I found some information who stated that cohorts were 1st introduced during manipular legion and it was made up from 3 maniples (1 hastati, 1 priceps and 1 triarii and some velites), a smaller, but more mobile combined force usefull during the campaigns in Iberia.
The first mention of cohortes in the Roman army is from 210 BCE, and you are correct that it is from fighting in Iberia (Livy 25.39). Livy goes on to mention cohorts in Spain several more times in following paragraphs, referencing years in the range 210-195 BCE. He never mentions the maniple in Spain in this same period. Very strange. Of course, Livy was writing centuries later, when cohorts were the norm and maniples had been forgotten. The historian Bell ("Tactical Reform in the Roman Republican Army," *Historia 14, 1965) argues that this probably can't be carelessness on Livy, or his invention of narrative. Accept or reject that assertion as you will.
In these mentions by Livy there were apparently ten cohorts per legion, each apparently made up of a maniple of hastati, principes, and triarii. Each maniple had roughly two centuriae worth of soldiers, so the cohort had 2x3 = 6 centuriae (roughly), which is how we find it in later times.
It isn't exactly clear why it was introduced, but in the 1st century BCE it seems that there was more demand for a smaller subdivision than the legion. Caesar frequently operates in terms of cohorts, not legions, sending three or four cohorts here and there. This was, presumably, more difficult to do under a manipular system. Some think the new organization had more to do with supplying the legions, especially in distant and difficult conditions in Iberia: the cohorts were themselves little mini-legions, and could operate as such in the field. Three maniples of hastat, meanwhile, could not operate under the same tactics as the whole legion. It makes a sort of sense. Cohorts could also occupy separate camps, when required to for reasons of supplies or expediency. I've seen it visualized as a more top-down rather than horizontal slice of the legion, which makes sense, but I've often thought it has more to do with the homogenization of the legion over time, especially during the time of Marius. There was less socio-economic stratification, fewer and fewer velites, and (perhaps) less stratification by age. We also see the gradual disinterest in using different types of weapons for different classes of troops. By the time of the Social War, every legionnaire has standard equipment: a scutum, a gladius, a galea, and a couple of pila. I think it started to make less and less sense to divide soldiers into hastati, principes, and triarii (and velites) by that time.
I've got few more questions on the subject...