That was activated by gamma rays. Like X Men something has to happen to activate the mutant gene. The hidden strengths that all humans have seems like a mutant gene, probably why Banner didn't die of radiation poisoning.
In the comics it's implied you have to have a gene to surive gamma exposure and become a gamma mutant.http://marvel.wikia.com/wiki/Gamma_Mutates
The episode 'Kindred Spirits' dealt with the concept of a prehistoric Hulk, and direct gamma ray exposure from the sun causing the genetic change for some unknown reason, but this wasn't followed up on much later on, since the plant that may have cured it was now extinct.
shareIn the pilot they magnified people's DNA a million times and found the gene to make them strong, after looking at Banner's DNA they found he had the same gene as the others but never found the strength as they did so they figure there had to be something external operating, Banner figured out it was gamma rays from sunspots that did it.
shareIn the Comic Banner had the gene to keep the Gamma Rays from Killing Him .
His Split personality took advantage of that .
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In the 50's and 60's radiation was enough to give somebody superpowers or create giant ants but now it's now genetic mutation. Modern retellings still have the gamma exposure but the gamma is more of a create factor or external force for something else like a mutant gene or failed recreated Captain America serum.
Not just to keep the gamma rays from killing him but interacting with the gene to cause the change. Not just Banner but anyone exposed to gamma rays has to have the gene to survive it.
Could've done more humans effected by gamma rays like from sunspots not just superhuman strength but other powers.
shareCould've had more people get powers from gamma rays from the sun.
shareHulk Season One graphic novel explains it as a damaged chromosome or a set of genes from his father.
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