Observation of individual neurons doesn't indicate they have intelligence however doe it means that intelligence of a human brain is emergence phenomenon?

Observation of individual atoms and molecules wouldn't revel any gravitation like properties either however we don't call that gravity emergence phenomena. Instead we argue that gravitation like properties of atoms and molecules are not observable. Could you conceder that we may grossly underestimate an "intelligent ability" of individual neurons?

In this case it is obvious. However, in general a system could be less complex than complexity of its elements. We don't have agreeable way to measure complexity but for now I would argue that complexity of molecule of water (its complete physical and chemical description) is more complex than sum of those molecules in form of a drop of water. Unfortunately so far there's neither accepted approach to measure nor define complexity.

I have always been intrigued that such a complex system like a living cell, could be reduced underlying physics and chemistry. Over time, my reductionistic curiosity was eroded by holistic views that embrace emergence phenomenon as an explanation of complexity. However, eventually, I was disillusioned with the emergence paradigm, as misleading and concluded that the popular interpretation of holism ‘The whole is more than the sum of its parts,’ is profoundly deceptive if used within scientific explanation. My current view is that emergence is a perception caused by part’s properties not observable in isolated parts. These properties become observable only during interactions in the system; a system acts as ‘litmus test’ or a ‘magnifying glass’ that just reveals the parts’ properties not observable otherwise.

Regarding ant colony how much we know about individual ants to deprive them from ability of complex behavior? In my book the complexity of any system resulted from collective complexity of its elements.

The typical rhetorical argument in favor of emergence is the question: ‘Is water more than one atom of oxygen and two atoms of hydrogen?’ The intuitive respond, is yes, because in our perception, water, the way we directly experience it, is very different from an abstract theoretical model of atoms of oxygen and hydrogen. However, from a scientific point of view this question is misleading. The correct question would be: ‘Is a molecule of water more than one atom of oxygen and two atoms of hydrogen, interacting among each other?’ This time the answer is no: the molecule of water is no more that the sum of its components.

I would be interested in any example in which complexity of the system "emerges" from elements we know (or at least we believe we know) everything about.