If the average age of giving birth is 25, Mark Antony would be about 80 generations ago. With each generation back doubling the number of ancestors (2 parents, 4 grandparents, …), this person has a theoretical 2^80 ancestors - somewhat more than the number of people that have ever lived. So clearly that logic is flawed, and the family tree must in fact be a family web - but potentially a very, very wide one. I wouldn…
I speculate that when times were hard for humans in the course of history, those who were better off were more likely to successfully raise kids to adulthood, who in turn had better chances of establishing their own families