How do you explain the evolution of indentured servitude to hereditary slavery?

Indentured serviture was usually done by white people who were in debt to rich people from the colonies. Once they paid off their debts, their serviture would end. The debt was usually not so big that the children of them had to work. Hereditary slavery however meant that when a child is born it inherits the slave status from its mother, and it was common during the slavery period in the US.