Life started from single cellular organisms (very primitive), around 3.5 billion years ago. Though no nervous systems, in the beginning, these unicellular organisms had natural tendency to detect, absorb energy necessary for life, and reproduction (cell division). This natural tendency is important. Very important. In fact, if you compare those unicellular organisms with us today, that tendency to look for energy and reproduce is the commonality. Of course, you can say that unicellulars didn’t have any tendencies (as no nervous systems). Their cells were adapted to absorb food and undergo cellular divisions in appropriate natural conditions. With time, organisms started developing more complex systems to efficiently process energy, reproduce, and most importantly survive. Those groups who were better at survival under those conditions at that time got naturally selected. It is a good time to point out that in some other conditions (say present), some other group of organisms might have been better at survival and the natural selection probably could have led to a whole different line of organisms (no humans possibly).
So, the key thing is finding energy, reproduce and hence, survive. Now, imagine this. The whole earth is undergoing ecological and environmental changes. Life has moved from oceans to lands. Wide chains of organisms are evolving. Now what? There is food everywhere (algae, primitive plants). Now there is competition. As the population of different groups increases, there is pressure for survival. Resources are limited. At some point in time, some sort of neurons developed in some primitive organisms (maybe in oceans). My speculation is a lot of complex cells, and systems were developing (are still developing). The difference is, environmental changes were more rapid in the early phases and natural selection was more brutal. So, the systems that offered advantages were preserved. Neurons were the next big step. Why?
Earlier, cells and cellular systems were just absorbing energies by cellular mechanisms (e.g. responding to glucose concentration difference, photosynthesis). Neurons rendered them an ability to sense stimulus. This was big. Now there was more input. With time, cellular systems adapted themselves (learned) to pair their response with different stimulus for energy and survival gains (e.g. responding to a smell for locating fruits). Again, a good time to point out, a smell is nothing but a neuronal response to the chemicals coming from the object. So, when you smell a fruit, it’s just certain sets of chemicals. We have learned to associate those chemicals in form of smell with energy gains. This happened with every sensory system over time (billions of years). Organisms developed ears, eyes, sense of touch, smell. Obviously, these organs offered them survival advantages. However, during the same period, parallelly a new organ was developing. My speculation is, with the development of neurons, there was a bulk of sensory input. Now, whichever the organ is (ears, eyes, nose), the nature of inputs and outputs from (to) neuronal cells are quite similar (electrical potential). So, it was quite natural that a new organ was developing to process these signals so that organisms can make better use of it for survival. And that’s when evolutionary natural selection hit its next big jackpot. Jackpot though technically isn’t the right word but describes this situation aptly. Why is it not right? Simply because evolutionary mechanism doesn’t care. It doesn’t care if apes are evolving to humans or dinosaurs to donkeys. It doesn’t really care. It just makes a momentous choice (again no one makes any choices, but to understand it better, I’m referring to the mechanism as “it” and the flow of events as choices) and over the long run, these choices lead to some different versions of organisms (apes, chimpanzees, Homo erectus, Homo sapiens).
Going back, the reason I used the word jackpot is because this new organ was none other than “the brain”. And it was the single most organ that not only was going to change the life on earth but immensely affect the ecology, environment, and now the global climate of the earth itself.
Brain offered numerous survival advantages. Better sensing of food, shelter, danger, reproductive partner, intelligence, eliminating competition, etc. With the brain, now not only organisms choose better food for survival but they were choosing better mates for reproduction. Mostly these choices were made by females. The result was again a heavy natural selection of the “brainiacs”. This made brain one of the fastest evolving organs in the history. It has evolved enormously fast. This can be understood by the fact that 500,000 years back, it was on an average around 1000ml and now it’s around 1200 ml (since Homo sapiens).
Homo sapiens: they are the ultimate product of the evolution (till now). There are more powerful, more beautiful species. But none of them intelligent enough compared to us. Capabilities of our brains had grown leaps and bounds. From just basic hunting and mating instincts to music, dance, art, science, religion, philosophy, weapons, technology, etc. We are at a phase where humans are so powerful that they can end the life (and the evolutionary process) on this planet. We are a result of complex, brutal natural selection over billions of years. Now we are eliminating fellow survivors of this natural selection (over 60 percent of species gone extinct in last 200 years due to humans influence on the planet). Temporarily, this creates more space for us (eliminating forests, animals). But in the long run, the climate change can be fatal. Not only for us but for the whole planet. It can truly be apocalyptic.
This is the thing with natural selection. Based on the environment, fit genes are conserved and selected. But what these genes can do in return to the environment is none of its business (well the changed environment can affect further natural selection, but then those supposedly fit genes will be in the majority).
Natural selection for billions of years resulted in humans. But humans can destroy the whole process in just a fraction of that timeline.