But my point also hinges heavily on population density. Taking the upper end estimates for human populations 30,000 years ago, before the Harvard study, which put the human population of the planet at 20,000 to 25,000 individuals, and being generous for the sake of argument, that's 10,000 on the Eurasian land mass. Divide the area of the Eurasian land mass by two to account for glaciation, and you still get a figure of one human for every 2000 square kilometers. A band of 25 people would have a range of 50,000 square kilometers. It's not as though there would have been any friction due to population density.
I agree that humanity almost went extinct at one time, and that the human population on this planet plunged to some four- or five-digit number during this near-extinction. (Most sources I've read put the time of this extinction 80,000 years ago, not 30,000.) And I agree that this near-extinction is reflected in a remarkable dearth of genetic variation among humans today. (The worldwide variation among humans is smaller than the typical variation among chimps on one plot of African forest.)
That being said, I think you have succumbed to some wrong intuitions about what that means. In particular, the genetic evidence does not
mean that humanity spent a whole lot of time going through this low-population bottleneck. Also, it does not
mean that the average human family lived in a neighborhood with only a few other human families in it.
Just to illustrate the point with a concrete example, let's say humanity's near-extinction was caused by a plague. This could have crunched the human population to almost nothing within a generation or less. The 10,000 survivors could have been a lucky minority who developed a resistance just in time before a total collapse. Breeding from those survivors, the human population would then have recovered within a few dozen generations --- a blink of evolution's eye. And humanity wouldn't have spent much time at all in this bottleneck. For what it's worth, most sources I've read attribute this population bottleneck to one catastrophic die-off, not to a steady state.
Now to your intuition about population density. You may know that at one time in recent American history, a lethal fungus made the population of American Elm trees collapse to a ridiculously low number. After this collapse, the American Elm's population density across the American continent was even more ridiculously low. Nevertheless, the few American Elms who did survive often had plenty of other American Elms around them. That's because they survived in small, isolated niches such as Central Park, New York City, where the fungus could not reach them. Within Central Park
, the American-elm population did not change at all.
Hence, it would be a fallacy for an ecologist to conclude that, since the population density of American Elms across the continent was so low, the average American elm must have lived in a neighborhood devoid of other American elms. I respectfully submit that you are succumbing to the same fallacy about the human population 80,000 years ago (or 30,000, as your source would have it). Even at a world population of only 10,000 humans, the average human family could still have lived in a neighborhood with plenty of other human families in it.