Often, in the Bible, people used to tear clothes, sprinkle their heads with ashes, and tear their hair as a sign of humility, penitence, and repentance.
Few examples:
Job 2:12 KJV
And when they lifted up their eyes afar off, and knew him not, they lifted up their voice, and wept; and they rent every one his mantle, and sprinkled dust upon their heads toward heaven.
Judith 4:15 KJV
And had ashes on their mitres, and cried unto the Lord with all their power, that he would look upon all the house of Israel graciously.
1 Maccabees 11:71 KJV
Then Jonathan rent his clothes, and cast earth upon his head, and prayed.
Lamentations 2:10 KJV
The elders of the daughter of Zion sit upon the ground, and keep silence: they have cast up dust upon their heads; they have girded themselves with sackcloth: the virgins of Jerusalem hang down their heads to the ground.
2 Samuel 1:2 KJV
It came even to pass on the third day, that, behold, a man came out of the camp from Saul with his clothes rent, and earth upon his head: and so it was, when he came to David, that he fell to the earth, and did obeisance.
Ester 4:1 KJV
When Mordecai perceived all that was done, Mordecai rent his clothes, and put on sackcloth with ashes, and went out into the midst of the city, and cried with a loud and a bitter cry;
Luke 10:13 KJV
Woe unto thee, Chorazin! woe unto thee, Bethsaida! for if the mighty works had been done in Tyre and Sidon, which have been done in you, they had a great while ago repented, sitting in sackcloth and ashes.
Jeremiah 6:26 KJV
O daughter of my people, gird thee with sackcloth, and wallow thyself in ashes: make thee mourning, as for an only son, most bitter lamentation: for the spoiler shall suddenly come upon us.
These are all gestures that clearly exhibit the feelings, but how/why did those behaviours originate, and why exactly those (and not other kinds of gestures/behaviours)?