Decreases in Brain Size and Encephalization in Anatomically Modern Humans

Abstract

Growth in human brain size and encephalization is well documented throughout much of prehistory and believed to be responsible for increasing cognitive faculties. Over the past 50,000 years, however, both body size and brain mass have decreased but little is known about the scaling relationship between the two. Here, changes to the human brain are examined using matched body remains to determine encephalization levels across an evolutionary timespan. The results find decreases to encephalization levels in modern humans as compared to earlier Holocene <i>H. sapiens</i> and Late Pleistocene anatomically modern <i>Homo</i>. When controlled for lean body mass, encephalization changes are isometric, suggesting that much of the declines in encephalization are driven by recent increases in obesity. A meta-review of genome-wide association studies finds some evidence for selective pressures acting on human cognitive ability, which may be an evolutionary consequence of the more than 5% loss in brain mass over the past 50,000 years.

Social robots as depictions of social agents

“…As Homo increased its reliance on physical artifacts, our genus' bodies grew less muscular and robust as a result (Ruff, 2005). The same may be true of cognitive tools: Homo sapiens have experienced more than a 5% reduction in brain mass throughout the Late Pleistocene and Holocene (Stibel, 2021) and that loss of brain mass has been linked to an increased use of cognitive tools (DeSilva et al, 2021). Modern technology, such as the internet and cell phones, have been shown to supplant thinking more broadly (Barr, Pennycook, Stolz, & Fugelsang, 2015;Sparrow, Liu, & Wegner, 2011).…”

“…Cognitive tools enable thought to move to and from the brain by offloading cognition from biological wetware to artificial hardware. Just as physical artifacts offload physical exertion, cognitive offloading may allow our expensive brain tissue to be selected against while enabling our intelligence to increase (DeSilva et al, 2021;Stibel, 2021).…”

Climate Change Influences Brain Size in Humans

“…However, despite general trends of increasing brain size in Homo across time, Homo brain size has not been on a persistent upward march. Periods during the early Middle Pleistocene (roughly 875,000 years ago) [Ruff et al, 1997] and the Holocene (roughly 12,000 years ago to present) [Henneberg, 1988;Henneberg and Steyn, 1993;DeSilva et al, 2021;Stibel, 2021] both reflected deceleration in brain size, and modern evidence shows a decrease in both brain size and encephalization [Stibel, 2021]. For humans, there is a clear adaptive advantage to larger brains, and speciation events have repeatedly yielded lineages with greater relative brain size; increases in cognitive ability have consistently outweighed the costs of expensive brain tissue.…”

How cultural framing can bias our beliefs about robots and artificial intelligence

“…As Homo increased its reliance on physical artifacts, our genus' bodies grew less muscular and robust as a result (Ruff, 2005). The same may be true of cognitive tools: Homo sapiens have experienced more than a 5% reduction in brain mass throughout the Late Pleistocene and Holocene (Stibel, 2021) and that loss of brain mass has been linked to an increased use of cognitive tools (DeSilva et al, 2021). Modern technology, such as the internet and cell phones, have been shown to supplant thinking more broadly (Barr, Pennycook, Stolz, & Fugelsang, 2015; Sparrow, Liu, & Wegner, 2011).…”

“…Cognitive tools enable thought to move to and from the brain by offloading cognition from biological wetware to artificial hardware. Just as physical artifacts offload physical exertion, cognitive offloading may allow our expensive brain tissue to be selected against while enabling our intelligence to increase (DeSilva et al, 2021; Stibel, 2021).…”