Here’s a subject that you probably never think of: numbers. They are so simple and obvious that they don't seem to need much exposition. Yet, the numbering system we use today has a long and complex history revealing a great deal about the development of human cognition.
Tally systems
The simplest possible system uses tally marks. One slash represents a count of one; two slashes represent a count of two; three slashes represent a count of three and so on. This works just fine for small counts, but quickly grows combersome. The Ishango Bone is a 20,000 year old example of a tally.
What were they counting? People? Cows? Electoral College votes? It’s impossible to know.
Shorthand tally
The next step up was the substitution of “shorthand symbols” for groups of slashes. This is most easily represented by the system used in Roman numerals, in which “V” stands for 5, “X” stands for 10, and so on. It was a small step forward. The Roman system was archaic; its long survival is surprising.
“Semi-decimal” tally
The Sumerians developed a slightly better system:
This was still a tally system at heart, but it added a baby version of decimal places. The Egyptians and the Chinese used a similar system; many other civilizations developed variations on this scheme.
“Kinda-decimal” systems
The Greeks went a step further. They used the letters in their alphabet to designate values. I’ll make their system easier to understand by using our own alphabet to demonstrate the system. The letters A through I represent the numerals 1 through 9. The letters J through R represent 10 through 90. S through Z represent 100 through 800. Lower-case a represents 900. Then b through h represent 1000 through 9000. They stopped there. Thus, the number 5,481 would be represented as fVQA.
Full-decimal system
The system we now use was developed in India. Its history is murky, but we know that it developed over a period of centuries. The crucial idea of positional notation (the rightmost numeral represents the ones place, the next represents the tens place, and so on) arose sometime in the period 600 BCE to 100 BCE.
Zero
Precursors to the idea of zero appeared in Babylonian texts, and the Mayans used a symbol that meant “nothing”, but the use of zero as a numeral is definitely attributable to Indian mathematicians sometime between 100 CE and 500 CE. This completed the decimal numeral system. Islamic mathematicians adopted the Hindu system; Europeans learned it from the Muslims. Hence they are called Arabic numerals in the West.