Intelligence in Nature - Jeremy Narby [28]
I wanted to reconsider things from the start and try to move away from my own prejudices. As an animal, I wanted to understand animals. For starters, I learned that not all animals have brains. The sponge, for example, does not even have nerve cells. It lives attached to the sea bottom, or to other objects. The natural sponge that can be purchased in a store is the skeleton that supports the sponge animal. Inside this skeleton, the body of the living sponge consists of a kind of perforated stomach lined by whiplike cells which create currents that draw in water and food particles. A four-inch sponge can filter one hundred liters per day in this way. Sitting stuck to a rock at the bottom of the ocean, a sponge just sucks in its food. Zoologists recently discovered that one type of sponge can respond to potential danger by generating electrical impulses similar to those that streak through the nerves of other animals, including humans. Electrical signals disseminate through the sponge body via a network of fine strands of cytoplasm, which are not divided into cells. The sponge uses these signals to shut down the intake mechanism when the water around it becomes murky with particles that would otherwise clog its pores. These electrical signals are part of a decision-making system that allows the sponge to gauge and exploit the world around it. Though sponges are brainless and nerveless animals, they appear to make correct decisions on a regular basis.
The hydra is another brainless, headless, and sedentary animal that lives in water. It looks like a thin, translucent tube about an inch long and has a nervous system called a nerve net, which crisscrosses its body without forming a particular concentration. The hydra lives attached to vegetation by the base of its tubular body. The bottom of the tube is closed, and an opening at the upper end both engulfs food and rejects residue. Around this opening is a circlet of retractable tentacles that sting and catch other small invertebrate animals such as crustaceans. When a hydra detects a prey, it extends its tentacles and reaches out to grasp it. How it carries out this precise action with no brain is not known. Studies reveal that the animalâs nerve net concentrates around its mouth area. This suggests that the earliest heads appeared about 700 million years ago in hydralike organisms that may have been the common ancestors of species from snails to humans. The early head was simply a net of nerve cells at the mouth of the organism. This concentration of neurons close to the mouth shows how important active feeding is for animals. We exist in our current shape, with heads containing brains close to our mouths, as a legacy of this.
I scratched my head thinking about the fact that my brain is close to my mouth. I used my predator brain to think about the long line of predators that had led to me. I could see an endless queue of mouthed ancestors stretching back hundreds of millions of years, snapping their teeth and laughing.
I looked into the origin of central nervous systems. They first developed in small invertebrates like nematode worms. The present-day nematode Caernohabditis elegans looks like a mere speck to the naked eye. It has a body made up of fewer than one thousand cells, some three hundred of which are neurons that form a ring-shaped brain around the digestive tube not far from the mouth. The nematode brain, which is among the simplest known, is shaped like a saintâs halo. Centralized nervous systems have shorter and denser connections between neurons. This makes for quicker reactions to changes in the environment and for more complex behaviors.
The brown garden snail Helix aspersa has a central nervous system containing only a few thousand neurons. This is not much, considering it has a body the size of a walnut. Consequently, nervous signals take time to travel through the snailâs body, and its muscles can take several seconds to react to an outside stimulus. In fact snails perceive the world in slow motion. But this does not mean they