Climate and Human Variation

The human body can respond, or adapt, to environmental stress in several ways to increase their chance of surviving, including acclimation, acclimatization, and developmental acclimatization.

• Acclimation: short-term changes that occur quickly after experiencing stress; Example: sweating to cool down the body. This response is not permanent and occurs in the person’s normal environment.
• Acclimatization: long-term changes to physiological stress, especially when the person has moved to a new environment; Example: increase in rates of metabolism, respiration, and blood circulation in hot climates.
• Developmental acclimatization: physiological changes during growth and development that help the individual thrive in their environment; Example: larger chest sizes in people who grow up in high altitudes.

These three forms of human adaptation demonstrate plasticity, or the ability to change physiologically in response to environmental stress to increase the chance of surviving.

Anthropologists typically turn to Bergmann’s rule and Allen’s rule to teach these concepts.

• Bergmann’s rule: Simply put, Bergmann’s rule argues that among mammals of similar shape, larger mammals will lose heat from their bodies at faster rates than smaller mammals due to more body surface. It also states that among mammals with similar size, those with linear shapes lose heat faster than those that are non-linear. This means that humans with smaller body sizes are better adapted for cold climates while larger body sizes are better adapted for hot climates.
• Allen’s rule: This rule states that mammals in cold climates more frequently have short, bulky limbs while those in hot climates tend to have long, slender limbs. Similar to Bergmann’s rule, these patterns of human body size and shape allow for the body to regulate their body temperature where short limbs allow less heat to leave the body and long limbs increase the loss of heat.

These rules also influence the size and shape of the skull. Anthropologists collect measurements and morphological information of the human skull to determine ancestry and study pattern of human variation as it is rich with information. Generally speaking, crania, or skulls, are wider relative to length for populations living in colder climates than those in warm climates. Again, this adaptive trait allows for the skull to release heat faster in warm climates and slow down heat loss in cold climates in order to protect the temperature of the brain and increase the chance of surviving. While this phenomenon initiated as developmental acclimatization, the trait became naturally selected over time to increase the overall crania shape and size for a population. This creates variation between groups of people.

One final notable trend in human plasticity is the shape of size of the nose. Both, anthropological and clinical research have found evidence of the relationship between nasal size and shape and climate, specifically temperature and humidity levels. Populations in cold climates tend to have narrower noses than those in warm climates. This allows the nasal cavity time to regulate the temperature of cold inhaled air before it reaches the lungs and potentially damaging the lungs. Additionally, populations in dry climates tend to have narrower noses than those in humid climates. This again allows the nasal cavity time to regulate inhaled air using the mucous membranes.

The image below,taken from the National Oceanic and Atmospheric Association website, depicts the temperature difference in 2018 from average temperatures between 1981 and 2010. If the world temperatures continue to increase and temperatures remain higher, we will start to see changes in physical form, such as nose and cranial size and shape, to adapt to the warmer environments. We can combine this type of climate data to anthropological research to observe how and why phenotypic variation exists.

It is important to understand how climate shapes human form over time and space to grasp the continuity of human variation. While populations may share their genetic makeup, one group may have lived in a very different environment for extended periods of time, causing significant phenotypic divergences from their ancestral lineage. Therefore, generalizing about phenotypes for large groups of people who share ancestral links without considering their environment is illogical.