By: Cameron Ward, Kellie Guerette, Meghan Doherty, and Benjamin Clark
(Stonehill College, BIO323: Evolution, Fall 2023)
Overview
Climate change is defined as long-term shifts in temperatures and weather patterns. Shifts can be natural, however, since the 1800s, human activities have been the main driver primarily due to the burning of fossil fuels. An article titled, “Climate Change Predictive of Body Size and Proportionality in Humans”, discusses how these changes have impacted the evolution of the human body over time. A relationship has been established between climate change and the human body shape and size, driven by natural selection in response to thermoregulatory demands. The studies suggested that smaller bodies with more of their weight distributed to the limbs dissipate heat better than larger and more rotund bodies with less relative surface area. This leads to larger species (endomorphic) in cooler geographic regions and smaller (ectomorphic) in warmer climates. Fossil records have shown that this trend holds true for a variety of species. Invertebrates, insects, and mammals all shrunk during past periods of global warming. Periods of global cooling have had the opposite effect, in which species increased in size to become better fit for their environment. There are several factors that could lead to the observed changes in body weight/ proportionality, however, climate change seems to be a major driver.
The Research
A 2023 study published to Springer by Jeffery M. Stibel explores the relationship between climate change and Homo evolution. Body size data were compared to a 100-year mean value surface temperatures derived from an ice core in Antarctica, sediment samples recovered from Lake Malawi in Eastern Africa, and water temperatures from two North Atlantic Ocean deep sea cores. The time period utilized was chosen both for availability of high-resolution climate data and to test associations across periods of global cooling, warming, and relative stasis. A major challenge was the lack of intact skeletons that exist, and body size estimates are difficult to ascertain with only fragmentary skeletal remains. To account for this, the study utilized 247 body mass (kg) estimates spanning from present day to 700 kyr BP and grouped by taxon, region, period, and sex. A variety of statistical analyses were conducted to determine whether there was a change in body proportions in response to climate over the past 700,000 years.
The Results
In the article, it is stated that “A significant relationship was found between temperatures at EPICA Dome C and body size estimates over the past 700 kyr BP, which held constant after controlling for geography, sex, and taxon.” Many adjustments were made to the data to create the most fair conclusions, while accounting for a multitude of confounding variables. Homo body mass during cooler periods over the past 700 kyr BP averaged 66.40 kg ± 0.86 as compared to 59.00 kg ± 0.73 for warmer periods, or a roughly 11.8% difference. The study also found that the effects of climate change on body size appear more pronounced near the mean and do not accelerate as temperatures grow more extreme.
The Consequences
Global warming tends to favor ectomorphic body sizes, whereas global cooling favors more endomorphic body types. There seems to be long term genetic changes influencing Homo evolution. In addition to body size, the results appear to indicate a longer-limbed body build during warmer periods as compared to cooler periods where weight distribution appears to concentrate at the body’s core. There are numerous other factors that may be underlying physiological changes in humans- such as precipitation, vegetation levels, water availability, culture, technology, etc. In modern times, we may look at caloric intakes, access to clothing/ shelter, and intensity of exercise. As a result of human reactions to environmental pressures, the consequences to the rest of the global ecosystem have the potential to be severe. Innovative cultural offsets to the genetic effects of climate change on human body size come, as a result, at a high ecological cost.
The Interview
When asked about the limitations of the study performed, Stibel responded that, “The biggest limitation is the number of fossils for prehistoric humans with enough bone fragments to estimate body mass and proportionality. Also, all these results are utilizing estimates as we cannot perform exact measurements on things that happened in the past. The climate proxies utilize ice cores, dust, and pollen records to estimate each variable. The body mass and proportionality estimates utilize bone fragments. While great efforts have gone into making sure the methods and formulas are representative, a perfect correlation will never exist.”
When asked about the effects of human advancement and how this influences the trend observed in the study, Stibel answered that we must “talk about recent cultural adaptations and modern technological innovations and how they offset the impact of climate on body size.” He explains that the correlation in variables has weakened overtime, however, even with recent studies it seems that “thermoregulatory demands are driving the effect more than food availability” or other confounding variables.
Article: Stibel, J.M. Climate Change Predictive of Body Size and Proportionality in Humans. Evolutionary Biology. 50, 461–475 (2023). https://doi.org/10.1007/s11692-023-09616-1