It is unclear where the Symbiotes actually originated, nor is it fully explained how the Symbiote (in its true form) manages to survive without a human host. However, it is known that the symbiotes have lived for millions of years - even longer than the dinosaurs. It is implied that while the symbiotes have lived for millions of years, they are able to remember and genetically encode information - to which the wearer of the symbiote is automatically gained access. They are actual thinking, plotting, scheming creatures.
While embroiled in a major crossover storyline fighting the Secret Wars on an alien planet, Spider-Man acquired an alien costume, the symbiote, in a lab. The application of a symbiote prevokes a cool feeling as the black plasmic ooze covers its host\'s body. It stimulates them, provides for them, and protects them by itself when the host is unconscious or unaware of a true symbiotic relationship. Symbiotes rely on human hosts because they are able to feed off adrenaline which is essential to their survival. They also feed off diseases fatal to humans, due to the type of energy or \"adrenaline\" it may produce; for example, the Venom symbiote was able to save Eddie Brock\'s life from cancer by feeding off it, since Eddie\'s cancer produced the exact type of adrenaline that the symbiote feeds on.
Each one is also considered to have a mind of its own, and so can actually think. They also are considered to be vicious and wild creatures, as they compel their human hosts to violence. They are much more effective if the human host has the same feelings and urges as the symbiote; for example, the Venom symbiote bonded successfully with Eddie Brock because they shared anger and desires for revenge towards Spider-Man. As this shows, Symbiotes have their own emotions and are able to react and reflect one\'s emotions on their personal desires. When bonding, they replenish the attributes of the human wearer, and over time, it replaces itself. By nature, Symbiotes try to permanently bond with their human hosts, ensuring that they live longer. They can, however, leave a human host (or at least try to) at their own will, although this is very rare. The only known case where a symbiote tried to leave its human host at its own free will was when Spider-Man taunted Venom\'s symbiote to return to him. The symbiote tried to leave Eddie Brock, yet the pain and trauma of doing so was too great, thus leading into Eddie\'s unconsciousness.
Symbiotes also are naturally weakened from intense sonics and infrared intense heat. They also have the ability to asexually spawn another symbiote and it is normal for a symbiote to hate its newborn spawn. Venom, for example, tries to kill and destroy its offspring, Carnage, when it bonds back with Eddie Brock, whereas Carnage tries to kill his son Toxin by leaving it for dead without a human host to find.
According to the mini-series Planet of the Symbiotes, symbiotes are an unfeeling, conquering race, taking over any species they come in contact with to feed off their emotions, mainly adrenaline rushes from death-defying feats. As a consequence, the hosts are short-lived. Venom was different because it wanted to bond with a host, to belong, instead of taking over, to dominate. As a result, it was studied in a lab, which was later transported to Battle World by the Beyonder for the Secret War.