Assuming Subnautica takes place in the late 22nd century
(2170-2199), it can be assumed the Precursors arrived at 4546B
around 1100 A.D, and discovering Carar prior to that.
The Sea Emperor Leviathan is approximately 1600 years old,
making it's date of birth at about 500 A.D.
How the Precursors obtained the Mongolian Blade is unknown, as
the data entry states it is from the 13th century, at least 100
years after the Precursors presumably were killed off by
A mass extinction event occurred at some point, possibly being
Carar, the meteor in the Dunes, or possibly Precursor interference,
although Carar seems the most likely given the timeframe of 1000