Everything that makes us human is located within 1.4 kilograms of yellowish tissue composing the human brain. It is here that our thoughts develop, here that we feel love or hate, and where the most creative and most evil ideas of humankind arise. This walnut-shaped structure is also the most complex organ nature has generated. The brain harbors about 86 billion neurons, or nerve cells, that have to be born at the right time, migrate to the right place, and wire up in the right way if we are to survive and thrive.