When I was in India I was interested to see that women wearing saris would use the veil to protect their heads when it got really hot.
I realised that Hindu women have used the veil either with the sari or chemise and trousers for protection from the hot Indian sun centuries before Islam arrived on the scene and made it mandatory for women to cover their heads.
Perhaps it was like this in the middle East and Asia minor too. Why then did it become a rule?