Evolutionary advancement in the animal kingdom

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It is not uncommon for people around this side of the world to call each other 'animal' in the process of trading abuses and insults. Whenever I witness such situation, one question always pops into my scientific mind - aren't we all animals? Yeah, you read that right. All humans are animal, if we want to put a kind of respect on it, we will call ourselves higher animals. What do you think makes us higher?

^{Public Domain, Link}

When it comes to classification of living organisms, the animal kingdom makes up one of the four eukaryote kingdoms. All living organisms in the kingdom Animalia are multicellular, the animal-like unicellular organisms have been placed in a separate kingdom - Protista. Biological organisms in animal kingdoms are quite unique and differ from those in plant kingdom in that they are heterotrophic, rather than being autotrophic. They are also unique from organisms in the fungi kingdom (which happen to be also multicellular and heterotrophic eukaryotes) in the way they obtain their foods.

While animals feed by ingesting their foods, digest such within the body and then egest undigested portion from their bodies, fungi digest their own foods outside their bodies by secreting digestive enzymes on the food and then absorb the digested food into their system. Furthermore, while fungi typically grow on their foods, animals have to move around in the quest of seeking their own foods, making locomotion important characteristics of the organisms in the animal kingdom.

Coming back to the question of why humans are respectfully described as higher animals, the answer lies in evolution. A whole lot of evolutionary advancement exist within the animal kingdom. When it comes to climbing the ladder of evolution, human beings happen to be at the highest end of it. A number of features exist within the animals which makes one group of animals to be more advanced than another. For the records, the animal kingdom is divided into several sub-groups or phyla. Each phylum has some features that makes it more advanced than the other. In order of advancement, the animal kingdom is divided into the following phyla:

• Porifera simply known as the sponges
• Cnidaria or Coelentarata
• Platyhelminthes/flatworms
• Nematoda or the roundworms
• Anelida or the earthworms
• Arthropoda
• Mollusca
• Echinodermata
• Chordata, the group that humans belong to.

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In what ways do these animal groups show advancement over each other?

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One of the evolutionary trends in the kingdom is the level of complexity of the animal's body. The simplest of them all, the sponges are just aggregates of cells with little or no interraction, that is they are not unicellular neither are they re multicellula but do not form tissues unlike other group of animals. Tissues are group of cells of similar origin and structure functioning as a unit in a process known as differentiation or division of labour. Thus, higher efficiency is achieved with the differentiation of the cells. At higher level of organisation, various groups of tissues aggregate and work together to form different organs of the body while the body of the most complex group of organisms is made up of several organs working together to form what is known as the system.

Along with the evolutionary development of tissues, organs and systems comes a lot of co-evolutionary advancements. Two body layers in certain animals were replaced with three body layers in others to accomodate more complexity level. While the former are said to be diploblastic with outer ectoderm and inner endoderm, the latter are said to be triploblastic with mesoderm forming in between the endoderm and ectoderm. In some organisms like the flatworms, the mesoderm completely fills the space between the ectodermal and the endodermal layer of the body. Such organisms are collectively known as acoelomates, that is, animals without coeloms. On the other hand, in more advanced animals like annelids, an extensive internal space or body cavity develops in between the ectoderm and the endoderm layer. Such organism are referred to coelomates and the cavity is specifically known as the coelom.

^{By CNX OpenStax - CC BY 4.0, Link}

The simplest animals like the sponges have no specific body symmetry. Animals like the cnidarians are radially symmetrical. The radial body plan is built along a single axis which other body parts attach to. Hence, the whole body can be divided into two identical halves along any plane or line of division. Radial symmetry also tends to be associated with organisms which do not show locomotion. However, majority of animals are bilaterally symmetrical; a body plan that is associated with locomotion, gives a more compact and streamlined shape and allows greater specialization of body parts. Animals with bilateral body symmetry can be divided into identical halves along a single plane only, as opposed to those with radial body symmetry that can be divided along several planes. Thus, the bilateria have dorsal(upper), ventral(side), anterior(front) and posterior(back) views.

^{By CNX OpenStax - CC BY 4.0, Link}

Even within each group or animal phylum, a variation of evolutionary advancement do exist. For example, some coelomates have metameric segmentation which is the transverse division of the body into a series of similar semi-independent parts or segments. The origin of the segments is within the mesodermal layer of the body and its usually associated with both mesoderm and ectoderm. Metameric segmentation is best illustrated in the earthworms in which the series of divisions distinctly appear externally as constrictions along the body surface. Each segment of the organism is bodered from the other by septa that extends across the entire coelom. Additionally, each segment contain its own muscles, blood vessels, nerve cells and reproductive organs in some cases. However, the nervous system and the blood system run through the entire length of the body.

^{An earthworm showing metameric segmentaion By David Perez - Vlastito djelo, CC BY 3.0, Link}

The development of the head otherwise known as cephalization is also an important evolutionary feature among animals. The head is totally lacking in some groups of animals while it is poorly developed in some. The most evolutionarily advanced group of animals have well developed head with brain.

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References

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origin and evolution of animals

Coelom: Features, Types and Mode of Transmission

symmetry in biology

animal kingdom

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