Natural selection gave a freediving people in Southeast Asia bigger spleens

Natural selection gave a freediving people in Southeast Asia bigger spleens

The new study suggests that people exposed to low-oxygen conditions have evolved "many pathways" for coping, says Gabriel Haddad, a physiologist at the University of California (UC), San Diego, who was not involved with the work. A larger spleen means that more oxygen gets released.

The spleen is the size of a fist and it's located close to the stomach. She points out that although the Bajau dive more frequently than most people, they are not necessarily under the water longer for any given dive. The study, published today (April 19) in Cell, is an uncommon example of natural selection in modern humans that likely occurred on the order of hundreds or thousands of years.

"This study is a wonderful example of the value of studying these small populations living under extreme conditions", said Professor Eske Willerslev of the University of Copenhagen, who carried out the genetic study.

Their lifestyle goes back at least as far as 1521: That's when a voyage led by Portuguese explorer Ferdinand Magellan - best known as the first to circumnavigate the globe - recorded coming across the seafaring Bajau.

The secret behind the ability of a group of "sea nomads" in Southeast Asia to hold their breath for extraordinary periods of time while freediving to hunt fish has finally been revealed - and it's down to evolution.

The Bajau people of South-East Asia are sea nomads who spend most of their workday under the sea hunting for fish and sea cucumbers. Ilardo measured their spleen sizes using an ultrasound machine and took saliva samples for genomic sequencing.

Astonishingly, these deep dives are performed only with a wooden mask or goggles and a weight belt.

"There's a human dive response that's triggered by holding your breath and submerging yourself in water". One way is to boost red blood cell production, which allows for more efficient oxygen delivery to organs and tissues, or to expand their lung capacity. "It's like a biological scuba tank".


Prior to her travels, Ilardo spent months learning the Indonesian language, Bahasa Indonesia, which the Bajau people speak in addition to their native language, so that she could communicate with them directly, and convey to them the research findings. "I asked very nicely for the people to let me look at their spleens", she said.

The spleen is vital for this, allowing divers to stay under for longer. "The first sign that we were maybe onto something was when we saw that both the Bajau divers and non-divers had larger spleens than the Saluan, a nearby, non-diving population". This response has also been found in diving mammals such as seals. She explained that the connection between genetics, enlarged spleen and diving has not been made before in humans.

"We also find evidence of strong selection specific to the Bajau on BDKRB2, a gene affecting the human diving reflex".

The researchers also stumbled across a gene called PDE10A, which is thought to control a certain thyroid hormone, in the Bajau but not the Saluan.

That gene variant was probably introduced into modern humans through ancient interbreeding (a process known as "introgression"), and then rose to high frequencies on the Tibetan plateau because of the advantage it conferred.

"It's not clear how long the Bajau have had this lifestyle, or when exactly the adaptation arose give the genetic data that we have now", said Dr Ilardo.

Other genes that varied among the Bajau more than would be expected in the general population were associated with the way the body responds to diving. In addition to genetically inherited adaptations, regular training could help prevent that effect. "There's still a lot of information to be gathered from these understudied populations". Another prevented high levels of carbon dioxide from building up in the blood.

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