| Description |
Brain size scales predictably with body size for most placental mammals. However, humans have brains that are about seven times larger than expected for their body size, with total brain volume being about 90% cerebrum and about10% cerebellum. The cerebellum is responsible for coordination of motor function and is extensively interconnected to the cerebrum. This is the first study to examine the relationship between cerebellar volume and novel motor behaviors that were crucial during hominin evolution-such as obligate bipedal locomotion, endurance running, fast, accurate throwing, and speech/language-as well as the relationship between cerebellar volume and total brain volume over time. Linear measurements of the posterior cranial fossa from selected hominin endocasts were used to approximate cerebellar volume because it is the region that contains the cerebellum. This study found that cerebellar volume shows significant variability, ranging from 9.4% to 21.1%, over hominin evolution. Additionally, when the average percentage of cerebellar volume for each hominin species was plotted over time, it peaks when novel motor behaviors are thought to be arising. The percentage of cerebellar volume declines following each peak. This decline can be explained in two ways-there is either a decrease in cerebellar volume or an increase in cerebral volume. The latter makes the most sense because total brain volume increases over time. Therefore, the influence novel motor behaviv iors have on the evolution of the cerebellum and ultimately on the evolution of large hominin brains cannot be discounted. |
