Salam Nihil est
I've heard how previous generations of believers had the Tawrat or the Injil. I assume their Islam entailed different things, different practices. For example the Sunna of the Prophet didn't exist yet, since of course he wasn't alive. Does that mean Islam has changed through the years?
The basic principles of Islam were always there. Oneness of God, concept of prophethood and divine books, belief in existance of angels, day of judgement etc. Before prophet Muhammad, Muslims even prayed and fasted.
002.183 يَا أَيُّهَا الَّذِينَ آمَنُوا كُتِبَ عَلَيْكُمُ الصِّيَامُ كَمَا كُتِبَ عَلَى الَّذِينَ مِنْ قَبْلِكُمْ لَعَلَّكُمْ تَتَّقُونَ
002.183 O ye who believe! Fasting is prescribed to you
as it was prescribed to those before you, that ye may (learn) self-restraint,-
Al-Qur'an, 002.183 (Al-Baqara [The Cow])
Text Copied from DivineIslam's Qur'an Viewer software v2.910
The Sunnah of prophet Muhammad wasn't there to guide these Muslims but they were guided by the Sunnah of their own prophets. So the fundamentals of Islam remained the same. However, the Islam today is not identical to the Islam at that time. We don't know how many times a day they prayed, when did they fast etc. So a few details have been changed. We can only speculate that the a few details have been different because the circumstances of the era are different. A few examples of these differences are given in the following hadith
Bukhari Vol. 1 Book 7 hadith 331
The Prophet, peace be upon him, said, "I have been given five things which were not given to any amongst the Prophets before me. These are:
1. Allah made me victorious by awe [by His frightening of my enemies] for a distance of one month's journey.
2. The earth has been made for me [and for my followers] a "masjid" [Arabic: a place for prostration] and a means of purification. Therefore, my followers can pray wherever the time of a prayer is due.
3. The booty has been made halal [lawful] for me [and was not made so for anyone else].
4. Every Prophet used to be sent to his nation exclusively but I have been sent to all mankind.
5. I have been given the right of intercession [on the Day of Resurrection].
Why is Muhammad the last prophet?
Prophet Muhammad is the last prophet simply because we don't need more prophets. You see, Allah appoints prophets to guide mankind to the right path. Now prophet Muhammad has been given complete guidance as the Quran says
This day have I perfected your religion for you, completed My favour upon you, and have chosen for you Islam as your religion. But if any is forced by hunger, with no inclination to transgression, Allah is indeed Oft-forgiving, Most Merciful.
Al-Qur'an, 005.003 (Al-Maeda [The Table, The Table Spread])
Text Copied from DivineIslam's Qur'an Viewer software v2.910
Hence we don't need more guidance and we don't require more prophets. Thus prophet Muhammad is the last prophet.
Is his message 'better' than the other messengers?
On the whole, his message is same as that of other messengers. However, as the hadith I quoted above says, prophet Muhammad's message is for the whole of humanity until the day of judgement but the message of other prophets was only for their people and for that time. Also, the previous books brought by other messengers were corrupted by their people. They added/subtracted a few things. But the message of prophet Muhammad i.e Quran has been guarded by Allah against corruption
015.009 إِنَّا نَحْنُ نَزَّلْنَا الذِّكْرَ وَإِنَّا لَهُ لَحَافِظُونَ
015.009 We have, without doubt, sent down the Message; and
We will assuredly guard it (from corruption).
Al-Qur'an, 015.009 (Al-Hijr [Al-Hijr, Stoneland, Rocky City])
Text Copied from DivineIslam's Qur'an Viewer software v2.910
The biggest proof of Quran being uncorrupted is that there is only one Quran Muslims have. Same cannot be said about Torah or the Gospel.
Peace