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The thumb is the innermost digit on the hand, and so there are two, one on each hand. We humans (in the hominid line) have what are called fully opposable thumbs which is to say that the thumbs can be used in combination with any of the other digits (fingers), or specifically in opposition to those digits, in a 'pincer movement'. This is an evolutionary development which enabled us (in the hominid line - from Homo habilis to Homo sapiens, the latter being the 'modern human') to use tools, leading to development of food collection and hunting, and more complex communication between each other through written language.
Other animals have opposable thumbs (not 'fully' opposable, as the cited webpage suggests), including gorillas, chimpanzees, lesser apes and some species of monkey. Many of these also have opposable 'big' toes, the 'big' toe being the equivalent on the foot to the thumb on the hand. That is, the 'big' toe is the innermost digit on the foot. Tree-dwelling animals (eg chimpanzees, monkeys, koalas) in particular use opposable thumbs and opposable toes to aid climbing, where chimpanzees and monkeys also have tails for balance when climbing. Since evolving away from forest dwelling, the hominid line is thought to have lost these climbing 'tools' as they became unecessary/superfluous. Anything that is superfluous nature usually 'trims off' as it takes the most efficient line, much like electricity and water running downhill.
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