The brain is 1 major developmental difference between us and these humanoids.
These humanoids start off with larger brains to begin with and so brain development in the womb happens at about the same pace as ours, just with more cells.
This larger brain means that babies soon go from not being able to move at all to being able to climb. Climbing is their first physical milestone because the species has an underground civilization.
As early as 3 months they can say single words like "Hungry" and can stand with support.
At 6 months they can walk with support and say phrases and short sentences like "Head hurts" and "I love you".
At 9 months they can walk without support and ask questions like "Why does my tummy hurt?".
At 1 year old they can run and jump and from 2-5 years old they start to understand why they are hurt and learn how to prevent themselves from getting hurt.
This is all during the gender neutral phase where there are no external genitalia but there are internal genitalia and gender has already been genetically determined.
Now I made these humanoids have increased intracranial space because of a viral infection called Viral TB which can infect every organ but unlike regular TB is not a lifelong infection when it isn't treated.
When Viral TB infects the brain it swells up. The increased intracranial space allows for more fluid balance and oxygen transport during any kind of brain infection.
But is the increased intracranial space beneficial for anything other than more fluid balance and oxygen transport when the brain swells up?