I was asked this question on Quora:
From an evolutionary standpoint, why would the early years of brain development be paramount in determining life-long neurological patterns, when those patterns can often be detrimental to long-term success in life?
Good question. We can restate it as follows:
Why would natural selection allow animals to be so sensitive to negative early experience?
I suppose the most important thing to say is that evolution may have nothing to do with this. Natural selection is not the explanation for all phenomena in biology. Some phenomena are the result of purely biophysical processes. In other words, natural selection is not magic — it cannot dissolve biophysical constraints. Natural selection is not an optimizing process — it produces solutions that are “good enough”.
Clearly an organism must have some developmental trajectory in order to become an adult that is capable of passing on its genes. The genetically encoded elements of this trajectory are traits that have undergone the pressures of natural selection. But an organism’s traits are not independent of each other. In other words, there are always going to be trade-offs among traits.
It may be that the long-term consequences of sensitivity in early life are the price organisms — particularly humans — pay for plasticity. One could argue that flexible intelligence is the signature trait of humans, and this requires a great deal of plasticity. One cannot learn about the world if all one’s brain connections are hardwired in the womb. Learning involves being modified by experience — and this requires being sensitive to experience.
So even if sensitivity were a trait that could be selected out, perhaps it doesn’t get selected out because the changes required to achieve steely robustness in the face of adversity might also make us… stupid. 🙂 Or if not stupid, then inflexible and unable to adapt to new circumstances.
And this is not just speculation. My lab has collected some indirect evidence that the most plastic (changeable) parts of the brain are also the most vulnerable to disruption. It is no coincidence that these are the parts of the brain associated with psychiatric disorders — the limbic cortical regions.
 Mirror trends of plasticity and stability indicators in primate prefrontal cortex.