Everyone knows about evolution, but how exactly did we even get to where we are today? It can be tricky to conceptualize the complexity of the evolution of our brain, but we aren't here today simply because we are here. It took billions of years to reach multicellular organism life and only until around ~500 million years ago did the first evidence of the origins of the brain come around. From then until now, numerous feats have been undertaken by nature to result in humanity's current brain composition. Until now, evolutionary progress has appeared linear. However, we have approached a crossroads upon which the ability for a coincidence of nature to inheret the ability of how we as humans will evolve.

While this chart is quite simplified, it captures the general evolution of the nervous system and the key features that eventually led to vertebrate nervous systems.  A notable point to mention is that "brains" before chordates were essentially mini brains called ganglia or ganglion. Another feature to notice is how bilaterians served as impetus for the development of more complex and advanced neural systems due to the mirrored halves of the body. 

Over time, the brain's ancestors evolved to serve a multitude of functions. Contrary to common representation and thought, the brain does not consist of a reptilian, mammalian, or neomammalian. In fact, many fundamental brain development genes have remained highly conserved from the first blueprints of brains hundreds of millions of years ago. Within early mammals and their precursors, genes for accelerated brain growth became more prominent, thus creating larger brains with more specialized divisions. Funnily enough, a crucial gene for rapid protein development is SHH, nicknamed Sonic Hedgehog. It creates a protein that allows for more complex tissue development and rapid tissue development. This is one of many gene evolutions that enabled denser, faster growing, and more complex brains that we all have now. 

The first Archaic primates and their brains have evidence of first appearing around 50 to 60 million years ago. Evidence suggests however that the first primate related species began roughly 80 million years ago, likely a small ~60 gram creature with a simple brain. These early primates were small with fast genetic mutation rates that began to slow once larger bodies and brains were developed. An interesting bit of information is how major groups of primates individually developed larger and more complex brains over time. In the image to the left, It should be taken note though, that it wasn't until much later in time (35-15 mya) that larger brains and bodies became more common. See split between Cebidae and old world monkeys, etc. Since mutation rates are strongly linked to lifespan, the concept of natural selection came into play, meaning more primates were living at a single point in time. It just so happens that the primates living longer had a larger Endocranial Volume and relative volume (space the brain actually takes up) as well as structuring more and more complex brains to master the world. 

Everybody knows that the large prefrontal cortex is what sets us apart from other great apes, but there are several other factors that played a large role in the structure of our brains. Several genes such as ARHGAP11B and NOTCH2NL are human specific genes that increase neuron responsible stem cell counts and delay stem cell development, respectively. These two combined are responsible for things such as gyrification (the folds in the brain), and a larger pooling of neurons. Utilities such as the internal structure and connectivity of neurons have permitted higher density and interconnectivity and interaction between neurons. Now, you may be wondering how other animals such as elephants or whales or dolphins have such massive brains, yet do not come close to humans on an intellectual scale. It comes down to many aspects, but the most paramount of those is the brain to body size and the structure. It takes many more neural resources for a massive body to function and even then, other factors are at work in the background. For example, a dolphin has a much larger neo cortex compared to humans, at roughly 40%, though the structure, density, and organization provide several pit falls that allow human brains to prevail against dolphin brains. A rough way to measure to measure intelligence is through Encephalization Quotient. It measures the estimated mass of the brain compared to the mass of the whole body. The final number is interpreted as how large the brain actually is compared to the predicted size. Though it does not accurately represent intellectual capabilities, it provides an easily perceived form of comparing different brains. It is worth noting that this system pretty much only works with mammals.

Humans: 7.4-7.8                                     Bottlenose Dolphin: 5.26

Chimpanzee: 2.2-2.5                              Raven: 2.49

Newborn Pig: 2.42                                  Elephant: 1.75-2.36

Gorilla: 1.39                                              Dog: 1.2

Brown Bear: 0.82                                     Mouse: 0.5