Humans and some higher primates form emotional attachments to primary caregiver. For humans, this develops during the first year of life. At 8-12 months separation anxiety illustrates that emotional bond has formed. Harry Harlow showed the importance of attachment in monkeys. They became abusive parents when they grew up.
Intellect grows through assimilation and accommodation .
Assimilation means using existing mental frameworks and patterns to understand a new information. Accommodation means that existing mental frameworks and patterns are modified to understand new information. Intellectual development is mostly nonverbal. Do not yet understand cause-effect relationships. Also, egocentric , unable to take the perspective of another person.