Why we have a brain

The brain is the ultimate machine, more complex than just about anything we have ever studied. There is much we don’t know about the brain, including exactly how it processes all of the information we receive. 

The brain—composed of ~100 billion cells known as neurons—is the central command of our nervous system, and is what we use to interact with the world. Sensory neurons relay information about the world around us. Motor neurons allow us to move around our environment. The brain integrates sensory information,
tests it against our stored memories and model of the world, and decides when and how to act; it’s also responsible for predicting possible future scenarios and giving meaning and emotional context to our daily experiences. 

Our brains weren’t always so complex. Over millions of years, brains have evolved from simple networks to become the intelligent, complex and efficient machines that
we have today.

The evolving brain

When multicellular organisms evolved from single-celled ones over 1.9 billion years ago, they found a way to communicate by using chemical and electrical messages. As nervous systems evolved, communication between neurons became faster, more precise, and increasingly complex.

The first nervous systems were likely ‘nerve nets’, which have no central command or brain. They are still seen today in simple organisms such as hydras and jellyfish. As evolution continued, neurons began grouping together around the mouth and sensory organs of free-swimming animals. This allowed signals to be processed and transformed, rather than just transmitted from one part of an organism to another—which enabled more complex behaviours such as hunting for prey, escaping from predators or finding a mate. 

Around 500 million years ago, the first vertebrates developed basic versions of a pattern of connections broadly shared by all species today. As four-legged vertebrates colonised land over 350 million years ago, the complexity of their brains, in particular the forebrain (see above), increased to be able to process information about their surroundings and remember and learn from experience. 

With the appearance of mammals about 200 million years ago, the  forebrain further developed in size and complexity. Connections between the left and right sides of the brain were further expanded in placental mammals by evolution of the corpus callosum—the main bundle of nerve cells linking both hemispheres.

A six-layered structure evolved, known as the neocortex—the wrinkly outer region of our brains, important for complex tasks like abstract thinking and planning. The neocortex is considered to be the key to our intelligence, including our ability to reason, plan ahead, communicate, store memories and solve complex problems. This intelligence, and the ability of our brains to handle more than just basic survival, has allowed humans to dominate the Earth.