For more than 2 million years, human neural networks kept growing and growing, but apart from some flint knives and pointed sticks, humans had precious little to show for it. What then drove forward the evolution of the massive human brain during those 2 million years? Frankly, we don’t know.

The brain is a complex system, but that doesn’t mean it’s incomprehensible. Our neural circuits were carved by natural selection to solve problems that our ancestors faced during our species’ evolutionary history. Your brain has been molded by evolutionary pressures just as your spleen and eyes have been. And so has your consciousness. Consciousness developed because it was advantageous, but advantageous only in limited amounts.

The neurobiologist Harry Jerison has made a long study of the trajectory of brain evolution since the advent of life on dry land. ...the origin of new faunal groups is usually accompanied by a jump in the relative size of the brain, known as encephalization. ...the first archaic mammals... were equipped with brains four to five times bigger than the average reptilian brain... primates are twice as encephalized as the average mammal. Within primates, the apes... are some twice the average size. And humans are three times as encephalized as the average ape.

Evolutionary psychologists suggest that, just as the eye is an evolved organ for seeing, and the wing an evolved organ for flying, so the brain is a collection of organs (or 'modules') for dealing with a set of specialist data-processing needs.

The increase in brain size and refinement of stone artifacts point to an unbroken advance in mental ability over the last two or three million years. ...No organ in the history of life has grown faster.

The human brain became large by natural selection (who knows why, but presumably for good cause). Yet surely most “things” now done by our brains, and essential both to our cultures and to our very survival, are epiphenomena of the computing power of this machine, not genetically grounded Darwinian entities created specifically by natural selection for their current function.

As David Eagleman describes it in his wonderful book Incognito: Your brain is built of cells called neurons and glia — hundreds of billions of them. Each one of them is as complex as a city. . . . The cells [neurons] are connected in a network of such staggering complexity that it bankrupts human language and necessitates new strains of mathematics. A typical neuron makes about ten thousand connections to neighboring neurons. Given billions of neurons, this means that there are as many connections in a single cubic centimeter of brain tissue as there are stars in the Milky Way galaxy.

We are thus faced with the following question: Why should an organ like our brain, with the sensorial system attached to it, of necessity consist of an enormous number of atoms, in order that its physically changing state should be in close and intimate correspondence with a highly developed thought?

The brain is the last and grandest biological frontier, the most complex thing we have yet discovered in our universe. It contains hundreds of billions of cells interlinked through trillions of connections. The brain boggles the mind.

The human brain is an exquisitely complex organ that evolved only once. The elephant's trunk, which can stack logs, uproot trees, pick up a dime, remove thorns, powder the elephant with dust, siphon water, serve as a snorkel, and scribble with a pencil, is another complex organ that evolved only once.

This is my experience, that the human brain is still organically evolving in the direction of the great mystics, in the same direction as the great geniuses. For this evolution a certain type of life is necessary.

Jerison argues that we should think of brains as creating a species' version of reality. ...As brains enlarged through evolutionary time, more channels of sensory information could be handled more completely, and their input integrated more thoroughly.

God has given to every one of us more than fourteen billion cells and connections in our brain. Now why would God give us such a complex organ system unless He expects us to use it?

Evolution is the long blur, a constant and living flow of branching relationships. One thing is always connected to another and another. Having no regard for our love of labels and organization, life rolls on as a continual stream of organic matter. The important thing about the origin of the human brain is not pinpointing some specific time, event, or fossil to declare a beginning in order to satisfy our desire for order. What matters is that we understand the process from which it emerged and how deeply rooted the modern human brain is to its past.