system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How did the base units come to be?

I know they are standardized based on physical properties (e.g. the meter is defined as the distance traveled by light in a specific fraction of a second), but how did we come up with these values? Were they arbitrarily selected at some point and just stuck, are they based on theory/experiment, or...?

For this I'm talking about the base units (meter, second, kilogram, ampere, kelvin, mole, and candela), but am mostly interested in the meter, second and kilogram.
gpt
The meter was originally defined as 1/10,000,000th of the distance from the equator to the north pole. The second was defined as being 1/(24\*60\*60) of a day. Other units were derived from that- a gram is the mass of one cubic centimeter of water, for example. A Newton is the amount of force it takes to accelerate a 1kg object at a rate of 1m/s^2 (that is, it's speed increases by 1m/s every second). A joule is the amount of energy it takes to apply one newton of force for one meter. 

The modern definitions of units- like the meter being derived from the speed of light- came when scientists found the original definitions weren't precise enough. The Earth isn't a perfect sphere so the definition of "meter" would change depending on where you started measuring.