Final answer:
All humans are genetically similar because we share a recent common ancestor from whom we've inherited most of our genes. Variations are limited due to the short evolutionary timeframe and gene mixing through reproduction and migration.
Step-by-step explanation:
According to the best genetic evidence, all humans alive today are so similar genetically because we share a common ancestor from a relatively recent point in our evolutionary history. Humans can trace their genetic lineage to a group known as Mitochondrial Eve and Y-chromosomal Adam. These aren't individual people, but rather a female and a male from whom all humans inherited their mitochondrial DNA and Y-chromosome respectively.
Genetic similarities among humans are the result of this common ancestry, and the relatively small degree of variation is due to the short time (in evolutionary terms) that has elapsed since we diverged from these common ancestors. Additionally, the mixing of genes through reproduction and migration has kept our gene pool somewhat uniform.
Learn more about Human Genetic Similarity