8.4: Brain Size
- Page ID
- 62183
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Bipedal locomotion became established in the earliest stages of the hominin lineage, about 7 million years ago, whereas brain expansion came later. Early hominins had brains slightly larger than those of apes, but fossil hominins with significantly increased cranial capacities did not appear until about 2 million years ago.
Brain size remains near 450 cubic centimeters (cc) for Paranthropus until almost 1.5 million years ago. At the same time, fossils assigned to Homo exceed 500 cc and reach almost 900 cc.
What might account for this later and rapid expansion of hominin brain size? One explanation is called the "radiator theory": a new means for cooling this vital heat-generating organ, namely a new pattern of cerebral blood circulation, would be responsible for brain expansion in hominins. Gravitational forces on blood draining from the brain differ in quadrupedal animals versus bipedal animals: when humans stand bipedally, most blood drains into veins at the back of the neck, a network of small veins that form a complex system around the spinal column.
The two different drainage patterns might reflect two systems of cooling brains in early hominins. Active brains and bodies generate a lot of metabolic heat. The brain is a hot organ, but must maintain a fairly rigid temperature range to keep it functioning properly and to prevent permanent damage.
Savanna-dwelling hominins with this network of veins had a way to cool a bigger brain, allowing the "engine" to expand, contributing to hominin flexibility in moving into new habitats and in being active under a wide range of climatic conditions.
In addition, there is a correlation between brain size and tooth size; as brains got larger, teeth got smaller. When an animal is born, the crania is in pieces to allow the brain to finish developing. Once complete, the skull fuses together. In animals like gorillas, who have immense chewing power, they also have a sagittal crest, or extra ridge of bone on the skull to hold large chewing muscles like the masseter muscles. This puts a lot of pressure on the skull and therefore means the gorilla’s skull has to fuse together soon after birth, limiting brain growth. Bipeds (including modern humans) do not have powerful chewing muscles and lack a sagittal crest. Therefore the skull can fuse later, allowing longer brain growth and development (and therefore a larger overall brain size). (Etheridge-Criswell, 2018).