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.
When referring to the Quran and hadith, one can definitely say that before Prophet Adam there was a generation or even generations of beings that were called "insans" or "nasnases", but there is no detailed information about them.
Allame Tabatabai says: "It is said in the history of the Jews that the human being has not lived any more than about 7 thousand years on earth, but geologists argue that humans have lived for more than millions of years. The fossils of things made by humans and skeletons of humans that are said to belong to 500 thousand years ago are presented as evidence for such a claim. "
The proof that backs this opinion is not firm and convincing; it cannot be proven that these fossils are the fossils of the bodies of people belonging to the current generation of humans, it is possible that these fossils belong to a time in which other homosapians lived on earth and maybe that period was not connected to the period of the life of this generation of humans, meaning that they lived before Prophet Adam and became extinct and this happened several times until this generation of humans were created. 
Therefore there were humans before Prophet Adam, and it was after the creation of other humans that the angels were commanded to prostrate before Adam. 
The Quran has not mentioned how we started our lives on earth and whether or not there were humans that lived on the planet before us.
Nevertheless some verses give us clues and suggest the existence of humans before us and that Prophet Adam was not the first human being, like this one: "" وَ إِذْ قالَ رَبُّکَ لِلْمَلائِکَةِ إِنِّی جاعِلٌ فِی الْأَرْضِ خَلِیفَةً قالُوا أَ تَجْعَلُ فِیها مَنْ یُفْسِدُ فِیها وَ یَسْفِکُ الدِّماء " َ "  . This verse implies that before the creation of Adam, there were others living on the earth. 
At the same time, there are hadiths that clearly say that there have been generations of humans before this generation and that the majority of mankind has lived before the creation of Prophet Adam that all went extinct.
For example there is a hadith narrated in "Tafsir Ayyashi" that reads: “Hesham ibn Salem says that Imam Sadiq (as) said: "If the angels hadn't seen the humans that shedded blood then why did they say " أَ تَجْعَلُ فِیها مَنْ یُفْسِدُ فِیها وَ یَسْفِکُ الدِّماءَ "  ".  (They have had to seen humans before to claim that humans shed blood, if not why would they ask Allah why he was creating such creatures?).
We couldn’t find any account that says Prophet Adam was the eighth person that lived on Earth, nevertheless there is a hadith that says the people that were born from Prophet Adam formed the eighth generation of humans. For example Sheikh Saduq narrates a hadith by Imam Sadiq (as) in his book Khisal in which the Imam 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.  ".  This hadith can be better understood when combined with other hadiths that say that there were many generations of humans before this generation and not only seven. For example Sheikh Saduq narrates a hadith from Imam Sadiq (as) in his book Tawhid in which the Imam explains: "You may assume that God has not created any humans other than you, but this is not true because he has created thousands of thousands of humans before you and you are the last generation of humans". 
There is also another hadith in Khisal by Imam Sadiq (as) that says: "Allah has created twelve thousand worlds and every one of them is bigger than the seven skies and seven earths together, none of the people of these worlds ever think that Allah has created any other worlds". 
Of course, there is a possibility that this final hadith is not about Planet Earth and therefore it will not contradict the hadiths that point out the existence of seven generations of humans on Earth before Adam.
But even if we believe that before Prophet Adam there were other humans, can we argue that they lived until the time of Prophet Adam?
There is a possibility that there were people from the previous generations that had become extinct at the time of Prophet Adam as some scholars have theorized. 
Regarding the marriage of the sons of Prophet Adam a contemporary scholar argues: "There is also another theory that says that the sons of Prophet Adam married people that had survived from the previous generations, because according to a hadith Prophet Adam was not the first person to live on earth. On the other hand, research shows that it is likely that humans have been living on earth since a few million years back, and we all know that it has not been a long time since Prophet Adam appeared on earth, therefore we have to accept that before Prophet Adam there were humans living on earth that were becoming extinct at the his time; it seems completely reasonable to believe that the children of Prophet Adam married people who survived from that extinct generation". 
Of course it is totally clear that Prophet Adam was the first person of this generation of humans.
The apparent if not direct and clear meaning of verses of the Quran assert that the current generation of humans were born from a mother and father named Eve and Adam, some of the verses are as follows:
 " وَ بَدَأَ خَلْقَ الْإِنْسانِ مِنْ طِینٍ ثُمَّ جَعَلَ نَسْلَهُ مِنْ سُلالَةٍ مِنْ ماءٍ مَهِینٍ "
 " إِنَّ مَثَلَ عِیسى عِنْدَ اللَّهِ کَمَثَلِ آدَمَ خَلَقَهُ مِنْ تُرابٍ ثُمَّ قالَ لَهُ کُنْ فَیَکُونُ "
 " إِذْ قالَ رَبُّکَ لِلْمَلائِکَةِ إِنِّی خالِقٌ بَشَراً مِنْ طِینٍ فَإِذا سَوَّیْتُهُ وَ نَفَخْتُ فِیهِ مِنْ رُوحِی فَقَعُوا لَهُ ساجِدِین "
As you can see these verses tell us that Allah (swt) has guaranteed the continuation of Prophet Adam’s generation through reproduction, although this applies to his children and on and not to himself and his spouse; they were created from mud/clay and were given reproductive organs in order to keep the generation going (therefore his children were created via conception and human reproduction). Thus, according to these verses, there is no doubt that the lineage of all humans traces back to Prophet Adam and his spouse, Eve (Hawwa). 
Regarding the question on whether or not there were any prophets among them or whether they were intelligent or not, nothing has been said in the Quran and hadiths, but because they too were called humans or "nasnases" it is likely that they were intelligent/rational and that there were prophets among them.
Related Question: The prophets of jinns before the creation of mankind, Question 371 (website:380).
 Muhammad Husein Tabatabai, translation of Al-Mizan, vol. 4, pg. 222.
2] Muhammad Husein Tabatabai, translation of Al-Mizan, vol.16, pg. 389.
 When your Lord said to the angels," I am appointing someone as myeputy on earth," they said (almost protesting)," Are you going to appoint one who will commit corruption and bloodshed therein?”
 Muhammad Husein Tabatabai, translation of Al-Mizan, vol. 4, pp. 222-223
 “Are you going to appoint one who will commit corruption and bloodshed therein?”
 Majlisi, Biharul-Anwar, vol. 11, pg. 117.
 Sheikh Saduq Khisal, vol. 2, pg. 652, hadith 54.
 With help from Question 2999 (website: 3297).
 Sheikh Saduq, Tawhid, vol. 2, pg. 277.
 Khisal, vol. 2, pg. 639, hadith 14, With help from Question 516 (website: 563).
 The imams said these things at a time where no one knew of any fossils signifying the existence of people tracing to millions of years back. Today after the development of technology we realize the true value of these sayings and that they had a connection with the immaterial world above.
 Naser Makarem Shirazi, Tafsir Nemouneh, vol. 3, pg. 247; Tafsir Kawthar, vol. 2, pg. 349.
 “He commenced man's creation from clay Then He made his progeny from an extract of a base fluid.”
 “Jesus' case with God was the same as Adam's: He created him from dust; then told him:" Be!" and he was”
 “Indeed I am about to create a human being out of clay. So when I have proportioned him and breathed into him of My spirit, then fall down in prostration before him”
 Muhammad Husein Tabatabai, translation of Al-Mizan, vol. 4, pp. 224-225.