Samples and scanning

We acquired eight brains from the Primate Brain Bank (PBB)—one prosimian primate (bushbaby), four New World monkeys (night monkey, woolly monkey, white-faced saki, and tufted capuchin), two Old World monkeys (black-and-white colobus and grey-cheeked mangabey), and one great ape (chimpanzee). Demographics are listed in Table ​Table1.1. All samples were returned to the PBB after scanning and remain available for future research. In addition, we used data from one rhesus macaque (Macaca mulatta, male, 4.03 years of age at death) from a previous study (Folloni et al. 2019) for comparison with this most commonly studied primate. Phylogenetic relationships between all species in this study and the human are displayed in Fig. 1.

Demographic information of the scanned PBB samples

Phylogenetic relationships between the species in this study. Numbers indicate median divergence times (in millions of years) based on genomic data (Hedges et al. 2015)

Prosimian and monkey data were acquired locally on a 7 T magnet with an Agilent DirectDrive console (Agilent Technologies, Santa Clara, CA, USA) using a 2D diffusion-weighted spin-echo protocol with single line readout (DW-SEMS, TE/TR: 25 ms/10 s; Matrix = 128 × 128; number of slices: 128; Resolution: 0.3 × 0.3 × 0.3 mm for the galago, 0.4 × 0.4 × 0.4 mm for the night monkey, 0.5 × 0.5 × 0.5 mm for the saki monkey, 0.6 × 0.6 × 0.6 mm for the woolly, capuchin, colobus, and mangabey; diffusion data were acquired over the course of ~ 52.5 h). 16 non-diffusion-weighted (b = 0 s/mm²) and 128 diffusion-weighted (b = 4000 s/mm²) volumes were acquired with diffusion directions distributed over the whole sphere. The brains were soaked in PBS before scanning and placed in fluorinert during the scan.

Great ape data were acquired on a 7 T whole-body scanner with an 28 channel knee coil (QED) using a 3D diffusion-weighted steady-state free precession (McNab et al. 2009) protocol (DW-SSFP, TE/TR: 18 ms/22 ms; flip angle: 50°; resolution: 0.6 × 0.6 mm²; number of slices: 176; slice thickness: 0.6 mm; EPI factor: 1; diffusion data were acquired over the course of ~ 50 h). 12 non-diffusion-weighted (q-value = 20 cm⁻¹) and 240 diffusion-weighted (q-value = 300 cm⁻¹) volumes were acquired with diffusion directions distributed over the whole sphere. Accurate modelling of DW-SSFP data requires the estimation of DW-SSFP dependencies (T1, T2 and B1) (Buxton 1993). To account for this, T₁-weighted data were obtained using a turbo inversion recovery (TIR, TIs: 30, 60, 120, 240, 480 and 940 ms; TE/TR: 17 ms/1020 ms; resolution: 0.8 × 0.8 mm²; number of slices: 112; slice thickness: 0.8 mm; TIR data acquired over the course of ~ 2 h), T₂-weighted data were obtained using a turbo spin-echo (TSE, TEs: 8.7, 17, 26, 35, 52 and 70 ms; TR: 1000 ms; resolution: 0.8 × 0.8 mm²; number of slices: 112; slice thickness: 0.8 mm; TSE data acquired over the course of ~ 2 h) and B₁ maps were estimated using an actual flip angle imaging (Yarnykh 2007) sequence (AFI; TE: 2.6 ms; TRs: 7.7 and 21 ms; resolution: 3 × 3 mm²; number of slices: 32; slice thickness: 3 mm; AFI data acquired in ~ 43 s).

MGE data for surface reconstruction for the prosimian and monkey samples from the PBB were also acquired on a 7 T magnet with an Agilent DirectDrive console (Agilent Technologies, Santa Clara, CA, USA) using a 3D multi-gradient-echo sequence (MGE3D, TE1/δTE/TR = 7.5 ms/11 ms/98 ms; resolution = 0.15 × 0.15 × 0.15 mm (galago), 0.2 × 0.2 × 0.2 mm (night monkey), 0.25 × 0.25 × 0.25 mm (saki monkey), 0.3 × 0.3 × 0.3 mm (other samples); data were acquired over 1 h).

Structural scans for the great ape were acquired on a 7 T whole-body scanner with a 28 channel knee coil (QED) using a 3D true fast imaging with steady state free precession protocol (TRUFI, TE/TR: 5.06 ms/10.12 ms; flip angle: 39°; resolution = 0.4 × 0.4 mm²; number of slices: 256; slice thickness: 0.4 mm; TRUFI data acquired over the course of ~ 22 min). 4 TRUFI datasets were acquired (phase increment: 0°, 90°, 180°, and 270°) and averaged via root sum of squares.