Question

Why do scientists consider cranial capacity when trying to determine the evolutionary history of humans?

Answer 1

because as the cranial capacity increases so does the size of the brain and throughout evolution our brains got bigger. 
As the size of the cranial capacity increases so does the ability to learn language and manufacture tools as the brain can increase in size therefore increasing intelligence.

-Hope this helped! :D

Answer 2

Answer:

Cranial capacity measures the interior volume of cranium or skull. It is usually measured in cubic centimetre. Cranial capacity has shown significant changes  in evolutionary history of humans hence it is studied while studying human evolution.

Increase in cranial capacity is associated with increased intelligence and development of more advanced skills as evident from the cranial capacity of humans which has increased from 440 cc in Australopithecus to 1350 cc in modern humans. Along with increase in cranial capacity; social complexity, communication skills and tool making skills also improved so they are often co related and studied together.