"A growing body of research indicates that babies are much more tuned into their environment than we realized."
This really shouldn't come as a surprise. Human babies are the product of tens of thousands of years of primate evolution; like all baby animals, they come hard-wired with inherited mental models that help them survive. A baby goat can walk in short order; baby turtles have natural directional tuning. Our latent abilities just run in a different direction.
Human babies are, physically, helpless - they can't even eat on their own. What they can do, though, is ascribe value to behaviour and infer what it means to them. We're built to be socially manipulative.
Mothers are sources of food and safety, so it's good to know who they are and give them some positive feedback (or withhold it to incite a behaviour change on their part).
Understanding happy and angry behaviour (in people, dogs, whatever) helps determine what's a threat, what's an opportunity and what's merely a curiosity.
Another cute little baby fact - babies get bored. In fact, they cry when they're bored. Their little brains are rapidly building synaptic connections, perceiving and linking the sensory stimulus of their world within their expanding mental reference library. If you give their brains nothing to do, it's like flooring the pedal with the emergency break on.
The thing to remember is that each of us started as babies; all our cognitive hardware has been built around that same, hard-wired reactive system. As such, we're all programmed to respond to certain stimuli in a pre-determined way. We can shape these responses, to a degree, when we understand the process - problem is, so few of us do.
If you doubt that at all, look at the four pictures here and think about how the different elements of each made you feel.
You see my point.