In order to understand brain research, it is important to consider three basic assumptions that neuroscientists make about brain evolution. For one, the brain is believed to carry traces of its evolutionary origins deep within its tissues. Just as we carry traces of our earliest childhood within our adult personalities, the brain carries the history of our whole species within its very anatomy. Secondly, the brain has evolved up and out, so that the lowest and deepest parts of the brain are the oldest. The outermost, uppermost, and the furthest forward brain regions are the youngest on the evolutionary scale. Thirdly, our brains have increased in complexity across evolution. The older structures tend to be simpler and more primitive, both in their anatomy and in the behavioral functions they control. Likewise, the evolutionarily newer structures tend to be more complex.