Hominin paleoneurology during the Stone Age–and before!

Paleoneurologists study the size, shape, and surfaces of the brains of human ancestors by producing casts of the interiors of their fossilized skulls (endocasts) and measuring the volumes of their braincases (cranial capacities). Cranial capacities from dated skulls show that brain size more than tripled in hominins during the Stone Age that began around 3.5 million years ago (ma). Endocasts replicate brain shape and, with luck, reproduce impressions of blood vessels and convolutions that were stamped on the inner walls of the braincase during life. However, the sulci that delimit convolutions often appear fragmentary and difficult to “read” on endocasts. Sulcal patterns differ noticeably in only two parts of the brains of great apes and humans –the lateral prefrontal cortex in the frontal lobes and the parieto-occipital association cortices located near the back of the brain. With respect to the latter, scholars have long debated whether a sulcus that approximates the anterior border of primary visual cortex in living monkeys and apes (the lunate sulcus) also did so in the brains of evolving hominins. This controversy has recently been addressed by considering what may have occurred underneath the surfaces of hominins’ evolving brains.

Because very few putative fossil hominins have been identified during the time between the origin of hominins, perhaps as early as 7.0 ma, and the beginning of the Stone Age, little is known about the size and morphology of hominin brains during approximately the first half of their evolution. However, brain evolution can still be studied during this period (the Botanic Age) by applying a variety of methods including comparative primatology and evolutionary developmental biology (evo-devo) that take into account not only brain development but also the evolution of locomotion including bipedalism. Such an extended paleoneurological synthesis (EPS) is applied here with special attention to the emergence of two related cognitive abilities that distinguish humans from other extant primates –the ability to sustain keeping a beat to an external auditory rhythm (auditory entrainment) and complex grammatical language that facilitates the expression of an open-ended number of ideas.