Global climate change and Homo brain size
Changes to brain size in hominins constitute a crucial evolutionary trend, yet the underlying mechanisms behind those changes are not well understood. In this dataset, climate change was considered as an environmental factor using four paleoclimate records testing temperature, humidity, and precipitation against changes to brain mass in 298 Homo specimens over the past fifty thousand years. Across regional and global paleoclimate records, brain mass in Homo averaged significantly lower during periods of climate warming as compared to cooler periods. To a smaller degree, humidity and precipitation levels were also predictive of brain mass, with arid periods associated with lower brain mass in Homo. The findings suggest a relationship between climate change and brain size in Homo that is driven by natural selection in response to environmental stress.
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373 independent cranial capacity measurements were utilized across 298 skulls. Brain mass estimates were derived from cranial capacity measurements, as cranial volume has been shown to highly correlate with brain size in both modern and prehistoric humans. Cranial capacity was converted to brain mass using a formula derived from a least-squares regression (r2 = 0.995) of 27 primate species (brain mass = 1.147 x cranial capacity0.976). To account for measurement differences, records were averaged across all individual measures. Incomplete measures, unusable skull fragment data, and juvenile specimens were discarded. Where precise dates or carbon-14 dating were available (n = 266), that source was used; otherwise, the average was taken across all date estimates (n = 32). To test for encephalization, articular measurements were converted to estimates of body mass based on the following equations: femoral head (FH): body mass = 2.262 x FH - 38.7; and stature/bi-iliac breadth (BI): body mass = 0.522 x ST + 1.809 x BI – 75.5 (Dataset S1B). Articular cross-sectional dimensions of long bones that bear weight are among the most used dimensions for deriving body mass estimates as there is a large body of evidence to support a strong association between these dimensions and body mass. Encephalization was measured using a variant of the method originally derived based on changes to surface area but updated to account for metabolic turnover (25) as follows: EQ = brain mass / (11.22 x body mass.76). Brain mass data were compared to 100-year mean value surface temperatures derived from the European Project for Ice Coring in Antarctica (EPICA) at Dome C. The climate change data were provided in increments of 100 years by the lead author of the EPICA Dome C study and were also catalogued for the present study into increments of 10,000 years. Surface temperatures (°C) derived from sediment samples recovered from Lake Malawi (10°–14° S in eastern Africa) over the past 1.3 million years were used as a proxy for regional temperatures in Africa. Grain-size analyses of siliciclastic marine sediments from the coast of Mauritania (core GeoB7920; 2,278-meter water depth) were utilized to derive a humidity index (mm) for Northwest Africa starting at 500 years ago and going through the entire 50 kyr BP period. To derive relative changes in humidity, log ratios of three core endmembers from the marine sediment were utilized [log(hemi-pelagic mud) / (coarse aeolian dust + fine aeolian dust)], whereas hemi-pelagic mud provided a measure of river runoff and aeolian dust records, which served as a proxy for subaerial erosion and continental vegetation cover. Pollen sequences (mm) were used to derive a time-series of mean annual precipitation level estimates by calculating the difference between the reconstructed absolute value of the pollen counts and modern precipitation values.