reduced reproductive success during life in order for the advantageous trait to have been selected for.
This example also provides an opportunity to discuss what conditions may be necessary for a cultural practice to act as a selective pressure, and to compare this to other examples of gene/culture coevolution, such as lactose tolerance.