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Biology 2nd Baccalaureate

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4.2. Evolution of living beings

The evolution of living beings

But, after life appeared on Earth, how did all species arise from a single cell? There have also been different theories throughout the history of mankind, although they can be divided into two groups:

Fixist theories

Until the nineteenth century it was believed that living beings that were known had always existed, without having any change.

The two predominant theories of the time were creationism and fixism.

  • Creationism: living beings have been created by God and, therefore, are not the result of evolution. Today, creationism as a theory is still studied in some schools in the United States on the same level as the theory of evolution.
  • Fixism: the species that exist today have remained unchanged, without evolving, since their appearance.

Both ideas postulated that all species of living beings appeared at the same time on Earth, were created by God and have remained unchanged throughout all time.

These ideas were called into question when the remains of fossilized organisms very different from those that exist today were discovered. George Cuvier, fixer, and founder of paleontology, observing the large number of fossils, deduced that species appeared on Earth, remained for a time without having any change, and disappeared when a catastrophe occurred, and so on, they appeared and they disappeared. One of these catastrophes could have been the universal flood, for example. In this way, a variant of fixism emerged called catastrophism.

Humor: Creationism in the Simpsons.

Humor: Evolution according to "Family Guy".

Evolutionary theories

Although there were some thinkers who questioned fixist ideas, it was not until the 19th century that they were most successful. Some of the early evolutionists were:

  • Anaximander (5th century BC). He defended that some species evolved from others. As for example, land animals came from fish .
  • Georges-Louis Leclerc, Count of Buffon (1707-1778). The species could undergo changes according to the environmental conditions, giving rise to hybrids.
  • Erasmus Darwin (1731-1802), grandfather of Charles Darwin. Some species are transformed into others according to the needs imposed by the environment.

Fixism was deeply ingrained for several reasons:

  • Evolution is a very slow process, invisible to the naked eye.
  • We did not have the scientific knowledge about the creation of new species that we have today.
  • Evolutionary theories went against religious doctrines. We must highlight the work of the theologian Pierre Teilhard de Chardin, at the beginning of the 20th century, who managed to reconcile evolutionary theories with the dogmas of the Catholic Church.

Biological evolution consists of the gradual and progressive transformation of some species into more differentiated and complex ones. Evolution has taken place over long periods of time and has given rise to the current diversity of living beings.

Currently around 1.9 million species of living beings are known, although thousands of new species are discovered each year, so it is estimated that there may be between 5 and 50 million different species.


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Biology and Geology teaching materials for Compulsory Secondary Education (ESO) and Baccalaureate students.