Anthropoid cranial base architecture and scaling relationships

Area of Science:

• Paleoanthropology
• Comparative Anatomy
• Evolutionary Biology

Background:

• The "spatial packing" hypothesis suggests basicranial flexion maximizes braincase volume relative to basicranial length in haplorhines.
• Previous studies indicate Homo sapiens may have less basicranial flexion than expected, implying other factors influence hominin cranial base variation.

Purpose of the Study:

• To investigate how basicranial length and cranial base angulation measures impact the relationship between basicranial flexion and relative brain size in anthropoids, including Homo sapiens.
• To assess if Homo sapiens' Index of Relative Encephalization (IRE) is inflated due to shorter basicranial length (BL).

Main Methods:

• Midline cranial floor sections from 157 specimens across 18 anthropoid species were scaled relative to the cube root of endocranial volume.
• Analysis focused on the posterior cranial base and planum sphenoideum lengths and various cranial base angles.

Main Results:

• Homo sapiens exhibit significantly shorter posterior cranial bases and planum sphenoideum compared to other anthropoids, contributing to higher IREs.
• Including the cribriform plate in BL measurement alters the comparison of Homo sapiens' basicranial flexion versus IRE with other anthropoids.
• Despite shorter BL and elevated IRE, Homo sapiens do not significantly deviate from the general anthropoid trend for basicranial flexion versus relative brain size for two tested angles.

Conclusions:

• Shorter basicranial length in Homo sapiens contributes to elevated relative brain size indices.
• The equivocal findings on basicranial flexion in Homo sapiens highlight the impact of measurement choices for cranial base angulation.