Have you ever wondered why the average Qatari makes about 192 times the average Burundian? Or why North America, Europe, and now parts of the Middle East are prosperous, modern countries with advanced technology while most of Sub-Saharan Africa are living about the same way they lived thousands of years ago?
These questions of national inequality are among the most controversial, political, and taboo questions of all. They incite the greatest feelings of frustration on the losing sides and concern for the winners (and sometimes immense pride {cough cough, Nazis, cough cough). The simple reason for this outcome is quite simple, and it has to do with efficiency.
Efficiency has a lot to do with the environments that people are born in or migrate to. The cultures that strike the jackpot in their environment and the animals around them are the ones that produced the Beethovens, Edisons, Benzes, Saudes, etc.
Two peoples are practically the same physiologically with differences largely present in physical appearances that have nothing to do with one people's prospects.
Differences in physical appearance are the easiest answer to the question because they are the most apparent besides differences in technology. Imagine Pizarro and 200 Spaniards marching towards the Incan capital observing first the poverty of these peoples and then the obvious physical differences in their color of skin, their quality and length of hair, etc.
However, these Incans did not have the advantage that these Spaniards had that was established by their posterity tens of thousands of years before through the patterns of their migration.
The Spaniards were privileged in the fact that their posterity had settled in the Iberian Peninsula likely coming from the Middle East or Africa through the Strait of Gibraltar crossing or through the Levant or the Caucuses. The Native Peruvian's posterity had settled through the Bering Trait or through Greenland and made it to Peru.
Peru had many fewer native domesticatable animals that were appropriate "beasts of burden". The only one was the Llama, but it had clear disadvantages to the Afroeurasian beasts of burden and domesticatable animals, including cows, sheep, horses, pigs, etc.
This fact contributed to much lower efficiency in the production of food in Peru and across the American continent. After thousands of years of much slower growth in America, the Spaniards were at a much higher advantage due to what historian Jared Diamond describes as "guns, germs, and steel".
This advantage culminated in the forty-year year conquest of the Incan Empire with one hundred thousand soldiers by the Spanish Empire by just 168 soldiers.
The rate of predicted growth of a nation over time is directly proportional to the efficiency of food production. It probably took those Spaniards about 0.2 days of work to get 1 day worth of food while it took the Incans about 0.9 or higher days of work.
This simple solution shows how one group of people is able to conquer another and why and how there is so much inequality between groups of the same species of people.