Final answer:
The mastoid process of the temporal bone and the cranial sutures developed from the fontanelles are the two most useful skull regions for determining ancestry due to their variability across populations.
Step-by-step explanation:
The two most useful regions of the skull in determining ancestry are the temporal bones, especially the mastoid process, and the cranial suture patterns, particularly at the fontanelles. The mastoid process of the temporal bone can be useful due to its prominence and variability across populations, serving as an attachment site for muscles and being easily palpable behind the earlobe. The fontanelles, initially the soft spots on an infant's head, give rise to the cranial sutures, which also vary among different populations and can inform about ancestral origins as they allow for brain growth and eventually ossify in a pattern unique to the individual.