Nature is the ultimate tinkerer when it comes to engineering solutions to problems. With a resume that goes back 3.5 billion years ago, this persistent employee has put in many hours designing life forms, of which the vast majority are gone forever. Old ideas that worked before are constantly used again. Like an engineering company that has many divisions for different disciplines, a functional result for one obstacle is eventually found and the overall system moves down the line to a finished product. When mammals were ordered up in bulk quantities to replace the oversized dinosaurs that had to be pulled from the shelves after 180 million years on this planet, they began as quadrupeds with tails. Over time they kept getting redesigned to try and gain an advantage over other mammals coming off the assembly lines. One of the organs that had existed before, and was enhanced in this creature, was the sense of smell. This was important for finding food and detecting potential predators before they, themselves, became a meal. Like all lifeforms, if food is taken in for energy, something has to exit, and in this case, it is semi solid waste known as poop. The 4-legged mammals were given tails to fan away the concentrated odors emanating from the anus to avoid detection. Later on, when more sophisticated species decided to stand up, so it could see farther and sample odors higher up, the tail got in the way. These bipeds crapped on their own tails when erect and carried the smell with them. Devoured mammals with fully intact, shit-coated tails lay upon the earth. Something had to be done. So modern primates, including man, shed their tails, but still needed a system to disperse the methane out like modern plumbing. Nature pulled out the drawing board and conjured up the beautiful butt crack. This gas channeling furrow is the perfect solution to vent the stench up to your T-shirt, where it is filtered and fanned out behind you for safety and stealth.