Origin of the Symbiotes

It is unclear where the symbiotes actually originated, but it is known that their species existed for millions of years prior to the arrival of the Venom symbiote on Earth. It is implied that Galactus, devourer of worlds, consumed a world which they had taken over and where they had thrived, based on Carnage's reaction to the Silver Surfer, former herald of Galactus. It was mistakenly believed that this was their homeworld. It is shown through the Carnage symbiote that this was a world whose dominant life forms had been overrun by the marauding symbiotes.

In Venom: Seed of Darkness -1, it is stated that when Krobaa was brought to Earth it bonded with the professor that had found him to acquire (through the symbiosis) knowledge of humanity to bring the information to a high galactic order that valued "Diversity of mind above all else", but Krobaa was infected with the madness of the human mind. Attacking all living things claiming to feed on their fear (much like Dreadface) and only after Eddie Brock's camera flash had weakened him, Krobaa regained control of himself and committed suicide to save other planets from the "plague of madness."

The mini-series Planet of the Symbiotes presents a different story (widely excepted as the canon story), symbiotes were originally an unfeeling, conquering race, taking over any species they came in contact with to feed off their emotions; this mainly involved adrenaline rushes from death-defying feats and, as a consequence, the hosts tend to be short-lived. This was also evidenced in the Fantastic Four series, when a black cube (imprisoned by Devos the Devastator as a trophy of the most dangerous species he has ever faced) escapes and is revealed to be a sentient alien symbiote (called Dreadface in the issue's title) that fuels (and then feeds off) the animosity between the Human Torch and the Thing before apparently being incinerated. The Venom symbiote was different because it desired a strong bond with a single host, and desired to belong rather than to dominate, leading to imprisonment by its own race, at which point it was brought to Battleworld during the first Secret Wars. The Venom symbiote describes itself as a mutation though it may just as likely be an atavism among its species. While the number of symbiotes in existence at present is unknown, Venom tells Carnage that he, Carnage, is the nine hundred ninety-ninth in his line, and making Carnage's offspring Toxin the thousandth. However, there are likely to be more than this because Venom was only talking about the amount in his personal family line, not the species as a whole. The symbiote that temporarily merged with Spider-Man was found in the Secret Wars.