Evolution

Evolution of the Nervous System in Animals



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The simplest multicelled animals, the sponges, have no nervous system, but they don't need one.  Their life style is also simple:  sit and sway in the currents while filtering food from the water.  They don't need to find food or avoid predators so they have no heads, no sense organs and no brains. 

The next steps up are the jellyfish and the most primitive worms.  Jellyfish are free-living predators and have nerve nets to help them sense prey and escape predators.  The cells are the first nerve cells, long and thin with connectors to allow them to pass messages throughout the body.  To find a true nervous system, though, one must go to the worms.

The simplest worms, the Platyhelminthes or flatworms, have only two cell layers, but they have something that no earlier animal had and that changed the path of evolution:  a head.  More primitive animals either had no symmetry in their cell development, like sponges, or were radially symmetrical, like the jellyfish.  Worms went for bilateral symmetry and stuck their sense organs and their nerve cells at one end of the body.  This turned out to be the best way to organise oneself for a predatory life style and worms quickly became the top predators of their primitive world.  Putting sense organs around the mouth allowed for the sensation of potential prey.  Sticking the nerve cells behind the sense cells enabled the messages to be quickly received and acted upon.  The beginnings of the nervous system were born.

I once studied polychaete worms and was amazed at what those animals could accomplish with nerves organised in a line down the body and concentrated at the head end into a nerve bundle.  It wasn't big enough to earn the name brain, but the worms made do.  They had simple eyes and sensitive tentacles that sent messages to the bundle of nerves about the world around them.  They could sense and capture prey and sense and avoid predators. They could sense potential mates and act appropriately.  Once the egg sacs were laid, they cleaned them and protected them, although once the larvae broke free and left the maternal burrow, they were fair game for her tentacles.

What interested me the most was that these worms seemed to be able to 'think ahead'.  If they ran out of food within reach of their tentacles, they would build mud tunnels to reach new food sources.  Interesting behaviour for animals with no brains!  Behind the nerve bundle, nerve cells ran down the line of the body through the worm's segments.  These nerves directed the actions of muscles and were the beginnings of the more complicated nerve chords present in higher animals.

By the time we get to fish, the nervous system is well developed, with a brain that is the body's control center and a dorsal nerve chord that is the message carrier from the control center to the body parts and back.  Signals from the fish's eyes, nose and other senses travel quickly to the brain via well developed pathways.  Messages then go to the muscles telling the fish to respond by fighting or fleeing.  The nervous system is essentially complete.  It gets more complicated in higher vertebrates but the system is the same:  brain, nerve chord and highly developed nervous cells to control all the actions and reactions of complex organisms.

In an example of parallel evolution, other animals have developed similar nervous systems separately.  Insects have a ventral nerve chord running from their brains to their bodies but the functions are the same.  The epitome of molluscan evolution, the octopus, has well developed eyes and behind them a strange but very functional brain.  From there the nerves go out to the muscles and tentacles.  This is an animal that can also 'think ahead'.  Stories abound of aquariums where fish were mysteriously disappearing.  It seems that octopi learn to leave their tanks and go to other tanks in order to catch a meal.  Then they slip back into their own tanks and play innocent the next day.  This is another example of an efficient, well-functioning nervous system in a so-called primitive organism.

The nervous system developed early in the evolution of animals.  The first worms used it to become the first predators and the arms race was on.  Prey needed to develop nervous systems in order to escape from predators.  Predators needed to become smarter and faster in order to keep up with the prey.  The result was an explosion of life forms in the early Cambrian period.  All the diversity of life has come about in the relatively short geological time span after that of about a half a billion years.  Sexual reproduction and the subsequent increase in genetic diversity was primarily responsible but the development of the nervous system and its effect on predator prey relations was important too. 

Whether brains will eventually prove to have survival value is still an open question as the world's top predator goes about destroying whole ecosystems in the name of progress but there is no doubt that the development of complex nervous systems is what got us this far in the first place.  Hopefully it will help us learn from our mistakes and survive longer than those much dumber dinosaurs.

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