What is the origin of throwing dust over head (as a sign of mourning) in the Bible?

Job 34:14 *If it were his (God's) intention and he withdrew his spirit and breath, 15 all humanity would perish together and mankind would return to the dust.* Read the chapter in context. Elihu presents a magnificent superlative of the superiority of God... It is He who invests the 'dust' of creation with aptitude and passion and beauty and free will...life and dignity? We receive from Him Blessings... (very important idea in the book of Job: blessings vs cursing) What is it within our sovereign integrity that we can return to Him? Blessings...! (He is worthy...!)

You are not just a lump of carbon. You have the capacity to "know that I am LORD..." (probably the single greatest purpose of Job and the Bible)

The man of faith picks up a fist full of dust and understands that it is the stuff of his substance, and it is the grace of God that gives him life... The throwing of the dust is a symbolic acknowledgement that I am nothing apart from the grace of God... It is He who preserves Job's life in the midst of Satan's attacks. I am unworthy... It is the attitude of humility and reverence and repentance and fear of the LORD...

It is interesting to note that it is Job's friends who are demonstrating this attitude of humility; true faith in God. It isn't until chapter 4 that the 'spirit' of Eliphez's dream enters into the friends thinking about self righteousness... that negates that idea of God's Righteous grace... (the throwing of the dust) Job's righteousness should oblige God's blessings rather than vice versa. And God ultimately disapproves of the 3 friends (not Elihu) and provides a remedy thru Job's faithfulness: prayer and the sin offering. The sin offering that gives Job 'innocent, blameless' status before God, in spite of the fact that Job is a sinner, buttresses the book in chapters 1 and 42. God has provided the way for man to be 'perfect' before Him.

The attitude of humility and reverence... involved in the throwing of the dust is the 'book ends'. It opens the book with Job's friends and it ends the book with Job's repentance in dust and ashes.

The practice of throwing dust upon the head symbolizes humility and repentance. By casting the dust over one's head, they are equating themselves to the same level as the dust of the earth and are seeking God's pity and forgiveness.

The Hebrew word translated as 'dust' and 'ashes' is עָפָר "a'phar". In Esther, it uses the word אֵפֶר "e'pher" which is essentially the same thing as "a'phar"

(Psalm 103:13-14) "As a father shows mercy to his sons, Jehovah has shown mercy to those who fear him. 14 For he well knows how we are formed, Remembering that we are dust."

(Joshua 7:6) "At this Joshua ripped his garments and fell with his face to the ground before the Ark of Jehovah until the evening, he and the elders of Israel, and they kept throwing dust on their heads."

(Esther 4:1) "When Morʹde·cai learned of everything that had been done, he ripped his garments apart and put on sackcloth and ashes. Then he went out into the middle of the city, crying out loudly and bitterly."

It would seem that is the earliest known Biblical reference to ashes being in Genesis, when Abraham accepts that he is nothing but dust and ashes before the LORD, so it is a symbol of humility before God.

Then Abraham answered and said, “Indeed now, I who am but dust and ashes have taken it upon myself to speak to the Lord..."

Genesis 18:27

Nevertheless, I believe there is also another symbol of great significance. It is important to note that ashes were taken from the burnt offering in the earthly sanctuary:

And the priest shall put on his linen garment, and his linen trousers he shall put on his body, and take up the ashes of the burnt offering which the fire has consumed on the altar, and he shall put them beside the altar.

Leviticus 6:10

Ashes could symbolize something very important, purification:

Then a man who is clean shall gather up the ashes of the heifer, and store them outside the camp in a clean place; and they shall be kept for the congregation of the children of Israel for the water of purification; it is for purifying from sin.

Numbers 19:9

Many times sackcloth and ashes were put on for mourning a death, but other timer for mourning over sin and showing outwardly the repentance that should have been happening inwardly.

Here is the case of Ahab...

But there was no one like Ahab who sold himself to do wickedness in the sight of the Lord, because Jezebel his wife stirred him up. And he behaved very abominably in following idols, according to all that the Amorites had done, whom the Lord had cast out before the children of Israel.

So it was, when Ahab heard those words, that he tore his clothes and put sackcloth on his body, and fasted and lay in sackcloth, and went about mourning.

And the word of the Lord came to Elijah the Tishbite, saying, “See how Ahab has humbled himself before Me? Because he has humbled himself before Me, I will not bring the calamity in his days. In the days of his son I will bring the calamity on his house.”

1 Kings 21:25-29

And also the case of the city of Nineveh where even the animals were covered in sackcloth and ashes...

So the people of Nineveh believed God, proclaimed a fast, and put on sackcloth, from the greatest to the least of them. Then word came to the king of Nineveh; and he arose from his throne and laid aside his robe, covered himself with sackcloth and sat in ashes. And he caused it to be proclaimed and published throughout Nineveh by the decree of the king and his nobles, saying,

Let neither man nor beast, herd nor flock, taste anything; do not let them eat, or drink water. But let man and beast be covered with sackcloth, and cry mightily to God; yes, let every one turn from his evil way and from the violence that is in his hands. Who can tell if God will turn and relent, and turn away from His fierce anger, so that we may not perish?

Jonah 3:5-9

And Job's repentance...

Therefore I abhor myself, And repent in dust and ashes.”

Job 42:6

They symbolize the blood of Christ.

For if the blood of bulls and goats and the ashes of a heifer, sprinkling the unclean, sanctifies for the purifying of the flesh, how much more shall the blood of Christ, who through the eternal Spirit offered Himself without spot to God, cleanse your conscience from dead works to serve the living God? And for this reason He is the Mediator of the new covenant, by means of death, for the redemption of the transgressions under the first covenant, that those who are called may receive the promise of the eternal inheritance.

Hebrews 9:13-15

We should take the example of how people mourned and repented and put ashes on their head, as repent in the same way. Instead of covering ourselves with ashes, claiming to be covered by the blood that Jesus spilled in the cross of Calvary for us. For that is how we are cleansed.

But if we walk in the light as He is in the light, we have fellowship with one another, and the blood of Jesus Christ His Son cleanses us from all sin.

If we say that we have no sin, we deceive ourselves, and the truth is not in us. If we confess our sins, He is faithful and just to forgive us our sins and to cleanse us from all unrighteousness.

1 John 1:7-9

TL;DR - ashes are representative of Christ blood which cleanses us from our sins.