Some of the issues related to animals are usually linked to certain controversies because, eventually, the evidence is not sufficient or clear to establish definitive positions, as occurs, for example, in certain cases with taxonomy. But probably one of the most controversial and complex aspects is that related to the evolution of animals.
Without a doubt, it is not easy to take for granted how the different processes that triggered the existence of so much diversity of these living beings occurred. However, after years of in-depth studies and with a lot of dedication, scientists in the world have tried to offer us an overview on this subject and, although much remains to be understood and discovered about it, in this article on our site we want to present some generalities about the origin and evolution of animals We invite you once again to join us to continue reading about this interesting topic.
Origin of Animals
The origin of life is a complex process linked, in turn, to a dynamic of many aspects, such as chemical, physical, geological, atmospheric and obviously biological. In this way, the above leads us to argue that the origin of animals is inescapably intertwined with the very emergence of life on the planet. In this sense, from the first life forms, which were characterized by being unicellular, anaerobic and prokaryotic, after many transformations through time, eukaryotic cell forms were originated. For this, according to some positions [1], the process took place, among others, based on the theory of endosymbiosis , which generally refers to the possibility of new structures, organisms or species originating through symbiotic associations that last over time. This would give rise to the appearance of eukaryotic cells, which later led to the first multicellular beings, from which would give rise to the first animal Phylum.
The ancestors of animals (metazoans) are found in the protists, having the first an impressive explosion of diversity, which according to the fossil record [2] occurred in an event known as the Cambrian explosion, which it developed between about 570 million years ago and about 530 million years ago (early Cambrian). During this event gave rise to what some call a big bang zoo , since the various groups or phyla of animals that we currently know arose, such as annelids, molluscs, arthropods, echinoderms, chordates, among others that still exist, in addition to many that disappeared.
The beginning of this explosion of diversity that occurs in the Paleozoic (within which the Cambrian period is found), occurs around the development of marine life, which spreads relatively rapidly in the Cambrian and Ordovician. Extinct marine animals such as the trilobites were dominant in the first period mentioned, while in the second the brachiopods (lamp shells) had a greater role.
Evolution of Animals
If the origin of animals is already a highly complex process, their subsequent evolution is not far from this aspect. The evolutionary novelties in the animal kingdom are associated with genetic changes and adaptive-type processes, which, without a doubt, promoted the emergence of the various forms of life. Then, multiplication processes occurred and, therefore, the different groups diversified evolutionarily.
In the ancestors of metazoans there were already certain genes that had an impact on multicellularity and also on the development of animals. In this sense, the function of certain proteins that today are suggested as belonging to animals must have played a key role in their evolution. On the other hand, phylogenomic studies have suggested that, although there are certain doubts inherent to this entire evolutionary framework, it is known that various unicellular and eukaryotic forms, such as choanoflagellates, lineages of Capsaspora and Ichthyosporea, are closely related to animals, since that are part of their unicellular ancestor.
Evolution from aquatic to terrestrial animals
Once animal life in the sea had diversified, came the conquest of the terrestrial environment, since the latter is reported to be devoid of simple life forms in the early Paleozoic. In this way, it is later that the adaptation to life on earth begins. The occurrence of some events allowed a development of animals from the sea to the land, thus, for example, the presence of oxygen levels similar to the current ones and the protection of solar radiation from the formation of the ozone layer offered ecological conditions for transition
The first land animals were invertebrates, then vertebrates joined this adventure, which was started by amphibians. The fossil record reveals that the extinct genera identified as Ichthyostega and Acanthostega were the first terrestrial vertebrates, although in the case of the first it is located intermediate between a fish and an amphibian that had legs, but not as efficient to move on land.
For this entire process, evolution undoubtedly played a determining role, since it was necessary to development of adaptations that would allow animals to be able to live on land, for which they would require particular anatomical characteristics to breathe, move, reproduce, feed and, ultimately, to be able to live outside the aquatic environment.
Evolution of invertebrate animals
Invertebrate animals were the first to make the transition from water to land. Myriapods such as centipedes and millipedes, which are descended from crustaceans, became the first group to conquer land, in fact, they were huge animals compared to their current relatives, with dimensions of around two meters. On the other hand, the marine scorpions gave rise to the terrestrial ones, and the latter had an ecological role of predation on the aforementioned myriapods.
In the Carboniferous another particular event occurs related to the evolution of invertebrate animals, and that is that land animals can fly from the development of wings by insects, so these were the first to perform this new action on earth.
The evolution of invertebrates entailed a complex process for the development of an extensive diversity of life. Thus, over time, animals with different types of symmetry, absence of bony skeletons, hydrostatic structures, in some cases hardened covers known as exoskeletons, in others the formation of shells, etc., emerged within the group. In short, adaptations that allowed them to conquer practically all the habitats on the planet.
Evolution of Vertebrate Animals
As for vertebrates, they had representatives in the marine environment with bony fish, but it is through the evolution of amphibians, who come from crossopterygian fish, which already breathed air in the Devonian, when vertebrates begin to develop on solid ground. Vertebrate animals had structures adapted for marine life, then they had to develop others for a new challenge: living out of the water.
In this sense, it was necessary to be able to avoid drying out, optimize breathing on land and the possibility of moving around in this environment. However, the independence of animals from the humid environment actually took place in the Carboniferous period, when animals of reptilian origin developed shell eggs, which provided them with the necessary protection to the embryos to stay away from the water. The presence of the scales protected their bodies from exposure to wind and sun.
On the other hand, records indicate that the transformation of the fleshy fins of ancestral fish such as the Sarcopterygiansgave the formation of legs , so these are estimated to have been the ancestors of the first tetrapods (currently represented by four-limbed animals that include all amphibians, reptiles, birds and mammals). This has been understood from the identification of the fin bones of the aforementioned fish, which have a homology with the bone system of the legs in current tetrapods. Additionally, it is known that the same genes that are involved in the formation of leg bones are also involved in the formation of fins.
Other evolutionary traits in vertebrate animals that supported the transition from water to land, in addition to those mentioned, were the transformation of the middle earto perceive sounds through the air, as well as the independence of the head from the rest of the body, so certain bones would no longer be fused and it could move more freely, a relevant aspect for the terrestrial environment.
Examples of animal evolution
In addition to some cases already mentioned, let's learn about other particular examples of animal evolution:
- The first fish were small in size, without jaws and finsHis protection consisted of a framework of bony plates. Evolution induced the formation of toothed jaws, transformation of the aforementioned plates into scales, emergence of lateral fins and swim bladder.
- Birds arose in the Jurassic from tetrapod reptiles, dinosaurs characterized by being bipedal and carnivorous. Among other things, the fossil evidence of these feathered reptiles proves this relationship. Thus, in some way, birds would be the current dinosaurs. Find out more about these incredible animals in this other article: "Types of dinosaurs that existed". Likewise, we invite you to learn why the dinosaurs became extinct.
- It is estimated that mammals originated from the Therapsids, formerly called mammalian reptiles, which ranged from the size of a mouse to that of a a hippopotamus.
- Domestic animals originated from prolonged interaction with human beings, for example, the dog descends from the wolf, cats from the wild cat, chickens from the red jungle fowl, among many others examples.
What were the first animals?
Although with opposing positions, some evidence [3]suggests that, since sponges (Phylum Porifera) are the most known basic species and the presence of the metazoans that correspond to the Animalia kingdom, are sea sponges the first animals that populated the Earth, which makes them the ancestors of the kingdom. In addition, one aspect that is consistent with the explosion of animal diversity in the oceans is that the oldest fossils of marine sponges date back to the Cambrian.
It is from the Precambrian biota, also known as Ediacaran, that the transformation from unicellular to multicellular forms occurs, which then dominate the dynamics of the planet. Although there is still much to know about it, about 140 genera have been identified, but it remains to be determined whether they were animals, fungi, algae or lichens, among others. However, in some fossils it has been possible to establish links with the group in question, as is the case of Dickinsonia, where the presence of a lipid exclusive to animals such as cholesterol was identified. Another case is that of Kimberella, which had bilateral symmetry and is considered a possible ancestor of molluscs.
Curiosities about the origin and evolution of animals
Since the aforementioned is a summary of the evolution of animals, we end with some more curious facts:
- Many ancient groups became extinct without leaving representatives, which makes it difficult in some cases to specify details about the origin and evolution of animals.
- From certain studies [4]it was possible to show that 55% of the genes present in the genome of human beings it was already found in the first animal, which, although its appearance is unknown, its genome was identified.
- It has been estimated that there would be about 7 770 000 species of terrestrial animals on Earth and 2,150,000 animals in the oceans, of which only approximately 953,434 terrestrial and 171,082 marine species have been described, which, without a doubt, makes clear the great evolutionary event that this kingdom [5]
- Although animals are not characterized by photosynthesis, the diversity of this kingdom is such that there are certain exceptions, due to the incorporation of functional chloroplasts into their organism, an event that is undoubtedly characteristic of evolution.
- Finally, the presence of human beings on Earth has had a significant impact on the evolution of animals, due to the use of science with genetic manipulation and domestication of these, so their future is definitely influenced by us.
