Hominin brain expansion

A compact teaching figure for the claim that hominin brain size did not merely rise with body size. The main panel shows endocranial volume through time; the inset contrasts observed brain enlargement with a body-size-only allometric expectation.

Teaching caveat. The fossil record is sparse and taxonomic assignments are debated. This figure uses representative anchors and deliberately emphasizes the human-through-line, while using a few gray side branches to keep students from reading the plot as a simple ladder.
Hominin brain expansion through time A teaching figure showing endocranial volume in hominin evolution from early hominins to Homo sapiens, with change-point annotations and an inset showing allometric body-size expectations. Hominin brain expansion A teaching plot for the shift from body-size allometry to hyperallometric brain enlargement Representative values, simplified taxonomy, and approximate dates. This is not a claim that every plotted taxon is a direct ancestor. early Homo / Oldowan neighborhood 400 600 800 1000 1200 1400 1600 4 3 2 1 0 Millions of years ago (older to present) Endocranial volume / cranial capacity (cc) 2.1 Ma rate uptick 1.5 Ma rate uptick Lomekwian 3.3 Ma Oldowan 2.6–3.0 Ma Acheulean 1.76 Ma steeper trajectory afterearly Homo early hominins A. afarensis H. habilis H. ergaster / early H.erectus H. heidelbergensis Late Pleistocene H.sapiens Recent H. sapiens Paranthropus H. naledi Neanderthals Magnitude of change about 3x from A. afarensis to recent humans about 4x from earliest small-brained hominins Body size alone does not get you there 35 50 70 400 800 1600 Body mass (kg, log scale) Brain / endocranial volume (cc, log scale) Neanderthal A. afarensis H. habilis H. ergaster / earlyH. erectus Recent H. sapiens hyperallometricresidual body-size-only expectation(slope ≈ 0.75) observed trajectory How to read this: The main panel shows the visible acceleration in cranial capacity after the earliest Homo interval. The inset asks what would happen if larger bodiesalone carried brains upward along a standard allometric expectation; the modern-human point sits far above that body-size-only line. The gray points are reminders of bushy evolution: small-brained hominins persisted late, and Neanderthals were a large-brained sister lineage ratherthan our direct ancestor. Sources summarized in accompanying notes: DeSilva et al. 2021/2023; Puschel et al. 2024; Will et al. 2021; Australian Museum; Harmand et al. 2015; Lepre et al. 2011.

Caption. Endocranial volume increased slowly among earlier hominins and then rose more steeply during the early history of Homo. DeSilva et al. (2021) report positive rate changes near 2.1 and 1.5 Ma; newer phylogenetic work emphasizes accelerating relative brain-size increase through time and cautions that within-species trends, body size, and taxonomy must be separated.

Source notes. Core teaching values are drawn from museum-level summaries and recent reviews: early hominins around 350 ml; Au. afarensis around 450 cc; H. habilis around 610 cc; H. ergaster / early H. erectus around 860 cc; H. heidelbergensis around 1250 cc; recent H. sapiens around 1350 cc. The allometric line is not a fitted fossil regression; it is a visual baseline anchored to Au. afarensis with a slope near 0.75, used to ask how far body size alone would move the brain.