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On the authority of the Declaration of Independence, it is a self-evident truth “that all men are created equal.” Yet we hear that biology has demonstrated conclusively that men are unequal. Does biology contradict what the Declaration of Independence holds to be a self-evident truth?
(Source: Dobzhansky T. (1966) Biological Evolution and Human Equality. In: Steinhardt J. (eds) Science and the Modern World. Springer, Boston, MA)
How are the Bajau people genetically disparate from the average person in terms of the ability to deep sea dive over extended lengths of time?
Recently the claim, “Does biology contradict what the Declaration of Independence holds to be a self-evident truth?” was constructed. The claim discussed the link between inequality between humans and the Declaration of Independence. As biology disputes; all men are not equal as after initial research different races have been found to have a large disparity in genetics in terms of their abilities. This claim however did not provide a specific example of a scientific case study where cultural and biogenetic variances contradict the Declaration of Independence.
This then posed a second question “Is there a genetic difference between different races?”. No two people are genetically identical however, genetics does differ between different races. Homo sapiens is a relatively young species and has not had as much time to accumulate genetic variation compared to the vast majority of species on earth, most of which predate humans by enormous expanses of time. Nonetheless, there is considerable genetic variation in our species. The most notable cause of this is natural selection. Natural Selection is the differential survival and reproduction of individuals due to variances in phenotype. It is a key mechanism of evolution affecting the change in the heritable traits and characteristics of a population over generations. A striking example of natural selection is the Bajau people of Southeast Asia, who generationally have developed larger spleens
To confirm this theory, additional scientific evidence and a more specific research question were necessary. In doing so this was further refined to specifically consider genetics and biological evolution. In consequence, a more relevant and detailed question was developed; “How are the Bajau people genetically disparate from the average person in terms of ability to deep sea dive over extended lengths of time?” The Bajau are a tribe of seafaring people, who have lived exclusively on the water for centuries. They reside in Southeast Asia, in the waters of the southwest Philippines. The Bajau “dive repeatedly for about eight hours a day, spending about 60% of their time underwater. So this could be anything from 30 seconds to several minutes, but they're diving to depths of over 70m,' quoted Dr Ilardo. These abilities aren’t merely the results of training, the Bajau population have lived their lives at sea for generations, so much so, that they’ve developed adaptions to their oceanic lifestyle.
An international team of researchers studied the Bajau extensively, and through a portable sonogram, were found to have significantly larger spleens than a neighboring village that primarily farmed as a source of food as opposed to fish. The scans showed that the spleens of the Bajau, whether they are divers or non-divers, are 50% larger than their close geographic neighbors, the Saluan, who have minimal interaction with the marine environment. This suggests that it is an inherited or genetic trait rather than simply a consequence of a lifestyle existing in diving. An increased spleen size is relatively significant as it is a reservoir in which red blood cells are stored. Ensuring that when diving, the spleen contracts and injects additional red blood cells into the circulating blood, increasing its capacity to carry oxygen.
As seen in Figure One, the boxplots are not closely related and it is clear that there is a vast difference between the sizes of the Saluan population's spleen and the Bajau people’s spleens. The maximum size of the Bajau spleen is estimably 275 cm3 whilst the maximum Saluan spleen size was 175cm3. This is a significant dissimilarity in size.
Notably, this difference is not significant when comparing Bajau divers to Bajau non-divers, suggesting the difference between the Bajau and Saluan is not simply driven by the fact that more Bajau individuals are divers.
A DNA analysis revealed a gene variation frequent in the Bajau population assists their ability to deep-sea dive. This gene controls levels of a hormone called T4, which is produced by the thyroid gland. T4 causes an increase in metabolic rate which can help to combat low oxygen levels. Melissa A. Ilardo quotes “We believe that in the Bajau they have an adaptation that increases thyroid hormone levels and therefore, increases their spleen size”. It has been shown in mice that thyroid hormones and spleen size are connected. Tests have shown that if mice have an absence of the thyroid hormone T4, their spleen size is drastically reduced, however, the results are reversible with an injection of T4.
Genetic Variation is a gene thought to be associated with increased peripheral vasoconstriction which oxygenates important tissues like the brain, heart, and lungs, thereby potentially increasing the Bajau’s population ability to dive over an extended period.
The spleens play a vital role in extending the time to free dive as it is a form part of the human dive response. The human dive response is when the human body is submerged under water for any amount of time, this response is triggered as a method of assisting the boy to survive in an oxygen-deprived environment. When this happens the heart rate slows down, the spleen contracts, and blood vessels shrink to preserve blood for vital organs. This allows more oxygenated blood cells to be circulated in the bloodstream and has been found to provide a 9% increase in oxygen, thereby prolonging the time underwater.
An international research team consisting of academics from the University of Copenhagen, Cambridge, and Berkeley eliminated the possibility that larger spleens were just a response to routine diving and began to investigate their genetic data. It was discovered that the Bajau have a gene called PDE10A which was not found in the Saluan’s genetics. The PDE10A gene controls the levels of the thyroid hormone T4.
An increased thyroid hormone PDE10A level in the Bajau population is believed to control their levels of the thyroid hormone, inevitably increasing their spleen size. Dr Racimo said “This gene existed before the Bajau because the Saluan have it too, but once they diverged from each other, at some point in history, it became advantageous for Bajau individuals to have this after the Bajau adapted to this particular lifestyle. Individuals who had it tended to survive more than others or they had more children. The fact that both Bajau divers and non-divers have the large spleen points to the fact that this is something they have from birth rather than something they acquired through experience over time.” This is a clear example of natural selection, where the PDE10A gene has been inherited and passed down ensuring only the fittest pass their genes onto future generations.
Dr Ilardo’s research team also discovered other genetic adaptations. One of these includes BDKRB2, which is the only gene that has been previously linked to diving in humans. It affects the constriction of blood vessels in extreme conditions and how much oxygen reaches the core organs like the brain, heart, and lungs. FAM178B was also found which affects the levels of carbon dioxide in the blood which is an important factor when holding one’s breath. This specific gene that is common in the Bajau seems to have originated from a group of ancient hominids who lived in Asia called the Denisovans. It is clear that when modern humans entered Asia they inherited some of the Denisovan DNA which was then passed down to the Bajau population.
The results of the population genetics analyses suggest that a selection scan comparing Bajau and Saluan would be appropriate to detect Bajau-specific positive selection as a lower number of Bajau would likely be unreliable. Furthermore, previous studies have shown that a larger selection across a range of surrounding populations is highly powerful in detecting local adaptation. Therefore, by merging the data from the Bajau and Saluan a more reliable and unbiased scan on detecting natural selection was performed.
The results of our population genetics analyses identified two seaside villages, Jaya Bakti and Koyoan, primarily inhabited by ethnic Bajau and Saluan populations. 59 Bajau individuals and 1 Saluan were found to be closely related through a genetic test, these individuals were excluded from further analyses. This demonstrates how meticulous the researchers were when collecting data to refrain from being biased. Additional factors were also taken into account, which included, gender, age, weight, height, and whether the individuals are divers as covariates. Two forms of measurements on different machines were taken to collect a range of valid data to corroborate a portable ultrasound was taken and measurements were able to calculate the volume of the spleens which was then correlated with the results using a computed tomography scan. Subsequently, the same tests were taken on the Saluan people and were then compared to the Bajau results which led to the conclusion that the Bajau have larger spleens than the Saluan which makes them more equipped for diving.
This source is from the University of Cambridge which is reliable since it is an educative and scholarly source. This article was published recently on the 20th of March 2018, further supporting the reliability of this source. In 2015, an international research team led by academics from the Universities of Copenhagen, Cambridge, and Berkeley, along with Melissa Ilardo a post-doctoral researcher in the Department of Molecular Medicine at the University of Utah collaboratively experimented on the Bajau population.
According to the University of Cambridge, there is a potential for further research on genetic adaptations to diving lifestyles such as the Thai Moken population and the Haenyeo diving women of Jeju in South Korea. Studying these similar populations will continue to shed more light on the connection between human physiology and natural selection.
This source is relatively recent as it was published on the 20th of April 2018. The same researchers are quoted in this source and the data is also from the same experiment. The team traveled to Indonesia three times to collect relevant data and information on the tribe before experimenting. The research was very thorough and strove for accuracy, precision, and unbiased data. They accounted for confounding factors like age, sex, and height as well as separate testing for any individuals who were related to the Saluan. Therefore, this source is very reliable.
Based on the analysis of evidence and evaluation of sources detailed above, a clear conclusion in response to the research question “How are the Bajau people genetically disparate from the average person in terms of ability to deep sea dive over extended lengths of time?” has been confirmed that the Bajau in their ability to deep-sea dive are dissimilar because of their genetically enlarged spleens. This conclusion also confirms the claim provided, does biology contradict what the Declaration of Independence holds to be a self-evident truth? This conclusion contradicts the Declaration of Independence that “all men are created equal” as this specific example verifies that this population is biologically different from other populations. However, the evidence supporting this claim has varying limitations that may have affected the quality of this investigation. If more research had been done on this topic by scientists a larger understanding could have increased the reliability of data as well as a wider range of sources. In conclusion, the Bajau population in Indonesia has genetically larger spleens enabling them to deep-sea dive for a longer period.
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