why are all human brains so similar in brain structure, specialization of their parts and functioning? As a result of my growing interest in neuroscience, I have begun to try and understand the functioning of the brain. However, I am always confronted by the  following questions: 


The brain is a highly specialized organ with innumerable nuclei and pathways, and yet the brain of one individual is surprisingly similar to that of another. Hence, it must be the case that the structure of the brain must, in part, be determined by our genes. If this assessment is correct, how much is our brain structure dependent on our genes?
Now consider the case of a newborn and suppose his/her brain structure has been established somehow. As he/she grows, the brain starts taking sensory input and learns to process it better through neuroplasticity. If so, then why does everyone's, say amygdala only process fear, anxiety, etc. What prevents it from maintaining autonomous functions? Are the functions of different parts also determined by our genetics? (Is neuroplasticity the only force dictating learning?)


In other words, why are all human brains so similar in brain structure, specialization of their parts and functioning?

Thank you
Up until the 70s and 80s, there were decades of hot debate about whether some phenotype such as brain structure or some behavior were caused by genes or by environment. Wikipedia has a decent summary of the history and content of that debate, and I've linked to the section on estimating the relative contribution of genes vs. environment: https://en.wikipedia.org/wiki/Nature_versus_nurture#Heritability_estimates

The current consensus is that the #1 question isn't very meaningful. Genes and environment are both 100% necessary conditions for a brain to develop. Yes, the similarity between brains (and between species) show that a lot of the structure is conserved, and much of that design depends on genes. But those genes wouldn't develop into a brain without very specific environmental conditions.

There are three quite distinct questions under #2. Cells and regions differentiate in the body; brain plasticity isn't 100%. Some structure is conserved. Consider that "modularity", one type of which is that different areas of the brain do distinct operations, is at least partially false. The brain is not at all like a computer with a motherboard with distinct components. It might help to consider how you would answer your question for why people's thumbs don't look like their other fingers. What prevents a thumb from developing into a finger? Are the functions of a thumb determined by genetics? This might help you to zero in on the specific processes of the brain that are the mechanisms that explain how genes and environments interact to produce phenotypes.