Table 2.

Calculation of IMMR in fossil hominins

Ape modelHuman modelIntermediate model
SpeciesAge (Myr)Mean adult brain mass ± SD, cc*Mean adult female mass, gMean neonatal brain mass ± SD, g*Mean NBM ± SD, gIMMR (×100)Mean NBM ± SD, gIMMR (×100)Mean NBM ± SD, gIMMR (×100)
Ar. ramidus4.4300 (280–350)50,000126.5 (120.4–141.3)1,265.0 (1065.0–1600.1)2.1–3.21,028.4 (865.8–1,300.9)1.7–2.61,129.4 (950.9–1,428.7)1.9–2.9
A. afarensis3.0–3.7455.6 ± 79.429,000170.2 ± 21.21,702.1 ± 211.95.2–6.71,383.8 ± 172.34.2–5.41,519.7 ± 189.24.6–6.0
A. africanus2.0–2.7463.3 ± 50.631,000172.5 ± 13.41,724.6 ± 133.84.9–6.31,402.1 ± 108.84.0–5.11,539.8 ± 119.54.4–5.6
P. boisei1.4–2.4498.4 ± 34.234,000181.8 ± 9.01,818.3 ± 90.14.7–6.11,478.3 ± 73.33.8–4.91,623.5 ± 80.54.2–5.4
P. robustus1.4–2.0530.0 ± 032,000190.1 ± 01901.0 ± 05.2–6.81,545.0 ± 04.2–5.51,697.3 ± 04.7–6.0
H. habilis1.7–1.9612.2 ± 63.232,000210.6 ± 15.72,105.6 ± 157.15.8–7.51,711.9 ± 127.84.7–6.11,880.0 ± 140.35.2–6.7
Dmanisi1.8670.0 ± 74.340,200224.6 ± 17.72,246.2 ± 176.84.9–6.41,826.2 ± 143.74.0–5.22,005.6 ± 157.84.4–5.7
H. erectus0.7–1.78886.0 ± 107.446,000274.2 ± 23.82,742.2 ± 238.75.2–6.82,229.4 ± 194.14.2–5.52,448.4 ± 213.24.7–6.1
Atapuerca0.531,245.0 ± 134.360,600350.0 ± 27.03,500.4 ± 269.55.0–6.62,845.9 ± 219.14.1–5.43,125.4 ± 240.64.5–5.9
Skhul-Qafzeh0.091,498.4 ± 106.058,400399.8 ± 20.53,998.2 ± 205.46.0–7.93,250.6 ± 167.04.8–6.43,569.8 ± 183.45.3–7.0
Pan troglodytes384.2 ± 39.853,600 ± 2,200152.8 ± 16.61,766 ± 369.13.3
H. sapiens1,336.1 ± 127.757,100 ± 8,300373.8 ± 100.63,111 ± 461.45.7
  • *Based on average of conversion of adult brain (cc) to brain (g) using refs. 25 and 26. Calculated using regression equation in ref. 9 and adult cranial capacities listed in Table S4. Extant data are weighted averages from sources listed in Tables S1, S3, and S6.

  • Because Ardipithecus is represented by only a single skull, the values in parentheses are based on the full range of estimates of 280–350 cc.