It merely transmits signals around the body. If you touch a hydra in one place, the nerve net spreads the signals indiscriminately, and the hydra twitches as a whole.Ī nerve net doesn’t process information-not in any meaningful sense. They are tiny water creatures-transparent, flowerlike animals with sacs for bodies attached to many arms-and belong to the same ancient category as sea jellies. The earliest nervous systems may have been simple nets of neurons laced throughout the body, interconnecting the muscles. A wave of electrochemical energy sweeps across the membrane of the cell from one end to the other, at about 200 feet per second, and influences another neuron, a muscle, or a gland. These numbers may change with new data, but as a plausible, rough estimate, it seems that neurons, the basic cellular components of a nervous system, first appeared in the animal kingdom somewhere between sponges and sea jellies, a little more than half a billion years ago.Ī neuron is, in essence, a cell that transmits a signal. Sea jellies don’t fossilize very well, but by analyzing their genetic relationship to other animals, biologists estimate that they may have split from the rest of the animal kingdom as early as 650 MYA. In contrast, another ancient type of animal, the sea jelly, does have a nervous system. They are thought to have shared a last common ancestor with us between about 700 and 600 MYA. Sponges seem to be poised right at the evolutionary threshold of the nervous system. In sponges, the same genes may be involved in simpler aspects of how cells communicate with each other. And yet sponges do share some genes with us, including at least 25 that, in people, help structure the nervous system. They sit at the bottom of the ocean, filtering nutrients like a sieve. They are the most primitive of all multicellular animals, with no overall body plan, no limbs, no muscles, and no need for nerves. I will begin the story with sea sponges, because they help to bracket the evolution of the nervous system. The key to the theory, and I suspect the key to any advanced intelligence, is attention-the ability of the brain to focus its limited resources on a restricted piece of the world at any one time in order to process it in greater depth. In the attention schema theory, consciousness depends on the nervous system processing information in a specific way. For most of Earth’s history, life remained at the single-celled level, and nothing like a nervous system existed until around 600 or 700 million years ago (MYA). Self-replicating, bacterial life first appeared on Earth about 4 billion years ago.
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