The bulk of the sophisticated Roman military was made up of heavy infantry units called legions. The structure of these legions experienced a number of changes throughout Roman history, with the biggest change coming about in the early first century BCE by Gaius Marius. Before Marius, the legions were known as "Manipular" legions. These legions were comprised of multiple smaller units called maniples, with each one usually containing 120 men. There are two main accounts on the organization of these manipular legions: one by the Roman historian Titus Livius, or Livy, (c. 59 BCE - 17 CE) and one by the Greek historian Polybius (c. 200 - 118 BCE). Marius's reforms in 107 BCE replaced the maniple with cohorts and increased the size of this basic unit to 480 men. Cavalry in the Roman military was always considered secondary, and mainly served in scouting or skirmishing missions.