Symmetry, Steering and the Brain

Animals have one of two kinds of symmetry – radial or bilateral, points out Max Bennett in A Brief History of Intelligence. 

Animals started by having radial symmetry. Why then did so many diverge into bilateral symmetry? Simple answer: Radial symmetry works fine if the approach is to wait for food. But it is a terrible setup if you want to navigate towards food. He expands on that.

 

A creature with radial symmetry would detect signals from and move in all directions. Bilateral symmetry, on the other hand, is designed for movement in two directions – ahead and left/right. The former is very complicated; the latter is so much simpler. (Which is why human engineers have designed everything that moves with bilateral symmetry – cars, planes, submarines).

 

While simpler on one front, bilateralism creates a new need – a decision-making capability. Which direction should one move in? Thus, all bilaterals, even the tiniest ones, have brains. Two rules are the minimum: (1) If food signal increases, keep going in that direction; (2) If it decreases, turn and repeat.

 

Soon enough, other creatures, including predators evolved the same capabilities. So now the rules had to be edited, move towards food, and/or move away from signals of predators.

 

Take a step back and you realize signals need to be classified as good (e.g. food) or bad (e.g. predator or other undesirables). The term neurobiologists use for assigning good/bad values to a signal is valence. Initially though, assigning valence values did not develop in the brain! Rather, the sensory neurons themselves assigned valence values to the signal they were sending.

 

But that wasn’t enough. Food always has positive valence; predator has negative valence. But other signals, like temperature are not boolean:

“A warm bath is miserable in scorching summer but heavenly in a cold winter.”

The valence value, at times, had to context sensitive. This logic didn’t start in the brain; rather two separate neurons evolved for “too hot” and “too cold”.

 

Ok, so we have multiple neurons all assigning valence values. Inevitably, as more and more of them got added, they began to contradict each.

“What if the nematode smells something yummy and something dangerous at the same time?”

Scientists tested just that with nematodes. The answer? It depends. On the relative strengths of the signals. Even the simplest brains are capable of doing such trade-offs.

 

This then is the answer to the question of why brains evolved.

“All these sensory inputs voting for steering in different directions had to be integrated together in a single place to make a single decision… The first brain was this mega-integration center.”

 

Soon, to this mix of external signals, animals began to add internal signals. How hungry are you? This too began to influence the decision making. Everything else being equal, if not hungry, not worth the risk. Else, probably worth the risk.

 

To sum it up, steering was the first driver that led to the evolution of the brain. It started with bilateral symmetry, valence neurons that assigned values to the signal, an integration center to handle contradictory signals (the brain), and later the ability to include internal signals in decision making.