“The common Y chromosome ancestor lived 250,000 years ago, not 4,000.”
Measuring time is a peculiar thing. If you take a spaceship and go at nearly the speed of light to the edge of the galaxy, and return, it would take you about 40 years. But in that period, about 100,000 years would have passed on earth.
Yet you would only be 40 years older.
I think we can’t measure yet with any accuracy how “old” the earth is, or how much time has passed between events.
Or so the theory of relativity claims.
That theory was deduced through pure mathematics, not direct observation, as in classical physics. Most modern cosmology is based upon the works of Einstein and other theoretical mathematicians, and its explanations and predictions about the behavior and make-up of our universe show it.
Black holes, dark matter, quasars, expanding universe, the big bang, dark energy, pulsars, gravitational waves, etc., are all inventions proffered by mathematicians to paper over the glaring holes in the Standard Model.
But I digress.
The 250,000 year old figure for the age of Homo Sapiens was arrived at by using 20 years for each generation as a baseline. It's only an approximation, but I think it's close enough to make the case.