Breathtaking evolution amongst Indonesian tribe - bigger spleens for free-diving

By Dr. Ananya Mandal, MDApr 19 2018

New evidence shows that the Bajau tribes from Indonesia, who are known to dive deep into the waters for up to 70 metres have evolved to develop larger spleens. Previous studies have shown that free dives and spleens have an association.

A larger spleen helps humans to free dive into deeper waters for longer periods of time. This is the first time that a study has shown that genetics and evolution plays a role in spleen size of the divers. The new study results were published in the latest issue of the journal Cell.

The researchers from the Universities of Copenhagen, Cambridge and Berkeley, write that the indigenous nomadic tribes of Bajau have been called “sea nomads” and they have travelled all over the Southeast Asian seas. They live in houseboats and forage for food in water using spears by diving deep into them. They have lived in these regions for a thousand years now. Mostly they are colonized presently around the islands of Indonesia. This tribe is well known for their incredible breath-holding capacities that allow them to work under water. They can dive in at around 70m of water with simple weights and wooden goggles. Researcher Melissa Ilardo explains that they never dive competitively so the duration they can spend under water at one-go has never been measured. However it can be to the tune of 13 minutes at one go she said.

According to first author Ilardo this ability to hold breaths for so long prompted the team to look at the spleens of these people. She explained that the connection between genetics, enlarged spleen and diving has not been made before in humans. However, Weddell seals which are deep diving seals, are known to have disproportionately large spleens. She explains that the same evolution theories could be working for these people as well.

She said that the human spleen plays a role in human dive response – or the body’s response to diving into water. When a person dives into water the body is submerged in cold water, the heart rate needs to slow down, blood vessels of the limbs need to constrict to allow more blood flow to the vital organs, and protective mechanisms that work when there is oxygen deprivation are triggered. The spleen contracts in these situations. The same works when a person is facing a situation of acute oxygen deprivation called acute hypoxia. The spleen, when it contracts during hypoxia periods lets out a spurt of oxygenated red blood cells into the blood stream. This leads to a 9 percent rise in oxygen and prolongs dive time.

The team spent some time in Jaya Bakti, Indonesia to examine the people of this tribe. They performed genetic tests by obtaining samples from these people and also performed ultrasound scans of the spleens. They compared the scans of these Bajau tribes with Saluan tribes that live nearby and are primarily land-dwelling. The genetic samples and scans were analyzed at the University of Copenhagen and results showed that the Bajau have a median spleen size 50% bigger than the Saluan. The diving as well as the non-diving Bajau had enlarged spleens which meant that there was genetics at play. A specific gene called PDE10A was found among the Bajau. This was absent in the Saluan.

This gene is also though to control the thyroid gland and its principal hormone Thyroxine or T4. Ilardo explained that thyroid and spleen sizes have been linked to each other before in mice. Genetically changed mice that had lower levels of T4 tend to have smaller spleen sizes.