Valence
Good and bad
New: a few cells that mean this is good and this is bad, and poison berries in the world.
Result: the animals learn over generations to turn away from poison before they bite it. They depend almost entirely on the bad side of the new circuit.
The animal of the previous stage steers by reflex. What it sees is wired, through a small ganglion, straight to its motor neurons, and it cannot tell what happened after it bit something.
This stage gives it cells for good and bad, a sorting of the world that is called valence. The world gets something to sort: two kinds of berry are now poison, and a poison bite hurts.
Point at or tap a box or an arrow for details. Dark arrows are designed, grey ones evolved; see reading a brain diagram.
The circuit
Five new groups of cells, each with a meaning fixed by its wiring:
valence_app, good: the sense of taste is wired to these cells, and reaches the brain nowhere else.valence_av, bad: pain is wired to these cells, and reaches the brain nowhere else.grasp: when it fires, the animal bites what is in front of it.no_touch, a gate: fires whenever nothing is within reach, and holdsgraspshut.no_feed: driven by the bad cells, it blocks the bite.
Good leads to a bite, but only when the food is in reach. The good cells excite grasp, and no_touch holds it shut until something is directly ahead. This is disinhibition, the release of an action by switching off its gate (background).
Bad turns the animal away and stops the bite, as in a worm that touches something harmful: the bad cells drive the turning motor neurons, and through no_feed they shut down biting.
The eyes give the value cells a head start. Taste and pain arrive only after a bite. So the value cells also get input from the eyes, and that input is evolved. Over the generations, the look of a poison berry comes to excite the bad cells before anything has been eaten.
Watching it work
One animal from the last generation, around the moment it bites a poison berry. Step through it one tick at a time.
At the bite nothing has changed yet: the pain arrives one tick later. Then pain is on, both bad cells fire, no_feed switches on and grasp falls silent. The animal turns away. A few ticks later the pain has faded and it is eating again.
Nothing was learned here: the animal will react the same way next time. What keeps it from biting poison often is the evolved input from the eyes, which acts before the bite.
Biology
In small animals most sensory neurons fall into two classes: some respond to things worth approaching, such as the smell of food, others to things worth avoiding, such as a harmful chemical. The two classes converge on a few interneurons that bias the animal towards carrying on or turning away.
In the worm C. elegans, with 302 neurons, a harmful stimulus does two things at once: it triggers a turn, and it stops the pumping of the throat with which the worm feeds. Max Bennett argues that this sorting into good and bad is what the first brains were for.
The worm is a stand-in. The animal this chapter is about, the common ancestor of all animals with a left and a right side, left no nervous system to look at.
The world
Six kinds of berry bush instead of four, each with its own colour. Four are food. Two are poison, and always the same two: a poison berry empties the stomach by about as much as a good berry fills it, and it hurts for a moment.
Because the poison kinds never change, nothing has to be learned within a life: which colours to avoid can be inherited. Learning it within a life comes in a later stage.
Did it work?
Look at the first two bars: without the bad cells, or without no_feed, the animals keep only about a tenth of their fitness. The good side matters much less, because the reflexes of the previous stage already bring the animal to food and make it bite.
The old brain falls far behind. It cannot feel pain, so it can avoid poison only if evolution happens to wire “this colour, turn away” into its reflexes, and in the generations given it did not.
That says how much easier the value cells make the solution to find, not what pain is worth in principle: the poison looks the same in every generation, and a brain without pain could evolve the same avoidance by colour. In earlier runs with many more generations, two of three controls eventually did, late and suddenly.
The difference shows in how the animals spend their lives:
Full results
The three main curves do not rise at the same moment: when evolution finds the wiring from the look of the poison kinds to the bad cells is a matter of luck. One seed found it within a few generations; the other two were still climbing when the planned generations were over, and were given as many again.
What the three rows measure: see how a stage is judged.
In the first runs of this stage the animals were born with a full stomach, and the good side was not used at all. A full stomach lasted most of a life, so there was little to gain from eating well. Born a quarter full, the animals use it a little.
One fault remains: the good side makes the animal bite whenever food is in reach, hungry or not, and many bites land on a nearly full stomach. Nothing in this brain can sense the stomach yet; a later stage adds a cell for hunger.