Whether for brains or bacteria, intelligence is all about food

If you wanted to write a computer program to simulate intelligence, you could start with two simple commands.

Get food: Yes. Be food: No.

Intelligence is all about enhancing your ability to survive, and getting food while avoiding being eaten are essential ingredients in any survival recipe. Evolution built brains to get food to the gut and keep bad things away. As Caltech neuro­biologist John Allman once expressed it, “the brain is the gut’s way of protecting itself — and making sure it gets good stuff into it.”

Actually, you don’t even need a brain; the most successful survivors in evolution history (by population for sure, perhaps also by biomass) are bacteria. Bacteria are in fact much like brains, possessing the ability to make intelligent choices. In the brain, information processing relies on nerve cells, or neurons, that are sensitive to specific chemicals in their surroundings. Each neuron’s outer membrane is spiked with antenna-like molecules, called receptors, that latch on to messenger molecules expelled by other neurons. Such signals induce neurons to fire electrical impulses, and the totality of neural signaling, electrical and chemical, translates into the brain’s thoughts and feelings, decisions and actions.

Bacteria do something similar. They are experts at a process called chemotaxis, the ability to move toward or away from certain chemicals. Bacteria spin their propellers (called flagella) to swim toward food or away from poison. Just as brain cells are activated by sensing certain molecules, the molecular machinery driving the propeller is activated by a bacterium’s own antenna molecules when they sense certain chemicals.

Sensing food starts the propeller spinning counterclockwise and the bacterium swims in a more or less straight line. If there’s no luck with food, or sensors detect poison, the propeller stops or reverses, causing the cell to tumble randomly in search of a better direction to swim. Most bacteria don’t simply sense the amount of food but check to see how that amount is changing over time, influencing the probability of changing swimming direction.

That’s the basic idea. But recently scientists have realized that textbook dogma about chemotaxis is in need of further review. For decades, most experts believed that bacterial swimming depended only on sensing food outside the cell, not what the bacterium was doing with the food it ingested. Newer studies show, though, that the cell’s metabolism can also affect swimming behavior, at least for some species. Internal sensors can gauge the cell’s energy needs and influence the signaling system that tells the propeller to move.

“Metabolism-dependent chemotaxis might be more widespread than previously thought,” Matthew Egbert of the University of Sussex in England and collaborators wrote last year in PLoS Computational Biology.

Viewed in this way, chemotaxis is not a simple stop-or-go behavior, but the result of complex cellular calculations, integrating all sorts of chemical cues from both outside and inside the cell. Which is, in fact, just what the human brain does. Instead of a single cell with molecular parts, the brain is a conglomerate of billions of cells, a coalition of neuronal calculators that gather external information from the various senses and merge it with internal conditions into a coherent (conscious) state of mind. Many species of bacteria rely on multiple molecules “to integrate signals about the external environment and the metabolic state of the cell,” write Steven Porter of the University of Exeter in England and Oxford University’s George Wadhams and Judith Armitage in the March  issue of Nature Reviews Microbiology.

Bacterial behavior thus shares essential similarities with the decision-making method of brains. “It’s the same story,” says Allman, “fantastically elaborated, but it boils down to the same thing.” Human intelligence owes a lot to life’s prokaryotic pioneers.

This basis for braininess comes at a price, though: the primordial memory of the food-poison dichotomy. Just as bacteria swim or not, nerve cells fire or not, human brains often view aspects of the world as right or wrong, good or bad, nothing in between.

Stephen Jay Gould, the late paleontologist, commented on this aspect of human intelligence in his magnum opus, The Structure of Evolutionary Theory. He considered thinking in dichotomies “a general mental error of oversimplified organizational logic … probably imposed by the deep structure of the human mind.”

Gould cited the late French anthropologist Claude Lévi-Strauss’ view that “human brains work best as dichotomizing machines at single levels” but are not so good at perceiving nuance and relationships among multiple levels of interacting systems. So it seems that humankind’s inability to cope intelligently with many of today’s problems stems from the bacterial origins of intelligence in the pursuit of food, as reflected in the basic division, as Lévi-Strauss himself noted, of food itself: raw and cooked.

SN Prime | July 15, 2011 | Vol. 1, No. 5