A study examining various primate species has found a surprising correlation between thumb size and brain power. The researchers discovered that humans’ unique adaptation of having opposable thumbs enabled our brains to grow larger and more complex, leading to greater sophistication in cognitive abilities and behavior.
Source: https://www.thetimes.com/uk/science/article/rule-of-thumb-suggests-longer-digits-mean-bigger-brains-l28bljpqz