"what accounts for the similarities among all human beings everywhere in the world, such as the universal capacity for language or loyalty to a family or clan? "

The genetic dispositions which have developed during the evolution of humans.

Explanation:

The gene frequency of a population tend to change with time. Through changes in the genetic frequency, evolutionary changes in a species might occur.

However, there are some genes that will always be passed from the ancestors to the children. Each gene carries story of the human evolution. Though gene frequencies tend to change but there are certain characteristics or genetic dispositions which will always remain common among members of a species. The human genome is 99% similar to one another. Hence, this accounts for the similarities among human beings everywhere in the world.