Wie entwickelten sich komplexe Strukturen wie z.B. ein Arm durch die Evolution?

The limbs of the terrestrial vertebrates (Tetrapoda) were created as well as the snags of birds and like everything else in the lively nature – in many small steps over generations.

Evolution is basically not a targeted process. She thinks and she doesn’t distract, she just happens. Evolution depends on the random mutations that create new variants of genes (so-called allele). The genes in turn lead to changes in the phenotype. In principle, a mutation is not “good” or “bad”, it is simply there. In order for a mutation to have an adaptation value, the natural selection still has to come into play. This depends on the prevailing environmental conditions. Only the environment decides whether a newly occurring mutation means a selection advantage for its carrier, has an adverse effect on its survival or whether it is neutral. New genes and the structures based on them are thus initially random and not intended. In evolutionary biology one speaks of so-called. Exaptation. A feature X thus arises notto do a thing Y. Lung caused notto breathe on land. And arms and legs notto run on land. None of the ancestors of the terrestrial vertebrates could finally know that one of his descendants would live many, many generations later on land.

In addition, legs, arms and lungs were a compelling prerequisite for the land vertebrates. Going to land and then developing legs and arms and lungs would not have worked because a land animal could not have survived without these characteristics on land. This means that these features must have already emerged beforehand, even while the ancestors of the land swirls lived in the water as fish-like beings.

We therefore have to ask ourselves what the value of the adjustment (Adaptation value) these structures must have existed in the water, which is why lungs and legs in the water should have had a higher survival success.

Let’s start with the lungs. If you go for a walk in the summer when it’s really hot, and you’ll come past a carp pond, stop for a few minutes and watch what happens on the surface of the water. What happens? From time to time, you’ll see one of the carp popping up and catching up. The fish actually do so that they do not suffocate. Because in summer the water temperature rises. With increasing temperature, the solubility of oxygen continues to decrease. At 0° C. it lies e.g. at 14.6 mg/l, at 20° C. only at 9.1 mg/l. The oxygen content in the water thus decreases sharply in the summer. The fish then additionally absorb atmospheric oxygen, which they absorb via the intestinal mucosa, so they breathe air with their intestine.

In fact, the lung is formed from a sacking of the front intestine. This sacking was certainly tiny at the beginning, but it increased the surface over which oxygen could be absorbed. It was a survival advantage to the individuals who did not have this sacking. The advantage was marginal, but he was big enough that these animals survived and were more successful. Step by step, the sacking became bigger and finally became a first lung. This must have happened very early in evolution, because very likely lungs belong to the Grubdbauplan of all Osteognathostomata (these are the “bone fish” including the land swirls). The lungs were an advantage in the water because, in times of oxygen scarcity, atmospheric oxygen could also be breathed. The Lung fish (Dipnoi), who are the closest relatives of mammals, make it so today. The genus of African lungs (protopterus) is thus even able to survive in ephemeral waters (i.e. water which does not permanently lead water). As soon as the river or tulle in which they live dry out, they dig in the mud, wrap themselves in a mucosacon and, thanks to their ability to breathe with their lungs, can even survive several years of dryness, as long as it rains again and its tulle fills up again.

In the course of evolution, many fish species lost their lungs or more precisely: they worked around them. The same plant, from which the lungs develop in the case of aerial vertebrates, is used for the fish Floating bubble. This must have happened twice independently, namely in the case of disturbances and in the case of actual bone fish (Teleostei). The origin of the floating bubble can be seen from the fact that in most species the floating bubble is still connected to the intestine via a channel. Some fish like the Arapaimas can even breathe with the swimming bubble. However, it mainly serves to regulate the buoyancy.

Thus, the ancestors of the terrestrial vertebrates already had lungs and they used them to survive periods of shortage of oxygen or drying out their tulip. Finally, the vertebrates were able to rework the lungs to survive on land. Such a feature that is later “reacted” is also called Preadaptation.

Now we look at the formation of the limbs. The land swirls belong to the family group of the Meat flakes (Sarcopterygii). In addition to the already mentioned pulmonary fish, the Quastenfloes (Coelacanthimorpha or Actinistia) into this group. The characteristic feature of the sarcoptery greeds are their “meaty fins” which already reveal similarities to the extremities of the tetrapodes. When in the 1980s Hans Fricke succeeded for the first time, the Komoren-Quastenflosser (Latimeria chalumna) to film, it was found that they already use their fins in the crossroads for the forward movement. This is probably an energy-saving way of moving. Possibly, the ancestors of the terrestrial vertebrates with their fins could also “run” energy-saving over the soil of the water (the two today’s quast flora species do not do this).

The ability to crawl with the fins over the bottom of the water may have been another advantage. Like the African pulmonary fish, the agriturismos probably also lived in ephemeral waters. When a tulle dries out, they could perhaps pull a few hundred meters far over land to reach another, not yet dried, tulle. Gradually, legs and arms developed out of the fins, with which the movement on land was even easier.

The landing of the tetrapodes was thus gradual over many intermediate steps. Each individual intermediate step was advantageous at the respective time of development. This process is also demonstrated by a whole series of former meat fossils and evolutionary “binding elements” (Bridge fossils), for example Tiktaalik or Ichthyostega.