The Diving Reflex and Vasoconstriction The "Diving Reflex", bradycardia, is a condition which occurs to the greatest degree in semi-aquatic and fully aquatic animals. It involves a decrease in heart rate and redistribution of blood through the body (vasoconstriction), to the brain and essential organs. It has been shown that for trained humans the diving response is equivalent to a semi-aquatic shallow diving mammal (Schagatay and Holm 1996). Experimental trained human dives included 40m depths for 2 minutes, and 7 min apneas at rest. It was also found that in humans the diving response is brought about by chilling the upper face (Schuitema and Holm 1988). Recent experiments have shown that the diving reflex, depends not so much on "absolute" water temperature, but more so on the "difference between ambient and water temperature" (Schagatay and Holm 1996). An interesting fact coming out of this research was that the diving reflex of a trained pig (an ex-aquatic), is equal to that of an untrained human. Much of Schagatay's research involves the Sea People of Indonesia, and the Ama in Korea and Japan. |
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However, this alone cannot explain why we can and do speak. Voluntary override of the autonomic breathing reflex must have taken place, and been advantageous for the species involved. It is important to note that communication among aquatic animals is often sound based. Humans communicate through sound, facial and body expressions, just as the other primates do. However, the extent to which both vocal and facial communication has evolved in Homo sapiens is not matched anywhere else in the primate family. If we consider a semi-aquatic episode, we have a reason for this behavioural change. When in water, most of the body is hidden, making body language useless as a tool for communicating (Morgan 1997). Our ancestor would have had to rely on something else to warn of danger or otherwise communicate with members of its group.
Again, this begs the question: What kind of environment did our ancestors evolve in (or were forced to exploit), that would force them to begin holding their breath and diving? If we go by the old chain of thought, a dry savanna, why would we possibly adopt a character that would, in that environment, serve no useful purpose? Even the model of the new "mosaic", (woodland/forests) does not provide the enormous stress required to acquire these features (Verhaegen and Oser 1997).