What is and how is the scale of geological time divided?

The geological time scale establishes the main biological events that have occurred through time. To understand the bases on which it was built, it is necessary to make some clarifications beforehand.

In the first place, to determine a historical sequence, there must be some element that indicates change. The elements that are repeated are not useful to determine the possible divisions and subdivisions of a scale, therefore, it is necessary to find "something" that arises at a certain moment, is maintained for a certain time, and then disappears and does not reappear (at least not as it was, not with the same morphology). In the particular case of the study of the history of life, an element that satisfies these characteristics and that can indicate the passage of time, are fossils.

Secondly, it is necessary to clarify that not all fossils are useful for this study. For this, it is necessary that the fossil appears in the record for a sufficiently long time to be considered characteristic of the time and, in addition, that it is widespread, that is, in numerous and diverse areas and not only in one region. Fossils that meet these characteristics are called index fossils and are extremely important because they make it possible to establish the necessary correlations between time and space.

It is one thing to be aware of time and another to elucidate the events that have occurred in its course.

The idea that the study of rocks and fossils found in the different strata of the earth's crust could serve to know the history of life was the product of a long process to which many scientists contributed. In the second half of the 15th century, there were already classifications of rock sequences produced in the field of mineralogy, but due to the repetition of some elements, it was not possible to establish a chronology.

In France, the naturalist George Cuvier studied in depth and detail the anatomy and functioning of living organisms and fossils (virtually establishing the study of modern paleontology) and acquired a thorough knowledge of them from which he developed the concept called correlation of parts. He considered animals to be perfect machines in which there is total integration and interdependence among all their constituent elements and in which the breakdown of any one of them causes the organism to no longer function properly.

According to Cuvier, since there is such an intimate correlation between all the parts of the organism when a change occurs in one of them, the whole body would have to be modified, which would be impossible for its survival; therefore, since an organism cannot change in a given environment, it perishes (it is understood that this does not happen with a single organism but with a species). From this idea, Cuvier introduces the concept of extinction.

Since the end of the 15th century -indeed since natural history began to be studied as a science-, the existence or not of extinction had been a matter of concern. Religion still had great weight in scientific research, so many scholars found it hard to believe that God had created a species and then eliminated it. Cuvier, through his research, established that extinction is a reality. Once it is accepted as a fact, fossils begin to be studied differently. They represent organisms that lived on Earth but no longer exist.

During the study of fossils, Cuvier explains extinction based on his theory of the perfect machine. The idea that when any organ changes, the animal can no longer live and suddenly disappears was supported by the evidence that in one layer of rocks certain types of fossils are found, which disappear completely in the next layer, in which the remains of different living beings are found.

Cuvier then spoke of catastrophes or revolutions, of sudden changes - which can be better understood if we take into account that he witnessed the horrors of the French Revolution. He pointed out that, surely, drastic changes in climate, volcanic eruptions, and other events destroy flora and fauna at a certain time and that subsequently, a repopulation begins. For Cuvier, the world is a game of creation, repopulation, and catastrophes.

Cuvier -together with Alexandre Brogniart- published an article in which he explained the above and in which we can say that the seed of the geological time scale appears. Both were able to determine the fossil sequence in the region of France in which they were working. This was possible thanks to the fact that the remains of living beings that they found in one stratum, in one type of rock, did not reappear in any of the following ones. Such clues indicated that these beings had lived at one time and then disappeared forever, which allowed them to use the fossil record as a criterion of time.

In other parts of Europe, other scholars began to do similar work, and their findings coincided with Cuviers. For example, William Smith produced the first geological map of an area of England. Unfortunately, due to his precarious economic situation, he was not recognized by other scientists, and his work was not given the relevance it deserved at that time.

Gradually, over time, scholars exchanged information and this allowed them to understand the importance of rock and fossil sequences. When working in isolation, naturalists thought that these were regional, but when they put the information together they realized that they range from the British Isles to Moscow and are found all over the world. Given this evidence, they began to name the formations that are characterized by a particular fossil population. The names by which they are known today will be given only in the third decade of the 19th century.

The first formal scale of geological time

John Philips, the nephew of William Smith, organized the sequences in a table. In it, periods spanning major events in the fossil record-which include rocks in which there are no drastic changes-were given the name eras. The separation of the different eras was not an arbitrary division but was based on the sequence of the known fossil record up to that time. The scale for measuring geologic time appeared at a time when geology, which had been born some 20 years earlier, was beginning to establish itself as a science and was in full growth.

How to read the geological time scale

The currently accepted scale is divided into three major stages or time marks called eras. Eras are subdivided into periods and these in turn are subdivided into epochs. The currently accepted eras are:

Paleozoic
Mesozoic
Cenozoic

Each of these divisions represents extinctions, disappearances of characteristic groups, and appearances of new groups. The changes in the era represent massive, large-scale extinctions. For example, it is estimated that in the last period of the Paleozoic era, called the Permian, about 96% of species were lost. At the end of the Cretaceous, in the Mesozoic Era, dinosaurs and many other types of living things became extinct, thus ending the period and era.

These mass extinctions do not imply the disappearance of all species (which would have meant that all life on the planet would have disappeared), many of them remained with mu- diffused descendants. Those that disappeared become extinct, never to reappear again. It is important to explain this with an example: dinosaurs belong to an order of the reptile class that became extinct 65 million years ago, however not all reptiles became extinct, and many of their modified descendants are alive today, such as snakes, lizards, and crocodiles.

The groups that characterize an era are called "dominant" because they are so in number and extent; they are groups that occur on a large scale during an entire epoch and are of great ecological importance. When one of these groups disappears, many others that depended on it also disappear, while others are favored and, with the extinction of the former, are left free, some of them becoming, in turn, dominant. For example, coexisting with the dinosaurs were small mammals that competed with them for space. When the dinosaurs disappeared, space was left empty and mammals proliferated in the following era.

It is very important not to lose sight of the fact that in all the biological history of the Earth there are elements of continuity. All organisms are linked by history, by genealogy; there is no spontaneous generation. All beings have an ancestor, all species descend from other species. When we say, for example, that mammals appear in the Mesozoic era, it should not be understood that they appear just like that, suddenly. Mammals are descendants of a certain type of reptile, in the same way, that dinosaurs come from other more primitive reptiles; birds, in turn, are also descended from reptiles.

The geological time scale allows us to know the history of life on the planet, or at least the most important transformations because in every historical interpretation there is always some distortion.

Caution should be exercised when looking at a scheme of the geological time scale because it sometimes lends itself to some confusion. One of the most common is due to the consideration of living beings as something static. For example, fish appeared in the Ordovician period of the Paleozoic era, and in most of the scales they are placed there, but in the rest of the scales, a fish does not appear again.

This may lead many non-scholars to think that fish have disappeared or that they have not undergone any change. But the typical Paleozoic fish groups were the ostracoderms and placoderms, which disappeared at the end of that era. In different later periods, other types of fish appeared, which is not indicated in almost any graph on the time scale.

Similar is the case of bacteria, which appeared about 3500 million years ago and dominated until 500 thousand years ago but have never disappeared, and even today they are the most abundant group of living organisms, they have great biological importance in ecosystems; however, on almost no scale they appear. Something similar happens with fungi.

Eras and periods

The oldest rocks with conspicuous fossils have been classified as belonging to the Cambrian period. Those more archaic have been placed in what is informally known as Precambrian, although the correct name is protozoic for those in which there are traces of the life of some marine invertebrates and archeozoic for those in which there is evidence only of prokaryotic life such as bacteria or cyanophytes.

The concept of the era is applied from the Cambrian onwards since it is in this period that a great profusion of phyla appears in the fossil record. The beginning of this period marks the beginning of the Paleozoic era.

The name of all eras ends with the suffix zoic, which comes from the Greek word zóon, animal, emphasizing the importance of fossils as a criterion for the historical reconstruction of life on Earth.

Paleozoic or Paleozoic Era

It began 570 million years ago and ended 325 million years later. The word Paleozoic derives from two words of Greek origin: paleo, ancient, and zoic, which, as already indicated, means animal; that is, "ancient animals", which can be extended to "ancient life". It consists of six periods: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian.

The criterion for separating the different periods is the same as for the eras but involves events of much smaller magnitude. In other words, between the periods there will also be some groups of living beings that appear and others that disappear.

Cambrian

Its name comes from Cambria, the Roman name for Wales, the country in which is located the region where rocks belonging to this period were first studied. It is characterized by the profusion and importance of trilobites. This type of arthropods almost disappeared at the end of this period, however, some groups remained although in much smaller numbers.

Other types of animals also appeared, many of which were continued in the following period and which represented very novel anatomical designs in the history of life. Among them, are a large number of invertebrates: brachiopods, mollusks (especially small ones), echinoderms, conodonts, ostracods, etc. Many of these animals were not dominant in this period and became important in the following ones. Cnidarians also appeared.

Ordovician

This term comes from the Ordovician, the Latin name for a tribe that occupied the region of Wales at the time of the Romans. The period began 505 million years ago and ended 438 million years ago, or 67 million years. Fossils have been found from this period that show the proliferation of articulated brachiopods, graptolites, conodonts, and bryozoans.

Mollusks diversify, for example, gastropods (snails) and corals. Towards the end of the period, all the phyla that will subsequently exist on Earth appear, such as the chordates, the phylum to which vertebrates belong. Towards the end of the period, the first vertebrates appear, the fish ostracoderms.

Silurian

It begins 438 million years ago and ends 30 million years later. It owes its name to the ancient Silurian people, which inhabited the region of Wales. Fossils indicate the appearance of ammonoids and jawed fish - fish were the only vertebrates - there were also graptolites, some trilobites, brachiopods, some echinoderms, and coelenterates. The first terrestrial plants appeared, which inhabited swampy terrain and reproduced by spores. Plants already existed, but they were all marine, and it was not until the end of this period that they began to invade dry land.

Devonian

It has an approximate duration of 43 million years since it extends from 408 to 360 million years ago. Its name comes from the English county of Devonshire, where fossils of this epoch were found. It is characterized by the progressive colonization of dry land by plants and the continuous appearance of new types. Rhynia, which appeared at the beginning of the period, is the ancestor of the higher vascular plants. It is no longer found in the Middle Devonian.

Towards the end of the period, there are already species that reproduce by seeds. The predominant fossils of this period are wingless insects, ammonoids, fish, and various types of arthropods and mollusks. The appearance of amphibians is one of the main phases in the evolution of vertebrates as it is the step to the colonization of dry land. Ichthyostega, a fossil belonging to this period, is considered the link between fish and amphibians.

Carboniferous

This period began 360 million years ago and ended 286 million years ago. During this period, many plants develop, mainly in swamps, called "coal swamps", pollen appears in the first gymnosperms, and there are many seedless ferns and ferns with seeds. This proliferation of plants is what will form, in a process of millions of years, the coal that is extracted from the earth and that gives its name to the period.

The first flying insects appear (such as dragonflies, some with wings of up to 50 cm) and the first fully terrestrial vertebrates emerge as reptiles, which expand and develop rapidly. Amphibians become larger. In the sea sharks become common and together with some cephalopod mollusks are the large predators.

Permian

It is the last period of the Paleozoic era, lasting 41 million years, starting 286 million years ago. It owes its name to a province in southern Russia. In this period reptiles become more independent of water and can lay their eggs - which already have amniotic fluid - on land. Perhaps the pelycosaurs, with their large sail, were the largest land animals. Some amphibians, such as the Eriops, reached more than one meter in length. There was a diversification of mollusks, reptiles, conifers, ginkgophytes, and gymnosperms. Crinoids and bryozoans also expanded.

Towards the end of the Permian, a great biological crisis occurred, producing a massive extinction that affected various groups, both marine and terrestrial, that had been evolutionarily successful up to that time, such as the ammonoids and the trilobites (which became extinct). Brachiopods, echinoderms, amphibians, and several groups of plants also suffered great devastation.

The Mesozoic or Mesozoic era

This era, which arose after the great mass extinction already mentioned, lasted 180 million years, beginning 245 million years ago and ending 65 million years ago. Its name, derived from meso, middle, and the Greek zoico, refers to the life forms that existed between ancient life (Paleozoic) and more recent life (Cenozoic). It is known as the age of reptiles since it is characterized by their diversification and expansion, and especially by the appearance of dinosaurs. In the plant kingdom, gymnosperms dominate the continental surface of the planet.

During the Mesozoic, there is a slow recovery of many types of living beings that were almost extinct at the end of the Paleozoic. For example, most of the mollusks disappeared at that time, but some groups managed to survive and adapted to the new conditions, so much so that at the end of the Mesozoic, and even in the Cenozoic, they achieved an even greater abundance than they had in the previous era. Something similar happened with the brachiopods, although they did not diversify as much. This era is divided into three periods: Triassic, Jurassic, and Cretaceous.

Triassic

The first period of the Mesozoic Era began 245 million years ago and ended 208 million years ago. The name derives from Triassic, a system of strata found in Germany. During its course, there is a great development of ferns. Initially, there is a scarcity of both marine and terrestrial fauna, but slowly some reptiles begin to expand, among which turtles and pterosaurs stand out. Another group of reptiles, called therapsids, is also of great importance since the first mammals were formed from them. Fossil corals from this period have also been found.

By the end of the Triassic, we can already find the first dinosaurs, which will constitute a group of individuals impressive for their size and size, and the selachian fish (sharks and manta rays) and arthropods such as crustaceans, insects, and arachnids continue their development.

Jurassic

The name of this period refers to the Jura, a mountain system belonging to the Alps and located between France and Switzerland. It begins 208 million years ago and ends 144 million years ago. The Jurassic seas no longer had the life forms that had existed in the Paleozoic, such as trilobites, foraminifera, and rugose corals. They are home to gastropods, ammonoids, sharks, bony fishes, decapod arthropods, and hexacorals. Gymnosperms are still the dominant plants and have a great expansion, especially conifers.

Flying reptiles appear and, towards the end of the period, the first birds appear, represented by Archaeopteryx lithographica, a characteristic fossil of this period, considered a link between reptiles and birds. Large amphibians disappeared but small amphibians such as urodeles and anurans flourished. An extremely successful group, which expanded greatly, is that of the dinosaurs, which presented a great diversity of forms.

Cretaceous

Its name comes from cretaceous, which is the Latin word for chalk. It is the last period of the Mesozoic Era and lasts 79 million years, beginning 144 million years ago and ending 65 million years ago. In the Cretaceous, the great reptiles of the Mesozoic reach their maximum development with forms well adapted to the different environments - terrestrial, aquatic, and aerial.

This period saw the appearance of vertebrate groups that still exist today, such as crocodiles, snakes, and turtles. Mammals are very small in size, which is attributed to the presence and dominance of dinosaurs. A group whose number decreases is the gymnosperms (ferns, conifers, etc.).

On the other hand, flowering plants make their appearance towards the middle of the Cretaceous and end up being the dominant group. At the end of the period, there was a biological crisis that led to the rapid disappearance of the dinosaurs. Ammonoids and other living things, both animals and plants, also become extinct. Other groups survive and manage to adapt to the new conditions.

The Cenozoic or Cenozoic Era

The drastic change in the Earth's conditions will provoke, the need for adaptation, the appearance, and the development of new forms of life, thus beginning a new era. The name derives from the Greek kainós, new, and zoico, animal. Stratigraphically, this era has been divided into three periods: Paleogene, Neogene, and Pleistocene.

Paleogene

This period, which, together with the following one, formed part of what was known as the Tertiary, began 65 million years ago and ended 24 million years ago. It is subdivided into three epochs: Paleocene, Eocene, and Oligocene. Marine life already closely resembles that of the modern world and fish with skeletons become the dominant group. Many organisms still appeared and there is a great diversification of mollusks such as gastropods (snails) and bivalves, as well as great adaptive radiation of birds.

Perhaps the most significant event in the evolution of animals is the great diversification of mammals, so much so that the Cenozoic era is known as the age of mammals. During this era, whales, primates, rodents, bats and many carnivores appeared and diversified. Towards the end of the Eocene, there was a global cooling that produced the extinction of many species, however, those that survived had a great development in the Oligocene.

Giant rhinoceroses abounded in this last epoch, as did felines, crocodiles, and other reptiles. The great expansion of flowering plants continued, which evolved into modern angiosperms. In the Eocene, plants that protect the seeds in a fruit already form the dominant group. A significant evolutionary event of this epoch is the origin of grasses. About half of the angiosperm species of the Oligocene still survive today.

Neogene

It begins 24 million years ago and ends 1.8 million years ago. It is subdivided into two epochs: Miocene and Pliocene. Concerning the plant kingdom, the most outstanding event is the dramatic increase in grass species, many of which still exist. Vertebrates diversify extensively while invertebrates undergo minor changes.

Songbirds make their appearance at this time. Marine life does not undergo many variations and many of the species present in this period still survive. Mammals stand out for their great evolutionary radiation; species of deer, antelopes, felines, and ovines, among others, appear. Primates had a great diversification in the Miocene, although later, in the Pliocene, they would lose species.

The appearance of the Proconsul, about 25 million years ago, and its subsequent evolutionary derivatives, opened the door to the appearance of hominids. The Australopithecus set foot on Earth about 4 million years ago. It was a small-bodied individual with a small brain about its body and teeth similar to those of apes.

Pleistocene

The Pleistocene, formerly known as the Quaternary period, begins approximately 1.8 million years ago and some scientists divide it into two epochs: Pleistocene and Holocene. In the first epoch, there were four major glaciations (Gunz, Mindel, Riss, and VWVúrm) interspersed by warmer periods known as interglacials. Hominids were in full evolution during this period.

Approximately 1.6 million years ago, the genus Homo appeared. Homo sapiens sapiens, modern man, has been present on the planet for at least 40,000 years and his traces are revealed by the presence of fossilized bones in well-determined geological layers or by clearly worked objects. In the Holocene, which begins about 10 000 years ago with the melting of the Wúrm glaciers, the climate stabilizes.

By Héctor Delgado Rodríguez, Source: Correo del Maestro, No. 32.