It seems, then, that our use of ashes at the beginning of Lent is an extension of the use of ashes with those entering the Order of Penitents. This discipline was the way the Sacrament of Penance was celebrated through most of the first millennium of Church history. Those who had committed serious sins confessed their sins to the bishop or his representative and were assigned a penance that was to be carried out over a period of time. After completing their penance, they were reconciled by the bishop with a prayer of absolution offered in the midst of the community.
During the time they worked out their penances, the penitents often had special places in church and wore special garments to indicate their status. Like the catechumens who were preparing for Baptism, they were often dismissed from the Sunday assembly after the Liturgy of the Word.
This whole process was modeled on the conversion journey of the catechumens, because the Church saw falling into serious sin after Baptism as an indication that a person had not really been converted. Penance was a second attempt to foster that conversion. Early Church fathers even called Penance a "second Baptism."
Lent developed in the Church as the whole community prayed and fasted for the catechumens who were preparing for Baptism. At the same time, those members of the community who were already baptized prepared to renew their baptismal promises at Easter, thus joining the catechumens in seeking to deepen their own conversion. It was natural, then, that the Order of Penitents also focused on Lent, with reconciliation often being celebrated on Holy Thursday so that the newly reconciled could share in the liturgies of the Triduum. With Lent clearly a season focused on Baptism, Penance found a home there as well.