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. 2011;77(1):33-44.
doi: 10.1159/000322729. Epub 2011 Jan 11.

Gorilla and orangutan brains conform to the primate cellular scaling rules: implications for human evolution

Suzana Herculano-Houzel  1 , Jon H Kaas
Affiliations

Gorilla and orangutan brains conform to the primate cellular scaling rules: implications for human evolution

Suzana Herculano-Houzel et al. Brain Behav Evol. 2011.

Abstract

Gorillas and orangutans are primates at least as large as humans, but their brains amount to about one third of the size of the human brain. This discrepancy has been used as evidence that the human brain is about 3 times larger than it should be for a primate species of its body size. In contrast to the view that the human brain is special in its size, we have suggested that it is the great apes that might have evolved bodies that are unusually large, on the basis of our recent finding that the cellular composition of the human brain matches that expected for a primate brain of its size, making the human brain a linearly scaled-up primate brain in its number of cells. To investigate whether the brain of great apes also conforms to the primate cellular scaling rules identified previously, we determine the numbers of neuronal and other cells that compose the orangutan and gorilla cerebella, use these numbers to calculate the size of the brain and of the cerebral cortex expected for these species, and show that these match the sizes described in the literature. Our results suggest that the brains of great apes also scale linearly in their numbers of neurons like other primate brains, including humans. The conformity of great apes and humans to the linear cellular scaling rules that apply to other primates that diverged earlier in primate evolution indicates that prehistoric Homo species as well as other hominins must have had brains that conformed to the same scaling rules, irrespective of their body size. We then used those scaling rules and published estimated brain volumes for various hominin species to predict the numbers of neurons that composed their brains. We predict that Homo heidelbergensis and Homo neanderthalensis had brains with approximately 80 billion neurons, within the range of variation found in modern Homo sapiens. We propose that while the cellular scaling rules that apply to the primate brain have remained stable in hominin evolution (since they apply to simians, great apes and modern humans alike), the Colobinae and Pongidae lineages favored marked increases in body size rather than brain size from the common ancestor with the Homo lineage, while the Homo lineage seems to have favored a large brain instead of a large body, possibly due to the metabolic limitations to having both.

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Figures

Fig. 1
Fig. 1
DAPI-stained granule and Purkinje cell nuclei in orangutan (a) and gorilla (b) cerebella can be distinguished by morphology. Horizontal arrows point to Purkinje cell nuclei; vertical arrows point to a few of several granule cell nuclei visible in the fields.
Fig. 2
Fig. 2
Scaling of cerebellar mass as a function of the number of cerebellar cells (a) or cerebellar neurons (b). Plotted functions (indicated in the graphs) apply to the 6 primate species described previously [Herculano-Houzel et al., 2007]; notice that great apes and human cerebellar data fall close to the plotted line. Addition of great apes and human data does not modify these functions significantly (which then become MCB = 2.96 × 10−9 × CCB0.971 and MCB = 1.86 × 10−9 × NCB0.999, respectively), and controlling for phylogenetic relatedness in the dataset changes these exponents only minimally, from 0.971 to 0.997, and from 0.999 to 1.010.
Fig. 3
Fig. 3
Deviation of observed mass, number of neurons and number of non-neuronal cells in the cerebral cortex from the values expected from the total number of cells in the primate cerebellum according to the equations MCX = 1.921 × 10−9 × CCB1.082, NCX = 3.634 × CCB0.898 and NNCX = 0.069 × CCB1.114 derived from data available for the 6 primate species shown in the left part of the plot [from Herculano-Houzel et al., 2007]. Bars indicate average deviation, 25th and 75th percentiles, and maximum and minimum deviations from the expected values. Bars for Pongo and Gorilla refer to cortical mass only.
Fig. 4
Fig. 4
Deviations of observed whole brain and body mass from the values expected from the total number of cells in the primate cerebellum according to the equations MBR = 1.018 × 10−8 × CCB1.020 and MBODY = 2.258 × 10−6 × CCB0.958 derived from data available for the 6 primate species shown in the unshaded part of each plot [from Herculano-Houzel et al., 2007]. Notice that the predicted brain mass for the orangutan and gorilla match closely the observed values, while the body mass of these species vastly exceeds expectations.
Fig. 5
Fig. 5
Time of divergence from earliest common ancestor for different primate lineages, in millions of years (Mya). a Relative brain mass for each group, expressed as percentage of body mass, is indicated on the right. Estimated numbers of neurons for great apes and hominin species are shown in billions. Based on Purvis [1995]. b Relative brain mass for individual species in each group plotted by time of divergence in millions of years (Mya).
Fig. 6
Fig. 6
Brain mass (a) and body mass (b) for each species in the different primate groups plotted by time of divergence (in millions of years, Mya) for each group from the latest common ancestor.
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