The human dive response is one of many examples of how humans have evolved to thrive in a semi-aquatic lifestyle. But how have we evolved to acquire this unique ability to spend a breathtaking (no pun intended) amount of time under water on a single lungful of air? It is estimated that today nearly 80% of the human population on Earth today lives near the ocean or other body of water. We have always relied on water but scientists are looking into how deep our connection with water actually runs. Were our ancestors more aquatic than we originally thought? How much has the aquatic environment shaped the course of human evolution?
The Aquatic Ape Hypothesis
This hypothesis is the idea that the ancestors of modern humans spend a significant period of time adapting to a semi-aquatic environment. It is not suggesting that we evolved form fish or exclusively existed underwater at any point during in evolutionary history. The hypothesis is simply suggesting that during a period in our evolutionary history, selective pressures favored traits that allowed humans to survive periods of frequent floods or dive beneath the surface of a body of water for an extended amount of time. The aquatic ape hypothesis offers an explanation for several traits in modern humans that could be derived from frequent interactions in an aquatic environment over the course of many generations. You were most likely taught in school that we descended from a common ancestor of apes and instead of hanging around in the forest, and found a niche in the savannah terrain. This is the most widely accepted hypothesis for how the lineages diverged and lead to acquisition of many unique human traits. However, it more or less fails to explain several key human characteristics that set us apart from even our closest ape relatives.
We Are Naked.
Most primate species, however, are not. You'll often hear the argument that we lost the majority of the body hair in response to the increased temperature of the savannah heat compared to the shaded terrain under tree canopies. But what about the animals that have fur coats in places like the desert? The fennec fox found in the Sahara has a fur coat that likely functions to protect it form the sun. Even camels, the most stereotypical heat-adapted animal, have a hairy fur coat. So how did our human ancestors evolve to lose virtually all the hair on their bodies? There is a reason why competitive swimmers shave their bodies before big races and have swim caps on their heads. Less hair is more efficient when swimming under water. Individuals with less hair on their bodies would have been able to swim underwater more efficiently and thus collect more shellfish, urchins, or other seafood to bring back to the surface. It would have probably been much easier to dive in shallow reefs for plentiful, inert seafood compared to sending hours or even days tracking large game in the savannah.
From Four Legs to Two
Walking on two legs isn’t common among mammal. In fact, humans are the only mammals that consistently walk on two legs after becoming old enough to do so. In periods of vast flooding, it would have been easier to survive on two feet and keep the heat above water. Those could cross flooded regions of lands to gather resources and survive the expedition would be able to reproduce with other that had this ability. Evolution isn’t perfect though. With the ability to walk on two feet came a much more narrow birth canal due to the changes in the pelvic bones required for this type of mobility. Before modern medicine, childbirth was a very dangerous, even fatal procedure. Today, some mothers perform what’s called a “water birth”. The newborn baby is born underwater and is said to be less painful for the mother. Although scientific studies haven’t been able to conclude whether or not this method is significantly safer for both the child and mother, the ability for the child to be born underwater and survive while making the process easier on the mother may suggest that this method was implemented at some point in the transition from walking on four feet to two.
The role of the aquatic environment in human evolution still remains controversial among evolutionary biologists. Yet for those of us that feel a deeper connection with nature when we are in or around a body of water like the ocean, it makes sense that who we are today has in some way been shaped by the most abundant natural resource on the planet: water.