The Cambrian Explosion: what it was and how it influenced biological evolution

This is what we know about the Cambrian explosion, a stage in which many forms of life appeared.

The evolution of living things, i.e., the set of changes in phenotypic and genetic characteristics of populations over generations, is something that is taken for granted today. The fossil record and genetic analyses leave us in no doubt: every living being today has a series of ancestors and living beings are grouped into taxa according to degrees of kinship.

This concept was in its infancy in 1859, and Darwin postulated in the book "The Origin of Species" the mechanism of natural selection that explained Biological evolution, but not without certain reservations.

This is where the not so well known "Darwin's dilemma" comes into play, which was a real puzzle for the thinker in his evolutionary postulations. Today we tell you about an unprecedented biological radiation, in a past and hostile epoch: welcome to the the Cambrian explosion.

What is the Cambrian explosion?

The Cambrian explosion or Cambrian explosion translated into English is a term that refers to the sudden appearance and rapid diversification of macroscopic organisms about 540 million years ago.. According to research sources, this radiation was especially significant for the "larger" taxa (i.e., the phyla).

This is because most animal groups arose during this time: arthropods, mollusks, echinoderms and chordates and 75% of the representative subdivisions on earth. Explained in a simpler way, one could say that the great "pinnacles" of life were established during the Cambrian, in addition to the small branches branching off from them, such as today's cephalopods or bivalves within the large group of mollusks.The cephalopods or the bivalves within the large group of mollusks could be today. In addition to this incredible diversification, other events took place:

• There was an escalation in the abundance of plankton.
• The presence of mineralized skeletons in living beings became widespread.
• The first reefs were formed.
• Animals began to exploit non-surface layers of the substrate, i.e., they began to practice digging as a method of obtaining food.
• Complex ecological interactions began and with them the establishment of food chains.

Throughout this evolutionary radiation, which lasted about 20 million years, at least 11 of the 20 phyla of metazoans (animals) recorded today appeared.. Of course, this is not the only "explosion of life" recorded throughout Earth's history, for example, the Triassic-Jurassic mass extinction allowed the subsequent dominance of dinosaurs. The replacement of species once many ecological niches are suddenly vacated is nothing new, although none of them has been comparable to the Cambrian explosion.

Among the most famous living beings belonging to this geological division are the trilobites, now extinct arthropods that comprised some 4,000 species and of which, fortunately, a large number of fossils have come down to us. Other common representatives were certain brachiopods, similar to clams and cockles, or the well-known anomalocaris, a genus of shrimp-like predators that trapped their prey with their hooked ends. At this time, these primitive beings were at the top of the food chain in the planet's oceans.

However, according to professional portals such as National Geographic magazine, there is a probability that this event was "exaggerated" a bit, since it is possible that the fossil record that has come down to us from this stage is much older due to mineralization of the skeletons. it is possible that the fossil record that has come down to us from this stage is much greater due to the mineralization of the skeletons of various species, with respect to the poor capacity of the oceans to support the food chain.The fossil record that has come down to us from this stage may be much greater due to the mineralization of the skeletons of various species, compared to the poor fossilization capacity of their soft-bodied predecessors.

Causes of Cambrian evolutionary radiation

At this point, it is necessary to point out that we are faced with hypotheses, all of them equally valid (with more or less scientific support), but which cannot be proven in their entirety. Several research articles postulate the possible reasons for the Cambrian explosion and we show them briefly in the following lines.

1. Environmental triggers

Water oxygenation and changes in its chemical composition have been postulated as the most likely environmental triggers that led to the Cambrian explosion.

Of course, the process of food metabolization in the presence of oxygen (cellular respiration) yields much more energy than anaerobic metabolic pathways. than anaerobic metabolic pathways, which is why an explosion of life makes biological sense when oxygen values reached near current levels in the Earth's oceans. Yet recent studies analyzing certain chemical compounds in rocks from these geologic stages report that the global oxygen increase probably did not occur as dramatically as most people believe.

Increases in environmental nutrients such as phosphorus or nitrogen could also partly explain this exaggerated radiation.Although this fact has not been directly correlated with a greater diversification of species or the appearance of so many phyla, as happened in the Cambrian.

Developmental genes and molecular clock

The Cambrian explosion could not only find its reason in environmental determinants such as the collision of continents, the greater presence of oxygen, a greater availability of nutrients and many other factors extrinsic to the individual, but part of the truth could also be found in the organisms' own genes.

A clear example of this are the Hox genes, which, during embryonic development, specify the body plan of living beings by activating or inactivating other genes in certain parts of the body (explained in a very rudimentary way). The evolution of Hox genes, for example, could explain the appearance of more complex bodies during this period..

Molecular clocks have shown that the radiation of the three bilateral animal clades during the Early Cambrian period occurred after the diversification of the Hox genes, so unfortunately, this theory has lost quite a bit of steam in recent times. If Hox genes diversified before clades, this would not be an adequate explanation for Cambrian radiation.

Darwin's dilemma

Perhaps we can understand the puzzle that this accelerated appearance of new species and phyla meant for Darwin in his time, since even today no completely irrefutable answer can be given on the subject.

Darwin's discourse always defended the gradual succession of modifications in living beings, a fact that ruled out the appearance of an "evolutionary Big Bang", which is exactly what the Cambrian explosion is. According to several studies, the acceleration of physical modifications during the Cambrian exceeded the current evolutionary mechanisms by 4 times, while genetic variations increased by 5.5 times.while genetic variations increased 5.5 times.

As indicated by professionals in the field who have studied this phenomenon during the last 20 years, "a moderately accelerated and sustained evolution during some tens of millions of years could lead to an evolutionary explosion", a fact that, to a certain extent, agrees with Darwin's initial postulations.

It is also true that colonization of new environments may promote a skewed and much faster evolutionary rate, since a taxon that acquires a new environmentIt is also true that the colonization of new environments may promote a biased and much faster evolutionary rate, since a taxon that acquires an innovative characteristic may require an evolutionary explosion to occupy new niches. For example, if a primary adaptation were the acquisition of legs and an exit to land, it is to some extent logical to think that evolutionary mechanisms would be accelerated for a time in the legged group, as it would find itself with thousands of empty niches to exploit after a unique evolutionary history in the water.

Summary

If one thing we have wanted to show with all this terminological conglomeration, digressions and hypotheses, it is that, unfortunately, there are questions that can surely never be answered in their entirety. This makes phylogenetics and paleontology sciences as exciting as they are frustrating, as debate and pondering reign in a world where we try to guess what happened 530 million years ago. The Cambrian explosion, impressive as it is, remains without a definitive answer.

Bibliographical references:

• Cambrian explosion, wikipedia.org. Retrieved November 15 from https://es.wikipedia.org/wiki/Explosi%C3%B3n_c%C3%A1mbrica.
• The Cambrian Explosion, Understandingevolution. Retrieved November 15 from https://evolution.berkeley.edu/evolibrary/article/side_o_0/cambrian101_01_sp
• Marshall, C. R. (2006). Explaining the Cambrian "explosion" of animals. Annu. Rev. Earth Planet. Sci., 34, 355-384.
• Morris, S. C. (2000). The Cambrian "explosion": slow-fuse or megatonnage?. Proceedings of the National Academy of Sciences, 97(9), 4426-4429.
• Cambrian Period, National Geographic. Retrieved November 15 from https://www.nationalgeographic.es/historia/periodo-cambrico.
• red.escubre, Boletín de noticias científicas y culturales, Universidad Complutense de Madrid (UCM). Retrieved November 15 from https://www.ucm.es/data/cont/media/www/pag-10072/Redescubre%20N%C2%BA7.pdf
• Wood, R., Liu, A. G., Bowyer, F., Wilby, P. R., Dunn, F. S., Kenchington, C. G., ... & Penny, A. (2019). Integrated records of environmental change and evolution challenge the Cambrian Explosion. Nature ecology & evolution, 3(4), 528-538.
• Zhang, X., Shu, D., Han, J., Zhang, Z., Liu, J., & Fu, D. (2014). Triggers for the Cambrian explosion: hypotheses and problems. Gondwana Research, 25(3), 896-909.