What first developed eyes or ears

Evolution: Reading sample: The invention of the senses

How nature has created highly sensitive sensory organs from simple sensors over the course of millions of years

Actually, it is extremely astonishing: Even if we remain motionless in one place and do not run back and forth to explore the room, our brain draws a detailed, current picture of the environment and what is happening in it. This is possible because we are equipped with highly sensitive sensory organs that literally absorb information from the environment.

For example, if we sit on a chair in the sun, we can use our sense of touch to feel the wood on our back, the armrests on which our arms rest and the ground beneath our feet. We feel the warmth, our sense of balance confirms that we are sitting upright, and the nose transmits the scent of a freshly mown lawn to the brain. The ears receive sound waves from tens of meters away, perhaps attesting to the lively palaver of a group of school children or the roar of a truck thundering past in the distance.

The information that our eyes perceive is even more detailed. They convey the children's facial features, the colors of their clothes, the silhouette of the truck and the swirling smoke from its exhaust.

Our eyesight reaches far into the distance. The rays of light that we receive from the sun come from 150 million kilometers away. And on a clear night we can even see the Andromeda galaxy as a faint glimmer, the rays of which travel to us from around two and a half million light years away (one light year equals about 9.5 trillion kilometers) through the vastness of space.

The fact that we can gain such a rich impression with the help of our senses does not serve our edification, however, but is simply a question of survival. Because neither humans nor animals could exist in the world if they did not have knowledge of their environment.

The need to obtain information about one's surroundings is therefore as old as life itself.

Only those who could orientate themselves in the world, use food sources and escape enemies could assert themselves and produce offspring. Therefore, the origin of the senses goes back to an era more than 3.5 billion years ago.

At that time, astonishing, extremely complex biochemical processes took place at hot springs in the depths of the primeval oceans. As in a witch's kitchen, a wide variety of molecules boil together, react, form ever more complex connections - and finally join together to form the first simple living beings. They are simple creatures, presumably built in a similar way to today's bacteria.

And yet these structures can already find their way around in their environment. They are able to discover food, identify harmful influences and avoid obstacles. But how can beings who have neither eyes, ears, noses, nor tactile organs, perform like this?

They probably do it in a similar way to bacteria living today. And thus create the basis for abilities that even more highly developed creatures will use for sensory perception billions of years later.

Although the early unicellular organisms are some of the most simply built and smallest living things, they contain thousands of different biomolecules. And on the shell that surrounds it and delimits it from the outside, there are dozens of special molecules that can do amazing things: They are able to detect chemical substances.

Some of these chemoreceptors recognize food or building materials - such as sugar or amino acids - while others react to poisons, such as dangerous heavy metals.

In this way, the tiny living beings are able to perceive a number of chemical substances - to taste, so to speak.

But these are not yet all the sensory performances that primeval creatures are capable of.

Because they can also "feel" a little: their membrane contains so-called mechanoreceptors that signal to them when they hit an obstacle. This is the simplest form of a touch.

But what use is such information from the outside world to primitive unicellular organisms? They are only useful for survival if the creatures are also able to react to the signals. And they actually can. Because at least some of them are able to move with the help of some kind of motor.

Presumably they have flagella, as they are found in many bacteria today - tiny protein threads that sit on the cell membrane, can rotate and thus propel the unicellular organisms forward like a ship's propeller.

The special thing about it: The flagella can be rotated in two directions. If they rotate counterclockwise, the cells move forward. If they turn clockwise, the bacteria stagger around aimlessly.

Even with this simple armor, the single-celled organisms have a good chance of survival. For example, if their receptors signal sugar molecules, they swim for a short time with the help of their motor. Then they measure the sugar content in the area again. If its amount has increased, it is clear that they are on the right way to the food source, the unicellular organisms continue their movement.

If, on the other hand, the amount of sugar has decreased, the tiny ones are on the wrong track. They stop, rotate their flagella in the opposite direction and lurch around a little. This puts them in a new position that dictates a new direction of movement. Now put them back into forward gear and see if they can get closer to the food source this time.

The protozoa behave in exactly the opposite way when they register harmful substances or encounter an obstacle. In these cases, they stop and take a new direction, hoping to avoid the poisons or the obstacle.

According to this principle, the bacteria-like creatures are probably already able to orient themselves in their oceanic environment in the early days of life. For some of them the chemical sense and the simple tactile faculty are sufficient.

Others, on the other hand, soon developed a momentous innovation: pigments that respond to light.

Whether these dyes initially only serve to protect them from too much sunlight - especially the destructive UV rays - can no longer be determined. What is clear, however, is that some protozoa manage to use the light-sensitive pigments to determine where it is light and where it is dark. It is the hour of birth of a new meaning that will one day prove to be the most important of all: eyesight.

developed until the first eye, but goes much time. Initially, the simple single-celled organisms dominated the seas for hundreds of millions of years.

Then, two billion years ago, completely new forms of life appeared. From today's perspective, they may seem unspectacular, because they also consist of just a single cell. But they are a hundred times larger than the bacteria and have a much more complicated structure.

Their genetic material is enclosed in a cell nucleus, and their inner workings have many separate areas - just like a large factory consists of several departments in which planning and designing or something else is manufactured in each case.

And their sensory performances also achieve new qualities. Because they have a large number of chemoreceptors that are distributed over their comparatively large organism, they can, for example, register when there are more nutrients on one side of their body than on the other.

So you can figure out the direction a flavor is coming from and then swim towards it in a targeted manner.

Some of them are already helped by effective means of locomotion - such as fine, hair-like cell extensions that densely cover the elongated body and are capable of coordinated, undulating movements. The single-celled organisms can reverse the direction of stroke of these tiny oars and swim backwards, for example to avoid an obstacle, escape an enemy or avoid a pollutant. You can also let the hairs beat differently on both sides and turn your body around in this way.

These are all amazing sensory performances and behaviors that allow even a single-cell organism to orientate itself in its world, to find food and to react to threats.

What is even more astonishing, however, is that in this early phase of life those basic principles developed according to which the sensory functions of much more complex living beings will later function:

• A receptor molecule on the unicellular shell registers a stimulus from the outside world - such as a flavor that binds to the molecule, mechanical contact or incident light. This stimulus triggers a change where the receptor is located; an electrical signal is generated.

• This electrical signal is propagated through a complex biochemical reaction along the cell envelope.

• The signal causes a reaction, such as a change in behavior. For example, the direction of stroke of the oar hairs of a unicellular cell is reversed. It is this combination of reception of a sensory stimulus, signal transmission and a change in behavior that will be decisive for the survival of a huge group of living beings.

The animals.

You can read the entire article in GEOkompakt issue No. 36 "Our senses".

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