There is a fair amount of evidence as I understand it on the importance of mothers to babies and young children.
Naturally, as described by Demicolon in her post about home education, children are primarily attached to mothers and from a secure base of that mother-baby/child relationship they begin to explore the world and form relationships with significant others in their lives and communities.
This would have been the norm for most of human history. Where there was interference in this norm the consequences would have been clearly observed as being negative.
The other people around the child would have also likely have been permanent members of the child’s community, so that the attachments formed would be lifelong.
We begin as one being with our mothers and separation is gradual. To force it before time or to hold back the process when the time comes for mother and child to move slightly further from each other will naturally cause problems for one or the other.
A secure, well grounded human being is usually someone for whom this process of gradual separation was respected. It makes up for a lot of other ills, problems from natural causes, disease, famines, etc etc
It seems that we have a generation which are feeling disconnected and untethered. Since our identities are naturally formed in large part by human relationships, our families and communities who have known us since birth, knew our mums and dads, maybe even grandparents a whole generation is missing out on this when they are brought up by transient carers.
We are known by those around us, it grounds us in the world and gives us a confidence in who we are, it means we don’t need to question our identities. Our family, friends, community validate us all the time implicitly through daily interactions over a lifetime and perhaps going back before our birth through knowledge of our ancestors.
Mothers bring us into the world gradually teach us that we are separate people worthy and loved and gradually introduce us to the significant people in our lives, gently, little by little, we build those relationships first with dad and siblings, with grandparents, aunts and uncles, with family friends with families nearby who have children the same age until we are fully integrated into our community.