Once you realize that every single human being there is has inside their skulls the most complex thing that we know of in the entire universe, it gets a little weird. There are over 7 billion of these brains out there right now, all over the planet, each vastly more complex than the universe it exists in (which, after all, is mostly empty space.) Dig these numbers: a human brain has about 86 million neurons, and roughly ten times that many glial cells, or upwards of a billion. Each of these neurons fires five to fifty times a second and each of these neurons has up to ten thousand connections with other neurons. The estimates for the total numbers of synapses (i.e. the connections) between our neurons run from 100 trillion to 1,000 trillion (or one quadrillion). These synapses connect via dendrites (little filaments that grow from the surface of a neuron) and there are more dendrites than are used by neuron at any given time, so the potential number of connections could be one million billion (or one quintillion). That difference between that maximum total number of actual synapses (one quadrillion) and potential synapses (one quintillion), means the brain hasn’t come close to maximizing its capacity. And it means that the brain will continue to grow in complexity (and size). The human brain currently uses but a tiny fraction of its synaptic capacity. There simply isn’t enough to think about yet to fill it up.
83% of your brain is cerebral cortex, the thing that makes you you and people people. That cerebral cortex has grown at an astonishing speed evolutionary-wise. In just a couple million years it has expanded from chimp size to what it is now. Indeed it has grown so fast that it developed the folds you see in a brain in a jar, in order to maximize the number of neurons that could be crammed in the area available inside the skull. These folds increase surface area inside a limited space (or skull size), which increases the amount of neurons and synaptic connections between them. The size of our skull is limited by the dimensions of the human female’s birth canal. Indeed, the difficulty of human birth is due entirely to the size of the homo sapien sapien cerebral cortex. Were the woman’s pelvis able to widen further (it can’t, or at least natural selection isn’t capable of widening it at the same rate as a continuously expanding skull size)–or were it detachable like a snakes jaw (it isn’t), the human skull might be even larger, since apparently skull size is one of the things that can change quickly in our species through time (look at a collection of us and our predecessors to compare.)
Now about those billion glial cells. There’s ten times as many of them because they are much smaller than neurons. We used to think all these glia simply held neurons in place–it is vital that neurons remain in place to keep the synapses firing correctly, since synapses are not actually linked together but are just close enough for an electro-chemical charge to cross between them. Glial cells also help to provide the neurons with nutrition, like oxygen or the minerals such as potassium used in neurotransmission, which neurons exhaust quickly. And glia also helps with repairs and supplies the myelin which, like the rubber around a wire, shields the current running from one neuron to another via each synapse. But now it’s known that much of the brains incredible plasticity is due to glial cells, and they are used in communication (and even breathing) and who knows what else. Glial cells, like everything else about the brain, just keep revealing more complexity.
And the complexity of all this is so vast that we are incapable of actually visualizing it. We fall back on huge numbers like quadrillion and quintillion, or compare it to the relative paucity of complexity in the known universe. What we have in our own skulls, and is our very essence, we can barely understand. But every person you see has something in their heads that is more astonishing than the entire known universe. I can tell you that without truly comprehending it myself, because it is not comprehendable. We can understand it as a fact, an abstraction, but not actually appreciate just what it means. Like how we know what infinity is, but we can’t truly comprehend what it is. Our brains have myriad capabilities beyond our capacity to understand, because our brains are smarter than we are intelligent. Basically we are apes with extraordinary cognitive abilities, but still apes.