Humans have 46 chromosomes. Our closest primate relatives have 48. So where did those extra two disappear to?...
... How did having 46 chromosomes then spread worldwide? It’s possible that having two fewer chromosomes than everyone else gave [THEM] ... a whopping evolutionary advantage, allowing them to out-compete the 48-chromosome sluggards. But probably not.
More likely, they happened to be living at a point when the human race nearly got wiped out.
Take your pick for the cause of our near-extinction—ice ages, plagues, Indonesian gigavolcanoes. But humans have far less genetic diversity than most other species, and the most reasonable explanation for this is a genetic bottleneck: a severe reduction in the population of humans in the past, perhaps multiple times.
One study suggested that our population, worldwide, might have dropped as low as 40 adults.