Doodh-ganga or the river of celestial ocean inundated the western part of the mountain, whoever drank from that river grew very strong physically and became almost immortal. This rendered immense power to the Hustlebucks over the Brazenheads, they grew aggressive and drove away the rest of the communities of early monkeys to far corners of the valley.
They plundered the forests, terrorized other small animals and became a menace. Their physical strength compensated for their lack of intelligence, yet at the same time power in the hands of witless is more dangerous than in the hands of the cunning.
The Brazenheads though small were wiser. They understood through stray incidences that drinking the water of the milky river one may help them to attain supreme strength that lasts for a long time. They devised a plan, instead of encountering the Hustlebucks head-on they decided to sneak upto the river one monkey at a time, drink the water of the celestial river and disappear into darkness quietly.
Before the Hustlebucks understood anything; almost all the Brazenhead developed immense strength and physical endurance. With their physical power and intelligence they soon conquered over Hustlebucks.
Being low in intelligence the Hustlebucks failed to comprehend that their opponents are matching them in physical strength, they fought continuously till their population was almost wiped off.
Some injured ones retreated to deeper ends of the forest, the Brazenhead's felt sympathy for them but knew that their lack of intelligence would incite them to fight till they die. So they left them alone.
The Brazenheads eventually became the ancestors of homosapiens and the Hustlebucks formed the missing link and went back to the star ship they came from. The inherent intelligence and associated compassion and emotions enabled the Brazenhead seeds to steer the course of life on earth. Based on this adage the theory of survival of the fittest came about several eons later.