Eli5: how is it that so many species, including humans, have the same motherly instinct that suddenly appears after birth? Where does this come from?
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There are basically two very broad approaches to having kids, or “reproductive strategies,” in biology, called the “k” and “r” strategies. (Each species has its own life history, so this is simplified — in nature, there’s a lot of variation on these themes.)
R-reproductive species focus mainly on producing as many offspring as possible. These species include fish, mice, insects, etc. In these species, the main focus is just on producing offspring in large numbers, and then turning them loose at some minimally viable age. Yes, very large numbers will fail to grow up, but the sheer numbers make up for that.
K-reproductive species focus mainly on helping a small number of offspring succeed in life. These species include us, other large mammals, etc. Here, there are much smaller numbers of offspring, and a huge amount of effort is invested into trying to make sure they grow up and succeed.
Maternal or parental instincts are crucial to the “K” species, like humans. Evolutionary theory predicts that basic instinctive behaviors could evolve where they make it more likely for offspring to survive to reproductive age. Put simply, this behavior evolved because without it, “k” species like us would have trouble getting our kids to survive long enough to have their own kids.
For that to work as an explanation, our DNA must contain instructions to help shape the brain in a way that will make us feel good enough about parental behavior that we are willing to do it. That part is easily explained by evolutionary theory. So far neuroscience is not advanced enough to explain exactly how the brain structure works to make that a reality.
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