Steering
Turning sight into movement
New: the first brain: eyes, a small clump of neurons, and motor neurons, all wired by evolution from scratch.
Result: the animals find food and eat it. No single neuron stands for anything; the behaviour is spread over all of them.
The first animal has no memory, no hunger and no idea of good or bad. It can do one thing: turn what it sees into movement. This stage evolves a brain that does that, starting from random wiring. Everything after this stage is added to this brain.
The circuit
- A ganglion: sixteen excitatory and eight inhibitory neurons (
ganglion_eandganglion_i). The eyes connect to it, its neurons connect to each other, and it connects to the motor neurons. - Reflex arcs: direct connections from the eyes to the motor neurons.
You do not need to follow the arrows. Every synapse here is evolved: which ones exist and how strong they are. Nothing is designed, nothing changes during a life, and no neuron has a meaning you could name. On later pages the ganglion is drawn as a single box.
The brain sees the outside world only. It does not feel its stomach and does not notice whether a bite gave it anything; those senses enter in later stages, through cells made for them.
Watching it work
One animal from the last generation, walking up to a bush and eating. The rows below the picture are the neurons of the ganglion. There is nothing in them to read, and that is the point: the behaviour is spread over all of them, wherever evolution happened to put it.
Biology
The oldest animals with a front and a back were small worms, and what set them apart from jellyfish and sponges was that they could go somewhere. A body with a front end, senses on it and a clump of neurons behind them can turn towards what is good and away from what is bad. In A Brief History of Intelligence this is the first breakthrough, steering.
In the worm C. elegans, with 302 neurons, sensory neurons connect to a few layers of interneurons and those to the motor neurons, with some sensory neurons wired to motor neurons directly. A direct connection is a reflex. The interneurons in between let several senses be weighed against each other before the muscles move.
The world
Four kinds of berry bush and wild onions. Everything is food and nothing is dangerous. Moving costs energy, so an animal that stands still starves, and so does one that walks without getting anywhere. The only way to live is to reach a bush with berries, bite, and go on to the next one.
Did it work?
The animals depend on their ganglion: without it they keep less than a fifth of their fitness. The direct reflex arcs alone are far from enough. There is no old brain to compare with, so this stage was run once: every later stage starts from this one population.
Full results
The jump in the curve is a change of setting, not of brain: the population first evolved in short lives and was then given lives four times as long, as in every later stage. In a long life an animal that eats once and then stands still does not reach the end, and over the following generations the standing still is selected away.
In earlier runs every bush was full when the animals were born. They filled their stomachs early and kept eating, the bushes were stripped by the middle of the life, and since a bush takes long to regrow most animals then starved together. Whatever an animal might have done better later in life could not show.
Starting most bushes empty, each at a random point of its regrowth, turned that stock into a steady flow: the share of bushes with berries stays about the same through a life.