Even before: learning to swallow in the womb. There's definitely no reinforcement from other humans involved in that, and it's fundamental that they learn it in the womb because it would be too late to learn it with milk.

That's been passed down over 3.5 billion years to form the DNA that creates that part of the brain in an infant. AKA pre-trained.

Not really, they develop it by sucking on their thumbs, drinking amniotic fluid, etc. You can see it develop, and you can see how premature babies have issues with it. Adding "breathe" to sucking and swallowing is even harder for preemies.

Starting to suck when they feel something on their lips _is_ a reflex though, it isn't learnt.

reflex = pre-trained?

More or less, yes. They usually disappear before the first year. Many later reappear in a more refined and voluntary way, which is learned rather than pre-trained.

All those things you described can be explained by having a pre-trained model built from DNA. Babies aren't a blank slate. Why would you even think they are? Do you think a baby learns to breath from blank-slate intelligence?

If what you are saying was true, then why all these skills require time to develop? If they are pre-programmed in the DNA, why are they not enabled on the day of birth? Clearly from evolutionary point of view an infant that is already able to walk and perceive depth would have vastly superior chances of survival? Why all the brain development, and much enhanced plasticity in the early stage of development? Why do neural receptive fields change rapidly in the first months of life and do depend a lot on the stimulus received?

Clearly there is stuff pre-programmed in the DNA and some animals e.g. are able to hold balance right after birth. But apparently most of this stuff in case of humans is instructions how to build a learning system, rather then encoding the complexity of the world in a strand of DNA. I grant you, the evolution did not have the time to encode ways to handle lego bricks and yet 2yr olds spontaneously acquire ability to manipulate these objects way beyond what any contemporary robot can do.

I hope it's clear that the answer lies somewhere between these two extremes. Most likely, there are instilled a number of prototypical reflexes that quickly make available the "right" decision for a given situation. Consider it that we don't know the objective function per se, but that we start in a place and with a gradient vector already pointed in such a way that we quickly reach the optimal location in action space with relatively few exploratory trials.

Yes, the walking movement is one of a newborn's 9 primitive reflexes. [1] However, developing the fine control skills required to balance while walking is a skill that babies learn, and it takes a long time to do so.

[1] https://en.m.wikipedia.org/wiki/Primitive_reflexes

> If what you are saying was true, then why all these skills require time to develop? If they are pre-programmed in the DNA, why are they not enabled on the day of birth?

I'd argue that we see clear examples of such in animals that do require those for immediate survival. We ought to think about it as a consequence of natural selection rather than a 'necessity' that is somehow intelligently pre-programmed for survival. It's far more likely that path to early survival traits were mutational (accidental) rather than supervised.