Answer – Chromosome
In eukaryotic nuclei, DNA and proteins are packaged into organized cellular structures known as Chromosomes. Genetic material is housed in the nucleus where is it tightly packed into linear chromosomes. Chromosomes consist of a complex of DNA and protein which is known as chromatin. Chromatins are organized into subunits called nucleosomes
This is only one part of the answer, but its important: high-altitude O2 absorbs much energy from the sun (including radioactivity), which forms the ozone layer. this process protects delicate molecules important to life - most notably dna
If you’re talking about too many people I’m guessing the limitation factors would be lack of resources and that would include lack of food and space.
