You are asking me to imagine the scenario at the top of the family tree, where my grandmother has several of her siblings, cousins, etc.
I am on the other hand asking what happened at the bottm, where the first branch took place.
But that's not what you're talking about. Let's make this into a scenario with similar terminology.
Let's say, just for the sake of argument, that your last name is Jones. You have siblings, cousins, even second cousins once removed. All these people can be traced back to a single woman, and to donate that common ancestor, we'll call her Jones-Eve. SHe's the most recent person all your siblings and cousins and second cousins once removed trace back to collectively. Now, there are a lot of other people that some parts of your family trace back to that other parts don't.
It's the same for Mt-Eve. Everyone living traces back to her collectively, but we also all trace back to other people who were alive at the same time, or eariler that other people living today don't trrace back to.
Essentially, all Mt-Eve and Y-Adam say is that all humans are related if you go back far enough. It never says that there was only one person alive at a particular time, nor does it ever say that we're all descended from only her. It only says that we're all partially decended from her along with hundreds or thousands of other people at the same point in time.
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