The reason humans have only two thumbs is related to the evolution of our forelimbs and the development of our hands. Thumbs play a crucial role in manipulating objects and performing fine motor tasks, which are essential for activities such as writing, using tools, and interacting with the environment.
Here’s a brief explanation of why humans have two thumbs:

1. Evolutionary Ancestors: Our early ancestors, which included primates and other mammals, had forelimbs adapted for grasping and climbing. The development of thumbs was a gradual process, with the thumb becoming more opposable over time, allowing for a greater range of grasping abilities.
2. Opposability: The opposable thumb is a key feature that distinguishes humans from other primates. It allows the thumb to touch the tips of the other fingers, providing a strong grip and precision in handling objects. This opposable thumb configuration is optimal for the dexterity required for tool use and manipulation.
3. Digit Reduction: While many primates have five digits in their hands, humans have reduced the number of digits to four, with the fifth digit (the little finger) being smaller and less opposable than the other fingers. This reduction in digits allowed for the optimization of the thumb’s functionality and the specialization of the hand for fine motor skills.
4. Human Locomotion: The adaptation of the hand for grasping objects was accompanied by changes in human locomotion, such as walking and running. The structure of the human hand, with its opposable thumb, is better suited for bipedal movement and the manipulation of objects while walking or standing.
5. Cognitive and Social Development: The dexterity provided by the opposable thumb has been crucial for the development of complex cognitive abilities and social interactions. The ability to create and use tools, as well as to communicate through sign language and other non-verbal means, is enhanced by the specialized hand structure.
6. Genetic Factors: The development of the opposable thumb is influenced by specific genetic factors. While the exact genes involved are complex and not fully understood, genetic variations that promote the proper development of the thumb have been selected for over many generations.

In summary, the presence of two thumbs in humans is a result of evolutionary processes that favored the development of hands capable of precise manipulation, which is essential for tool use, fine motor skills, and many other activities that define human culture and society.