Evolutionary changes to Homo body size and proportionality across 800,000 years of global climate change
Description
The challenge with testing the impact of climate change on hominins is a lack of comparable sample data. Few intact skeletons exist, and body size estimates are difficult to ascertain with only fragmentary skeletal remains. What measurements are available tend to be sparse and come from small samples. To test for evolutionary changes in particular, sufficient sample breadth across multiple measurements is needed throughout a large enough timespan where climate data are available at both small and large intervals. The present dataset addresses these issues by examining skeletal remains that are predictive of relative body size in Homo over a period of roughly 700,000 years. Measures of femoral head breadth (mm), proximal tibia plateau breadth (mm), and stature/bi-iliac breadth (cm) of skeletal remains were used to produce 247 body mass (kg) estimates that span from present day to 700,000 years ago (700 kyr BP). Body mass data were compared to five climate records. Global climate change was derived from the ice core at EPICA Dome C, which provides precise surface temperature measurements dating back 809,950 years. Regional climate change was derived from sediment samples recovered from Lake Malawi in eastern Africa and two North Atlantic Ocean deep sea cores located roughly between North America, Africa, and Europe. Pollen sequences in the Burundi Highlands were used to determine precipitation levels and humidity levels were tested using a datasource derived from grain-size analyses of siliciclastic marine sediments from the coast of Mauritania. When comparing Homo body size and proportionality against the climate records, evolutionary body changes correspond to temperature but not precipitation or humidity.
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247 body mass estimates were derived from measurements of femoral head breadth (mm) (n = 187), proximal tibial plateau breadth (mm) (n = 6), and stature/bi-iliac breadth (cm) (n = 3) skeletal remains, in addition to direct measurements from a modern sample (n = 51). Measurements were converted to estimates of body mass based on the following equations: femoral head (FH): body mass = 2.262 x FH - 38.7; proximal tibia (TP): body mass = 1.623 x TP – 52.7; stature/bi-iliac breadth (BI): body mass = 0.522 x ST + 1.809 x BI – 75.5. 87 estimates of stature were derived from lower and upper limb measurements. To determine body proportionality, a simple formula of stature as a function of mass (stature / mass) was utilized. A more robust formula for deriving surface area as a function of volume (SA:V or surface area / volume) was also tested, where body surface area (m2) = [√ (height (cm) x weight (kg))] / 3600 and volume (l) = (1.015 x weight) – 4.937. In addition, body mass index (BMI) was also tested given that it has been used in prior work as a proxy for body proportionality, where BMI = body mass (g) / (stature (m))2. Global climate change was derived from the ice core at EPICA Dome C, which provides precise surface temperature measurements dating back 809,950 years. Temperature records at EPICA Dome C were measured as a function of differences in past temperatures (∆T) relative to the most recent 100-year temperature average. Regional temperatures were derived from sediment samples recovered from Lake Malawi in eastern Africa over the past 1.3 million years as well as two North Atlantic Ocean deep sea cores located roughly between North America, Africa, and Europe, which produced sedimentary records of bottom water temperatures as a proxy for climate change between 10,400 and 3.2 million years ago. Paleoclimate records were provided over 100-year averages or greater, whereas fossil records were dated to the year, as best estimated by geochronologists without regard for variance. The temperature record was rounded down to match the estimated age of each skeletal remain. Given the considerable variability in fossil dating, efforts were made to ensure that the results accounted for potential geochronological errors. The body mass sample set included a large and diverse dataset and, while there was considerable overlap, the matched stature / mass sample provided an independent set of sources. Additionally, the fossils were clustered into 100-year and 10,000-year groupings, the latter of which can account for dating errors up to 10,000 years. In each of the above cases, the results remained significant.