Everything I have come across through the study of liturgical evolution, the cassock evolved from Byzantine court wear, which evolved from the Roman day-to-day tunic worn by non-citizens and citizens outside of official or formal duties. This tunic was called the “dalmatica” and originated from the Greek immigrants who spread throughout the Roman State beginning after the Battle of Corinth in 146 BC when Greece became a Roman Province. As such, the dalmatica was already ingrained in Roman society by the time of Christ. It was so wide spread that the tunic Christ was stripped of at the Crucifixion could have possibly been a form of dalmatica.
After the Edict of Thessolonica in 380, the Church hierarchy became part of the imperial court. As such, for official functions they were expected to wear garb which was appropriate for their positions. Basic priests usually could only afford cloth dyed with darker, cheaper colors, which differentiated themselves from the Bishops, who wore finer fabrics in brighter colors and wore the appropriate headgear (hat) which designated their station within the court. This headgear would eventually evolve into the modern miter in the Roman Church.
At this time, there was no standard for the style of this garb and styles of this day-to-day garment changed freely from region to region. It was only in France in the 5th-6th century that the basic design of the cassock was standardized by bishops and it spread through the Holy Roman Empire gradually, eventually becoming the basic standard for secular clergy. The religious habit in the west also developed during this time from this generic Byzantine garment, although it developed through the Benedictine Order separately in Italy, this is why there are many stylistic differences between the base to the Benedictine habit and the cassock.
As to the laxity of Britain with regards to length mentioned in the Catholicstraightanswers link, this was actually a point of great contention between the richer members of the clergy and the poorer country priests. The shorter cassock/tunic only fell to their knees because they regularly had to hike through deep mud through the countrysides to reach the rural chapels. I have been told that the situation was so bad in the countryside during the winters that priests who wore long cassocks would get them wet walking for hours to mass, catch pneumonia and die.
Hope this gives you a little more info.
God Bless,
Br. Ben, CRM
