Why Did Humans Evolve to Sweat More Than Other Animals Around 2 Million Years Ago? — Here’s What Scientists Believe

Have you ever wondered why humans sweat after we walk, run, or are exposed to hotter environments? You probably already know that it is our body’s way to cool us down. Other animals do the same thing, for the same reason. However, humans are unique in how much we sweat compared to other primates, and how well this regulates our temperature.

Scientists think that this is an evolutionary feature that early humans or homo genus species developed around 2 million years ago. But why exactly did they need these adaptations, and why do other primates, like chimpanzees, with whom we share a common ancestor, not sweat as much as we do? Let’s explore these questions and ideas more deeply.

Big Changes in Body Design

Around 2 million years ago, the Earth was undergoing environmental and climate changes. As suggested by the open savanna hypothesis, early humans may have been forced to spend more time in the open areas because the dense forests were receding, and savannas and grasslands were expanding over a long period.

These open spaces are hotter and have less shade, and in that scorching heat, having a way to cool down your body would have been effective and advantageous for survival. This is one of the main theories why we evolved sweating more than the other primates, because their ancestors likely stayed in forested areas, while ours may have been forced into different, hotter environments to scavenge and look for resources.

This also means that sweating did not occur in isolation; instead, it was likely a part of a long process of physical changes that happened in the earlier human body to transition from tree-dwelling creatures to terrestrial hunters and scavengers. Along with the development of efficient human sweating, we also lost most of our body hair.

For sweating to cool down the body effectively, it had to come into contact with the open air and evaporate. However, with thick fur, this is harder to achieve. So, as hominins spent more time in the open, hotter savannas and grasslands, the reduction in body hair over time became a vital adaptation as it was advantageous for survival.

Additionally, early humans also learned bipedalism, or the ability to walk and stand upright. This made more skin exposed to the air and wind, but less to the sun, allowing better air circulation overall and helping the heat to leave the body through sweating.

Read more: Wake Forest University

How Sweat Glands Differ in Humans vs. Other Animals

The Evolution of Human Physical Activity - Genetic Drivers of Human Thermoregulatory Skin Traits

The human cooling system, which involves not just our sweating but also the loss of fur and development of bipedalism, is unique from other animals because of its specialized and complex nature. But most significantly, it is the amount or density of sweat glands in the human body.

There are two main kinds of sweat glands in mammals, the eccrine and apocrine. Humans have their bodies covered up by millions of eccrine sweat glands, especially in specific parts like the palms and the forehead. These glands are what produce the watery and salty liquid that cools down the skin as it evaporates upon contact with air.

Most other mammals and all primates aside from humans do not have this many sweat glands. Although they still sweat, it is in much less density, and is limited by the presence of thick fur that restricts the sweat from evaporating efficiently. So, other species, like dogs, developed other ways to regulate their body temperatures, such as by panting.

Humans also seem to have more control of these cooling systems than the other primates. Our nervous system could effectively signal the sweat glands to work when it detects the body’s temperature, such as when we are running in the heat. Other animals either do not have as many sweat glands as we do, or they are not as well-controlled and suited for full-body cooling.

Why Sweating Became an Important Evolutionary Advantage

So, how did sweating and the full human body cooling system actually become an important evolutionary advantage? The combination of sweating, loss of body hair, and bipedalism all contributed to these reasons. First, it gave earlier humans more time for hunting and scavenging.

When most other predators would be finding a shelter or a shade from the heat of the sun to avoid overheating, early hominins could still hunt or scavenge midday. In other words, it led to our ancestors having more food and resources that allowed them to have better survival chances.

Another advantage would be for the persistence hunting hypothesis. Before fire or tools, humans developed a key survival strategy, and that is to stalk, track, and chase their prey, who may have been faster or bigger than they were, until they collapsed from exhaustion or the heat of the sun. This skill would have never been possible without the effective sweating and thermoregulation of the human body.

The third way it gave earlier hominins an advantage was for brain protection and development. The human brain is highly sensitive and produces a lot of heat. If the brain gets too hot, the overall performance of hominins goes down, or it becomes damaged, until they are incapacitated. The sweating and cooling systems they have developed were necessary to protect this organ and let it function optimally.

Read more: Smithsonian Magazine

Trade-Offs and Possible Costs

Running and Sweating - The Evolution of Human Physical Activity

Although sweating and our cooling system have a lot of advantages and are one of the most significant evolutionary adaptations of earlier humans to survive, it is not without downsides. One disadvantage was the risk of dehydration. When humans sweat in a hot environment, they lose water and salt in their bodies as the sweat releases it, and then it evaporates. This makes them prone to dehydration unless they drink enough water.

Additionally, the loss of thick fur made more skin directly exposed to the sun, and although this feature is helpful with sweating and thermoregulation, it has trade-offs. The earlier hominins became at risk of severe sunburns and insect bites.

Lastly, there is the energy and water expenditure of sweating and our cooling system. If the water is scarce in the environment, sweating too much could cause dehydration, as mentioned above.

Moreover, if earlier humans were sweating, it already means they were using energy, so the mixture of dehydration and fatigue could be disadvantageous. This may have resulted in our body balancing sweating so that it cools us down, but does not dehydrate too fast.

Author's Final Thoughts

Sweating and the development of the body’s cooling system are among the most significant evolutionary adaptations of earlier hominins. It may have started a cascading event beginning from being able to hunt, scavenge, and survive in the open and hot areas like a grassland, to the development of more advanced traits and abilities, such as persistence hunting and endurance running.

In other words, sweating is part of a long and gradual process of how our species survives, spreads to many different environments, and ultimately leads to the development of the dominant species on the planet today, Homo sapiens, or the modern human species.

Read next: Why Did Humans Evolve to Run Long Distances Around 2 Million Years Ago, Unlike Other Primates? — Here’s What Researchers Found Out

References & Further Reading

Lieberman, D. E. (2015). Human locomotion and heat loss: An evolutionary perspective. Comprehensive Physiology. (Open PDF) https://scholar.harvard.edu/files/dlieberman/files/2015a.pdf

Wheeler, P. E. (1991). The thermoregulatory advantages of hominid bipedalism in open equatorial environments: The contribution of increased convective heat loss and cutaneous evaporative cooling. Journal of Human Evolution. https://doi.org/10.1016/0047-2484(91)90002-D

Chaplin, G., Jablonski, N. G., & Cable, N. T. (1994). Physiology, thermoregulation and bipedalism. Journal of Human Evolution. (PDF via Penn State repository) https://cpb-us-e1.wpmucdn.com/sites.psu.edu/dist/8/10224/files/2014/02/Chaplin.1994.JHE_.497ff.pdf

Pontzer, H., et al. (2021). Evolution of water conservation in humans. Current Biology. https://www.cell.com/current-biology/fulltext/S0960-9822(21)00289-X