The date of the transition to the cohort formation is disputed. Conventional wisdom says to credit Marius for the innovation because he built Rome’s first professional army. But Marius never claimed he developed the cohort and we know enough details of his life to have that information. In addition, there are historical documents describing the use of cohorts long before Marius. There are seventeen references to cohorts by Livy writing about the period 210-195 B.C. So the debate is out there without a resolution. Let’s leave this issue and move on to discuss the cohort formation itself.
To be honest, we don’t know much about it and what we do know is controversial.