Why didn\'t Romans fought in single line?

[Frank]

Ave,

I have been told that perhaps the best representation of the deployment of a Roman legion can be found in the battle scene in the film Sparticus starring Kirk Douglas from the early 1960s.

https://www.youtube.com/watch?v=zgywD3XJaWU

Just a movie, but it shows, how the gaps can be easily closed for melee combat. The same gaps can be opened again on command. Of course not, if the line is under pressure.

Imagine, what happens, if they would not close the gaps. These gladiators would immediately occupy the gaps and attack every unit from 3 sides. They could even attack the 2nd line and encircle the units of the 1st line in worst case. Thats what I meant with: gaps are suicidal against barbarian hordes.