Why has the human body been used to generate units of measurement over the course of human history?
The answer is that the human body was a convenient and universal standard by which things could be measured or counted. The ten fingers of both hands represented a simple method to construct counting systems, and the base 10 is widely used even today. The length from the tip of a thumb to the first joint is about 1". The distance from the tip of the middle finger to the elbow is about 21", and that length, called a cubit, was a standard measurement which was used for many centuries.
Is the body a good reference point for standardized units?
The answer is a qualified yes. Actual body size is variable, and that fact was recognized early in human attempts to define measurements. What happened was that from general sizes, fixed units were defined. Those definitions have been continuously revised.
Can you use it to measure reliably?
Within reasonable limits, the answer is yes. But, due to variations in actual sizes of body parts, measuring systems were developed to eliminate the variations.
