Despite Megatron's crash landing discovered by Archibald Witwicky in the year 1897, Cybertronians had encountered the humans before. There are two main things the species have in common, They are both capable of great compassion... And great violence. And with that, the first encounter between the two species did not end well.
Around 17,000 B.C. At the location today we would call Egypt, the two species meet for the first time. At first, everything was fine as the Primes were initially harmless to alien species. However, things changed with their brother, The Fallen, formerly known as Megatronus. The Primes had started building a machine called the Harvester. It was capable of destroying a planet's sun to turn into Energon, the lifeblood of Cybertronians. Around the completion of the Harvester, one Prime discovered the human race and reported their existence to the other Primes. Once informed that the planet contained life, the Primes had to destroy the Harvester or it would break their one rule; Never destroy a planet with life. Upon hearing the needed destruction of the Harvester, The Fallen became enraged. He was tired of losing Energon, letting his species die, just so another could live. In an act of rage, The Fallen attacked his brothers, he had sparked the war by becoming the very first Decepticon.
His brothers were caught of guard as they never would've expected such a betrayal from one of the 7 Primes, and because of this, the first was viciously stabbed through the back. As he fell to the ground, the other Primes prepped their weapons for The Fallen's next attack. The Seekers, Cybertronians who found Suns to blow up, heard the fight. They looked up at the Harvester and watched as one of the Primes tumbled down the side wounded. Only a couple of seconds into the fight and the Primes noticed something strange. All the Primes were meant to be equal in power, but The Fallen was somehow able to take on the other 6 Primes at once. The Prime that had been stabbed and fallen, opened his hand to reveal the Matrix of Leadership, the key to activating the Harvester. The Primes called for help from the Seekers, but many of them sided with The Fallen. Over matched and outnumbered, the 6 Primes teleported to an unknown location while the Seekers fought. In the ultimate sacrifice, the Primes gave their lives to seal the Matrix away in a tomb made of their very own bodies. After helping the Decepticon Seekers effectively forced the Autobot Seekers to retreat into hiding, The Fallen sensed a powerful presence on a nearby planet, thinking it was the Matrix. The Fallen and his new Seeker Armada took the ship they used to travel the galaxy together, now named the Nemesis, and set out after the Matrix.
The Decepticons arrived at the planet Saturn where The Fallen felt the presence coming from. However, once they arrived, there was nothing. The Primes had made a fake powerful presence to lead the Decepticons away from the real location of the Matrix, and while on the journey to Saturn, a member of the Autobot Seekers who sneaked on the Nemesis had sabotaged it, causing it to crash-land onto the surface of Saturn. The crash killed nearly everyone onboard and badly injured The Fallen.
2009
Even after years, no one has found the tomb of the Primes, and The Fallen is still out there waiting for his chance to strike revenge. Now, after the battle in Mission City, the humans have created a new strike team called N.E.S.T. that works across the globe with the Autobots to find and destroy any remaining Decepticons in hiding.