By the tradition of Abrahamic faiths, be it Christianity or Islam, it is stated that God created Man, and formed our bodies accordingly.

And when God created us, He gave us two thumbs, in His infinite wisdom.

Now it is not stated in any religious text what the exact nature of our thumbs are for, but some scholars have given credence to the theory that God gave you two thumbs for a divine reason: one to place into your mouth, and the other to do a modern gesture known as "thumbs up", to signify your acceptance of your fate, no matter how dire it may be.

As a Malay-Muslim scholar of the religious texts (3rd Warrant Officer Ilmiizhar) used to summarise very eloquently to me during times of distress:

“Lan lan suck thumb bro, kena caught just own up”