Give a brief evolutionary explanation for salt retention by african americans.?

Answer 1

Sub saharan african people evolved in plains where salt NaCl is a limited resource, so a mechanism that alows them to retain salt gave an ecological advantage versus strains that did not. They survived and passed the trait to their descendants, so nowadays african americans tend to show this trait.

Answer 2

salt retention = water retention
In hostile enviroment they preserved salt and water.