"Our brain’s cerebral cortex—the seat of higher thought—is eerily similar to a clump of neurons inside the head of the lowly marine ragworm. The ragworm’s brain, which evolved some 600 million years ago, is so similar to the cortex that humans and worms must share a common ancestor..."
When scientists "compared the worm’s cells with those in a vertebrate cerebral cortex, they found they were too similar to be of independent origin....."
"...these tests challenge the standard notion that the ability to think evolved from complex vertebrate behaviors like predation.... that now appears to spring from something far more basic, he argues, like the ability “to distinguish between food and nonfood”—a feat the ragworm accomplishes with aplomb."
For years, I'd considered fear of predation to be our most primitive instinct. I'd missed the obvious. It's not merely eat or be eaten. It's knowing what is food and what is not that's the most basic instinct. Though, of course, photo-sensitivity and other visceral senses are more basic -- still, I'm referring to instincts.
Fear repels. And the three basic defense mechanisms interplay ( run, hide, fight).
On the psychological level, it's a combination of various psychological defense mechanisms.