https://en.wikipedia.org/wiki/Population_bottleneck may be a starting point for surveying the current hypotheses about the human population bottlenecks that have occurred in the past. You're also expressing a fundamental misunderstanding of evolutionary selection mechanisms, resulting in incoherent conclusions. (most egregious example: "Hard conditions permitted for only the most fit to survive so there was no need for extreme medical intervention." The "most fit" for a selection lie at a tail end of a distribution. There is "no need for extreme medical intervention" only when ignoring the concerns of more representative samples of the distribution.)