Why is circumcision done on the eight day?

Why is circumcision done on the eight day?

In an article on circumcision, an interesting question is asked.

Why in Judaism circumcision takes place on the eighth day?

The article goes on to provide a scientific explanation.

“This has been carried out by Jewish tradition for well over 3000 years
though the answer to the question ‘Why the eighth day?’ was not answered until
more recent times.

A new born child is prone to bleeding especially between the second and
fifth days of life. The blood clotting factor, Vitamin K is insufficiently in
evidence in the bloodstream until the seventh day of life and another blood
factor necessary for blood clotting,called prothrombin, is at its one time high
peak on the eighth day. Therefore, of all the days of a baby's life, the eighth
day is the optimum day for an operation to facilitate wound healing.”

What does circumcision mean?

My question is , how did people who lived 3000 years ago, know this?