Magnetic Field
So why is the Magnetic Field important to intelligent life? Let me make a rough comparison.
Take some macaroni noodles, or Lincoln Logs, or something else slender. Join the ends with chewed gun, or silly putty, and make a beautiful, spiraling double helix shape. Admire it's form and complexity, it's graceful curves, it's symmetry.
Then, shoot it point blank with a shotgun and see what happens to your pretty little sculpture.
OK. The macaroni picture represents our DNA - the collection of genetic information that resides in our cells that are, in essence, the blueprint for the human body. The shotgun blast represents high powered solar particles from the solar wind, as well as other particles and rays from extra-solar origin, such as cosmic rays.
So how does the magnetic field help out? Well, the particles from the sun have an electrical charge. So does the magnetic field. When the particles enter the field, they are pulled away from the surface and pushed around the planet (like water being shoved aside by the bow of a ship), where they can't penetrate tissue and damage our DNA, which would cause all kinds of nasty genetic mutations (not the Marvel Comics type), eventual sterility by destroying the genetic material in sperm and egg cells, cancers, and the nastiest sunburns this side of that one time I was in Arizona as a little kid.
So what causes the magnetic field? We're not entirely sure, but the leading theory is the dynamo theory[1]. Without speaking to fancy, it's caused by the molten outer iron core rotating against existing magnetic field lines organized by the coriolis effect[2] along the north-south axis.
It's believed that the reason Venus - our near twin when it comes to mass, density, radius, and composition - doesn't have a magnetic field is it's slow rotation rate - 243 Earth days. Without a magnetic field, high energy solar particles broke the atmospheric (and perhaps, at one time, surface) water into it's base components of oxygen and hydrogen/ The hydrogen was stripped from the atmospheric and blown into space. Without the hydrogen, water couldn't form, and the free oxygen atoms bonded with carbon, producing carbon dioxide, which in turn helped make Venus into the toxic hellhole of an oven it is today. Think about THAT the next time you wish there were more hours in a day.