Starting around the year 1...
Augustus, 31 B.C.E. - 14 C.E.
Everyone on earth today is related to just a single person.
That is, there was a person living in 1 A.D. who, due to the mathematics of division and a standard deviation or so of variance as a result of invasion and migration (e.g., Egypt being invaded by Western Asians (Peoples of the Sea), Nubians, Babylonians, Persians, Greeks, Romans, and Arabs; Spain and North Africa were overrun by Vandals (thereafter traveling to Spain from the North was known as 'vandalism'; the Navajo came from an area around Great Slave Lake in Northwest Canada/Yukon and settled in Arizona, and so on....)
The population of earth was much smaller in the year 1 than today.