A couple of points: - We don't know much about the details of what we would see down the chimp line. They were forest-dwellers, and things don't fossilize as well in forests as they do in the savannahs where the human line largely lived. - Your picture doesn't make explicit that the far-end mommies on each line are holding hands with their single mommy. This little bit may well be best saved for after you already have your correspondent agreeing that the lines each change as you go back in time, but it does need to be stated emphatically at some point in the conversation.
And we may always be missing her, particularly if she never left the forest.
As if there was any chance at all of the individual Mommy Of Us All being fossilized anyway, of course. We seem to be getting closer to the population she belonged to with each new dig in East and South Africa, but ther's a lot of Miocene/Pliocene rock in those areas.
That image, or a close cousin, is old enough to have been parodied in a New Yorker cartoon in 1925, during/after the Scopes trial. It was titled "the rise and fall of mankind" and showed a similar ape, a brutish "Neanderthal," the spear-carrier, Socrates, and finally William Jennings Bryan.