The metre was originally defined in 1791 (…) as one ten-millionth of the distance from the equator to the North Pole along a great circle, so the Earth’s circumference is approximately 40000 km. en.m.wikipedia.org/wiki/Metre
The kilogram was originally defined in 1795 during the French Revolution as the mass of one litre (1/1000 m³) of water. en.m.wikipedia.org/wiki/Kilogram
… and the last major SI unit is the second which of course you know is (originally was) 1/86400 day.
Please notice about the Celsius scale : the second reference point isn’t a mixture of ice and salt but rather pure water freezing point.
Now how can we as naked humans develop technology to figure this out is something of historical proportion, that’s a quite amazing story !