Gwyneth Paltrow and Liz Hurley may be beautiful, but if they don't smell good you won't want to reproduce with them.

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Hard-wired Human Survival Instincts

So are humans wired to survive? It sure seems like it. There are many examples of hard-wired human instincts that help keep us alive. Perhaps the most obvious case is the fight-or-flight response, coined by Harvard University physiologist Walter Cannon in 1915. When humans are faced with danger or stress, a biological trigger helps us decide whether to stay and fight or get the heck out of there -- flight.

When we're stressed or staring danger in the face, the brain's hypothalamus is activated. It initiates a series of chemical releases and nerve cell responses that gets us ready for the impending scenario. Adrenaline is released into the blood stream, our heart rate increases, blood is pumped more quickly into our muscles and limbs. Our awareness, sight and impulses all intensify and quicken. You can thank our caveman ancestors for this one. Early man faced a lot of dangers, and the fight-or-flight response evolved to help them evade or battle those dangers in order to survive. Today, it's what allows an ordinary Joe to rush into a burning building or a mother of three to lift a car off of one of her children -- a phenomenon known as hysterical strength. It also helps us out in non-life threatening situations like a boss screaming in your face or possibly fleeing -- or getting involved in -- a barroom brawl.

Another way we seem to be hard-wired to survive is in how we pick and choose our reproductive partners. The British Broadcasting Corporation (BBC) produced a show called "Human Instinct" that tested an interesting theory about how we pick our partners. You're probably thinking it's all about visual appeal. Would you believe it has more to do with your nose? Here's how it works: Humans all have different genes that help determine how our immune systems work. Some people are better at fending off certain sicknesses more than others. When we pick reproductive partners, they would ideally have a set of genes that supports an immune system different from our own. That way the offspring would get both sets of genes and be able to fight off a larger range of sickness and disease.

So that part is easy enough to understand. Here's where the nose comes into play. In its study, the BBC supposes that a human's smell has more to do with our instinctual attraction than sight. To test it, the BBC went to Newcastle University and recruited six women as test subjects. Their blood was tested and six genes were identified to indicate what kind of immune system they had. Then each woman wore a T-shirt to bed on consecutive nights. The shirts were placed in separate jars, and the show's host smelled each one to pick out which scent was most appealing to him.

The findings revealed that the two scents the host preferred shared none of his immune system genes. In this case, opposites attracted and the hypothetical baby they would produce would have the most wide-reaching immune system gene set. The host didn't know what any of these women looked like -- he only had his nose to do the work for him. The results indicate that humans have a hard-wired ability to choose a partner that would produce a robust, healthy baby and help to ensure the survival of the human race.