According to Islamic teachings firstly: there is no doubt that Prophet Adam was the first person of the current generation of human beings and that this generation are all born from him. Secondly, before Prophet Adam there were humans that lived and were called Insans or “nasnases”, but we do not have much information about them.
Therefore there may have been some people living when Prophet Adam was created, and some say that they were the ones who married his sons.
We have not come across any religious scripture that says Prophet Adam was the eighth person that lived on earth, nevertheless there is a tradition that says the people that were born from Prophet Adam were the eighth generation of humans. Of course, there is a possibility that the number only signifies abundance, not that there were actually seven generations before this one.
Sheikh Saduq has narrated a hadith in his book “Khisal” from Imam Sadiq (pbuh) in which he says: “When Allah (swt) created the earth, he created seven worlds in it (and made each one extinct); none of those worlds [generations] were of Prophet Adam’s generation. Allah created all of them from the earth’s crust and brought them one after another, and created a world for each one after the other, until He created Prophet Adam and ramified his generation from him.”
Regarding whether or not there were prophets among them and whether or not they were intelligent/rational individuals, the Quran and the hadiths have not said anything, but because they too were called humans or nasnases, it is very likely that they were intelligent and there were prophets to guide them.