A plesiomorphy It is the primitive or ancestral form of an organism, that is, its anatomy. In addition to morphological plesiomorphy, we also speak of genetic plesiomorphy; the genetic characteristics of ancestral living beings.
From animal fossils, bone comparisons are made with other living or extinct animals and possible evolutionary relationships between them are sought. With the development of molecular biology, comparisons can also be made with molecular markers (DNA sequences, chromosome analysis).
Traditionally, the taxonomy has been carried out with morphological characters, since the closer two species are phylogenetically, their morphological similarity should be greater.
Ancestral morphological markers can derive, through evolution, in different ways with appropriate functions for the adaptation of a given organism to the environment where it inhabits..
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Most mammalian limbs show the plesiomorphic morphology of five metacarpal bones and the "fingers" with a maximum of three phalanges each..
This characteristic is very conserved, however, there are notable differences with the human hand. The “hand” of cetaceans presents bone and soft tissue innovations which have resulted in a fin, with a greater number of phalanges.
Certain dolphins can have between 11-12 phalanges on a single "finger." This morphological change allows dolphins to adapt to their aquatic environment. The presence of a fin and the elongation of the phalanges, effectively, increases the surface of the dolphin hand.
This makes it easier for the animal to control its movements so that it moves in the right direction, counteracts its body weight and increases resistance when it wants to stop..
On the other hand, bats reduced the number of phalanges, but extended their length, which allows them to support the membrane of their wings. These wings act as a control surface so that the takeoff and the forces to balance the flight are optimal..
Other land mammals, such as the horse and the camel, lack phalanges, which allows them to increase their speed of locomotion..
Other studies have shown that the anatomical plesiomorphy also changes in the muscles of the neck, pectorals, head and lower extremities of some animals such as salamander, lizards, primates, among others..
In this regard, it is interesting to note that humans have accumulated more evolutionary changes than any other primate studied, but this does not mean an increase in their musculature.
On the contrary, these changes have led to a complete loss of some muscles and thus, the human musculature is much simpler than that of other primates.
From the above it is deduced that ancestral characters can be maintained or disappear in different species over time. Therefore, classifying organisms in the same species just because they possess a certain character is wrong.
That is, it may happen that an ancestral character is initially shared by several species. Then evolution separates the species, which may or may not have the ancestral character.
For example, humans and iguanas have five toes, but they are different species. Likewise, mammary glands are present in different mammals, but not all of them belong to the same species. Classifying in this wrong way is known as simpleiomorphy..
The classifications of living beings, according to their degree of complexity, have been done since Ancient Greece. Aristotle and his school were the first to systematically study nature with the aim of scientifically classifying the biological world.
Aristotle placed plants below animals because the latter could move, which was considered a very complex behavior.
Still, within the animals themselves, the Greek philosopher classified them according to a scale of complexity that was based on the presence or absence of blood or the type of reproduction.
This classification, progressively linear or scala naturae called "natural staircase" places the minerals, because they have no life, on the lowest rung of the ladder. According to religion, God would be in the superior position, which would lead the human being to climb the ladder in search of perfection
There is great diversity among living beings and over time it has been tried to describe and interpret. In 1859, it came to light The origin of species by Charles Darwin, who postulated that the existence of living beings has a unique origin.
Likewise, Darwin considered that between ancestors and descendants there was a time-dependent association. Darwin put it this way:
“We do not have pedigrees or coats of arms; we have to discover and trace the many divergent lines of descent in our natural genealogies from characters of any kind that have been inherited for a long time. ".
This idea was represented as a single root tree with different branches that in turn were separated into more branches from common nodes.
This hypothesis that frames the interaction between different organisms is represented as a phylogenetic tree and from then on, the classification of living beings has been carried out through phylogenetic relationships. This gives rise to the emergence of the aystematic subdiscipline that includes evolutionary taxonomy or phylogeny..
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