Imagination

From bacteria to mammals

After life emerged on Earth around 3.4 billion years ago, organisms gradually became more complex. Around 700 million years ago, neurons organized into simple neural nets that then evolved into the brain and spinal cord around 525 million years ago.

Eventually dinosaurs evolved around 240 million years ago, with mammals emerging a few million years later. While they shared the landscape, dinosaurs were very good at catching and eating small, furry mammals. Dinosaurs were cold-blooded, though, and, like modern cold-blooded reptiles, could only move and hunt effectively during the daytime when it was warm. To avoid predation by dinosaurs, mammals stumbled upon a solution: hide underground during the daytime.

Not much food, though, grows underground. To eat, mammals had to travel above the ground—but the safest time to forage was at night, when dinosaurs were less of a threat. Evolving to be warm-blooded meant mammals could move at night. That solution came with a tradeoff, though: Mammals had to eat a lot more food than dinosaurs per unit of weight in order to maintain their high metabolism and to support their constant inner body temperature around 99 degrees Fahrenheit (37 degrees Celsius).

Our mammalian ancestors had to find 10 times more food during their short waking time, and they had to find it in the dark of night. How did they accomplish this task?

To optimize their foraging, mammals developed a new system to efficiently memorize places where they’d found food: linking the part of the brain that records sensory aspects of the landscape—how a place looks or smells—to the part of the brain that controls navigation. They encoded features of the landscape in the neocortex, the outermost layer of the brain. They encoded navigation in the entorhinal cortex. And the whole system was interconnected by the brain structure called the hippocampus. Humans still use this memory system for remembering objects and past events, such as your car and where you parked it.

Groups of neurons in the neocortex encode these memories of objects and past events. Remembering a thing or an episode reactivates the same neurons that initially encoded it. All mammals likely can recall and re-experience previously encoded objects and events by reactivating these groups of neurons. This neocortex-hippocampus-based memory system that evolved 200 million years ago became the first key step toward imagination.

The next building block is the capability to construct a “memory” that hasn’t really happened.

Involuntary made-up “memories”

The simplest form of imagining new objects and scenes happens in dreams. These vivid, bizarre involuntary fantasies are associated in people with the rapid eye movement (REM) stage of sleep.