Invertebrates are mainly associated with simple organisms that can be found literally everywhere, including in our homes. How did they evolve? Let’s check.
It should be emphasized that the diversity of invertebrates can be astonishing. And it’s not just about differences in body shape and size, but also internal structure and reproduction. They managed to conquer all continents, oceans and seas. They were the first to populate the land, and scientists have no doubt that they will remain on Earth beyond the age of human rule. What characterizes these organisms?
Not everyone is aware of this the world of invertebrates has more than a million described species, and some say that the kingdom of these animals may be a hundred times greater. This means that at least 97 percent of known species are invertebrates† Their adaptation to the environment is very impressive – they can survive in the most unfavorable places where no other organisms can live. What else do we know about them?
What exactly are invertebrates? This concept should be understood as organisms without an internal skeleton. It is this factor that determines belonging to this group. In addition, they are characterized by a multicellular structure, and in terms of body structure – by two symmetries. The first is symmetry radiantcharacteristic of stingrays, ribs and echinoderms, and the other – double sidedwhich is typical of other invertebrates.
The great diversity of invertebrates means that in this case we cannot speak of a common structure of the body, internal organs or the same biology of life.
Invertebrates have mastered each of the Earth’s ecosystems and today they are an integral part of it. This is due to an extraordinary evolutionary adaptation. The importance of invertebrates in any environment is enormous and it can be said without a doubt that without them there would be no life. It is these animals that pollinate plants, control plant and animal populations, provide food for animals and humans, and loosen the soil. And this is only a fraction of the merit that can be attributed to these organisms.
The first living organisms developed in water, that is, in the environment where the best conditions exist. When plants appeared on the mainland, the animals were finally able to leave the sea, creating a whole new ecosystem. At that time, the conditions for further development were the best – the organisms that conquered land did not have to compete for food. There was an abundance of plant material, so reproduction went smoothly.
However, the process of mastering the mainland was not an easy one. The animals had to develop completely new mechanisms that they did not need in the aquatic environment. I’m talking about hard armor, the job of which was to isolate the water in the system. However, such a coating made gas exchange impossible, so it was necessary to develop a mechanism that would allow oxygen absorption from the air – spiracles.
It is not known which animal left the water first. The first fossils left arthropods, but traces of land movement dating back 500 million years have been discovered in China. However, it is unclear what creature they belong to. For now, therefore, we must stick to the proposition about the priority of arthropods, and more specifically – herbivorous millipedes and other twigs. However, these primitive organisms did not resemble modern ones. The largest were up to 2 meters long.
Although the millipedes were the first, they did not take over the planet. It was done by insects, which could consume a variety of foods and adapt to life in almost any environment. The oldest fossils have left wingless insects. Their tracks go back as far as 396 million years.
While the first phase of land colonization went smoothly, in time the predatory twins appeared. Contrary to appearances, the first fighters did not have an easy task. Eating meat was hindered by a mechanism developed by herbivores to retain water in the body – a thick armour. The victims were also not easily digestible. However, evolution found an answer to this problem – in a fairly short time, predators developed poison spikes that were used for the application of digestive enzymes.
Scorpions were also early land predators. 300 million years ago, these arachnids became advanced organisms, reaching 1 m in length. More or less in the same period, other representatives of arachnids – mites appeared.
The constantly growing population of predators forced herbivores to look for a new, safe place. It turned out to be the air environment. The dragonflies were the first to leave the ground. The wings they formed reached a wingspan of 70 cm. However, it was not a perfect mechanism, as the repeated molting process still exposed them to attack. In Late Carbon, almost all insects managed to eliminate moult in the flying stage.
When the first vertebrates appeared on our planet, the insects had to adapt to life under the bark of trees or in the litter. To do this, they had to learn to fold their wings. Beetles were one of the few invertebrates that remained in the open.
Time of Mesozoic changes
The next major evolutionary step was taken by invertebrates in the Late Cretaceous, when angiosperms appeared on our planet, producing the perianth and nectar to attract insects. Due to this, the pollen was transported – insects, flying from plant to plant, transferred it. This process continues to this day and largely determines life on Earth.
The variety of invertebrates is enormous. This group includes zooplankton, invisible to the naked eye, stinging beetles and echinoderms that live in the seabed, arthropods (including the largest spiders in the world, such as the Goliath tarantula) and insects that we deal with every day. There are also some giants in the invertebrates kingdom. An example is a giant squid, much larger than the longest snakes in the world. It should be clearly emphasized that so far science has discovered only a fraction of the existing invertebrate species† There are still more waiting to be discovered.