Exactly. But I want to also quickly clarify the math, because I realize I put lots of numbers out there - but it's all pretty straight forward and very helpful for modeling and understanding where things are headed. There are two main components to the math:
1) An effectively fertility rate of "N" will result in the next generation being N/2 times as large. So a fertility of 2 means the next generation will be exactly the same size (2 people, having 2 kids who have 2 kids...) while a fertility rate of 4 would result in a doubling of size, and a fertility rate of 1 would result in the next generation being half as large.
2) To reverse the multiplication of some value by "X", you simply multiply by 1/X. So if our population increases by 1.5 in a generation, then to reverse that and get from the new population, back to where we started, we'd multiply by 1/1.5 or 0.66. So for instance 100 * 1.5 = 150. And 150 * 0.66 = 100.
---
So taking the most complex example. Fertility rate of 2.5 = a population increasing factor of 1.25. Reversing that means multiplying by 0.8
So if our parent's generation had a fertility rate of 2.5, that would mean that their population would be 0.8 times the size of our population, which I take as the global population which is about 8 billion. And if their parents also had a fertility rate of 2.5, then the exact same would follow for another multiplication of 0.8. So if we go back "N" generations, we'd end up with a population, for that generation, of 8 billion * 0.8^N.
So going back 70 generations with a fertility rate of 2.5, we finally get the original formula: 8 billion * 0.8^70 = world population 70 generations ago. You end up with this problem that if you don't have a really low effective fertility rate, the population approaches 0 far too quickly, and that's only going back 2100 years!