Tail Loss and Brain Expansion in Hominoid Evolution: An Evolutionary Developmental Perspective

A comprehensive understanding of hominoid evolution requires more than fossil evidence and comparative genomics alone; it demands an integrated perspective that links morphology, developmental processes, and molecular mechanisms. In this review, we focus on two major morphological transformations that occurred on different timescales during hominoid evolution: tail loss, which arose relatively early in the hominoid lineage, and brain evolution, which proceeded over a long timescale and became especially pronounced later in hominins, particularly in the genus Homo. Fossil evidence indicates that early hominoids were already tailless, whereas human embryos transiently form an embryonic tail containing somites before undergoing a tail reduction process after the axis elongation ceased. Developmental studies suggest that hominoid tail loss involved coordinated changes in the termination of posterior axial elongation, somitogenesis, and tail reduction, and recent work has highlighted altered TBXT splicing representing one possible contributor rather than a single causal switch. By contrast, hominoid brain expansion is associated with expansion of cortical progenitor zones, prolonged neurogenesis, an extended developmental period, and cortical expansion and folding, and lineage-specific molecular changes affecting progenitor amplification, neuronal maturation, and gene-expression regulation. Although we do not assume a direct causal relationship between tail loss and brain expansion, comparing these distinct events highlights contrasting modes of developmental reorganization: reduction and termination of a posterior structure versus expansion and prolonged growth of the anterior structure. Together, these examples illustrate how integrating morphological changes/variations with embryonic development and genomic change can help reconstruct the mechanism underlying hominoid evolution.