occurred millions of years ago. We can work around it somewhat, but we cannot eliminate or change it.
Brains adjust through domestication, which is driven by artificial selection. Here, humans choose stallions and mares with certain traits to yield offspring who carry the same trait. Breeders can select for temperament and trainability, but often choose beauty, speed, or strength instead.
Brains mature during development from birth to adulthood. The human brain develops for 25 years before it is fully mature, longer than most of us realize. In horses, the length of brain maturation to adulthood is unknown. Physical maturity in general takes five to seven years, depending on breed. Experiences during development alter the brain significantly, so the early training we provide to a horse is critical.
Brains change physically throughout adulthood in response to daily learning. Every time you or your horse experience something important, new connections are formed among brain cells. With use, these connections form a persistent record in the brain. In addition, new neurons are born throughout adulthood.
Natural Selection
Bone and tooth fossils show that the earliest ancestors of today’s horse lived in North America 56 million years ago. The size of small dogs, they had wide-set eyes down near their noses and padded feet with several toes. Warm temperatures of the era had produced subtropical forest across most of the continent, and the lower leaves of those trees kept pre-horses fed and sheltered. Life was good.
But fast-forward 21 million years, when an Ice Age caused temperatures to drop. Polar ice caps formed, glaciers moved in, forests died, and prairies emerged—covered with hard ground, tough grass, open space, and predators. Pre-horses whose bodies could not withstand these new conditions died off. But those few individuals who happened to have warmer fur, bigger bodies, stronger feet, faster legs, and harder teeth survived. They reproduced, their offspring reproduced, and so on—altering the species’ bodies and brains over eons of time.
If you have a chance, try a speedy getaway across rough ground on soft padded feet and a bunch of cold toes. Not very fast, is it? So, through natural selection, the horse’s outside toes began to disappear. Today, the splint bones in our horses’ legs, the chestnuts above their knees, and the ergots on their fetlocks are vestiges of those ancient outside toes. Meanwhile, the central toes hardened and enlarged into hooves that could travel a long way on rigid surfaces.
Natural selection also streamlined the horse for speed, with longer-legged horses better able to survive and reproduce in the new conditions. The bones in equine legs became longer. The “knee” (or carpus) on a horse today is actually the equivalent of a human wrist. Everything below that carpus is the equivalent of a human hand with very long fingers. The horse’s ankle or fetlock corresponds to the large knuckles of your hand, where the fingers begin to protrude. With all this lengthening of equine bone came longer tendons. Today’s equine legs are lightning fast for running away, but length makes them fragile too. Long legs also lifted the horse’s head above the level of tall plains grasses—the better to notice predators lying in wait.
Brains Run the Show
With all of these evolutionary changes in the horse’s body, the brain’s sensory organs had to adapt. Peripheral motion vision and precise hearing tuned up so that important sights or sounds—the rustle of a predator’s movement through grass, for instance—would be noticed instantly. Smell became critical for safety from predators and navigation to water. Motor coordination and fast-twitch muscles became vital for escape. These increases in sensitivity were built partly into the eyes, ears, nose, and muscles. But they are even more apparent in the brain tissue where sensory signals arrive for interpretation and in the hard wiring that carries action commands to various destinations in the brain.
The internal operation of brain cells adapted too. Fatty tissue was produced to surround the long tail (or axon) of each neuron, so it could transmit information faster. (Neurons are a type of brain cell that transmit functional information.) In today’s horse, some axons are 10 feet long, stretching from the brain and looping around through the body. The fastest can transmit messages up to 394 feet per second. That’s nearly 250 miles an hour! Glial (“GLEE-ull”) cells—the brain’s janitors—multiplied to keep neurons healthy. The neural ability to form connections became faster and more efficient (fig. 2.1). So did the brain’s ability to kill off unused connections that would only interfere with the learning process.
2.1 Neurons transmit electrical impulses through equine and human brains. The dendrites of one neuron send electricity through its axon. Axon terminals then transmit that information to the dendrites of the next cell.
The brain adapted to collect glucose from food more efficiently, because brains hog glucose for fuel. The human brain represents 2% of your total body weight but uses 20% of your body’s glucose. Equine brains are downright gluttonous—they comprise only two-tenths of a percent of the horse’s total body weight but use 25% of the glucose. Too much glucose can harm our bodies—both equine and human—but too little harms our brains. That’s why we get confused when our blood sugar is too low.
Hard-Wiring for Safety
When dangerous sensory signals were detected on the prairie, a horse couldn’t twiddle his hooves deciding what to do. He had to run first and be alive to ask questions later. To manage that requirement, the equine brain evolved to connect perception directly to action. A nerve signal comes from the eye to the visual processing area of the brain, for example, and the equine brain instantly sends that signal to the motor control area with the command to RUN (fig. 2.2 A). These processing areas are in the surface, or cortex, of the brain. It all happens unconsciously.
2.2 A The horse’s brain detects sights at the visual cortex, then sends the new information to the motor cortex for immediate action.
2.2 B The human brain also detects sights at the visual cortex. But it sends that information to the prefrontal cortex for analysis and evaluation before the motor cortex springs into action.
The wiring between perception and action in the human brain is quite different. A nerve signal comes from our eyes to the visual cortex at the back of our brain, and is usually diverted to a slow path that meanders over to the prefrontal cortex just behind our forehead. There, an unconscious analysis is undertaken: “What have I seen? Have I seen that before? What does it mean? What should I do? Which option is best? Why? Did I have lunch yet? Oh…whoops, let’s pay attention… Hmm, option 17c has worked in the past. Let’s try that one again.” Finally, action kicks in—long after a lion would have punctured an equine throat and gobbled half a leg (fig. 2.2 B).
Innate Instincts
The process of natural selection over millions of years forms the hard wiring of a brain—its major pathways and structures. Evolution always lags behind the present day. So the human brain still functions according to its ability to hunt meat and gather berries, keep its body warm and dry, find mates in the savannah, and try to prevent the children from being eaten by lions. It doesn’t matter that today we drive to the grocery store instead of spearing a wildebeest for dinner, meet potential mates online, and try to prevent the children from being shot in their schools.
Some pathways make stops here and there along the route to their brain destinations—and often, these stops occur at places where the path ended a few million years ago. When scientists see an abandoned way station like that, we have evidence that
