Short answers, pulled from the story.
The metre was first defined in 1791 when a commission of French scientists declared that the distance from the North Pole to the Equator measured through Paris would be exactly ten million times the length of the new unit. This decision transformed the planet into the ultimate measuring stick and replaced centuries of arbitrary local standards with a universal constant carved into the Earth's geometry.
On the 20th of May 2019 the General Conference on Weights and Measures voted to redefine the kilogram not by a physical object but by the Planck constant. This historic move ended the era where the standard of mass was a physical artifact kept in a vault in Sèvres France and replaced it with a definition embedded in the fabric of the universe.
The Mars Climate Orbiter crashed in 1999 due to a unit conversion error between metric and imperial systems. This costly error occurred because American engineers had to navigate two parallel systems where the United States continued to use a hybrid system that combines metric units in science and industry with customary units in daily life.
In 1901 the Italian physicist Giovanni Giorgi proposed adding an electrical unit as a fourth base unit to resolve the anomalies in electromagnetic systems. Giorgi's insight led to the creation of the metre-kilogram-second-ampere system which became the direct forerunner of the modern International System of Units.
The metre was originally defined as one ten-millionth of the distance from the North Pole to the Equator measured through Paris. This definition was slightly shorter than the theoretical ten-millionth part because the scientists had to make compromises measuring the distance through Paris rather than the full meridian.