Cambrian explosion: what it was and how it influenced biological evolution

The evolution of living beings, that is, the set of changes in the phenotypic and genetic characteristics of populations through generations, is something that today is taken for granted. The fossil record and genetic analyzes leave us in no doubt about this: all current beings have a series of ancestors and living beings are grouped into taxa according to degrees of kinship.

This concept in the year 1859 was in his infancy, and Darwin postulated in the book "The origin of the species” the mechanism of natural selection that explained biological evolution, but not without certain Bookings.

This is where the not so well-known "Darwin's dilemma" comes into play, which was a real puzzle for the thinker in his postulations of an evolutionary nature. Today we are talking about unprecedented biological radiation, in a bygone and hostile era: welcome to the cambrian explosion.

• Related article: "The theory of biological evolution: what it is and what it explains"

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 (ie, phyla).

This is so because during this time the majority of animal groups arose: arthropods, mollusks, echinoderms and chordates and 75% of the representative subdivisions on earth. Stated more simply, it could be said that the great “pinnacles” of life were established during the Cambrian, plus the small branches cleaved from them, as could be today the cephalopods or bivalves within the large group of molluscs. In addition to this incredible diversification, other events occurred:

• There was an escalation in the abundance of plankton.
• The presence of mineralized skeletons in living beings spread.
• The first reefs were formed.
• The animals began to exploit non-superficial layers of the substrate, that is, they began to practice digging as a method of obtaining food.
• Complex ecological interactions began and with them the establishment of trophic chains.

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

Among the most famous living beings belonging to this geological division we find the trilobites, some already extinct arthropods that included some 4,000 species and of which, luckily, a large number have come down to us of fossils. Other common representatives were certain brachiopods, similar to clams and cockles, or the well-known anomalocaris, a genus of shrimp-like predators that caught their prey with their shaped ends Hook. 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 larger due to the mineralization of the skeletons of various species, regarding the poor fossilization ability of their soft-bodied predecessors.

Causes of Cambrian evolutionary radiation

At this point, it is necessary that we delimit that we are dealing with hypotheses, all of them equally valid (with more or less scientific support), but that cannot be fully tested. Various research articles postulate the possible reasons for the Cambrian explosion and we show them briefly in the following lines.

1. environmental triggers

Oxygenation of water and changes in its chemical composition have been postulated as the most likely environmental triggers for the Cambrian explosion.

Of course, the process of metabolizing food in the presence of oxygen (cellular respiration) yields much more energy than anaerobic metabolic pathways, which is why an explosion of energy makes biological sense. life when the values ​​of this gas reached almost the current levels of the oceans on Earth. Even so, recent studies that analyze certain chemical compounds in rocks from these geological stages report that the increase in global oxygen probably did not occur as drastically 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.

2. Developmental genes and molecular clock

The Cambrian explosion could not only find its reason in environmental determinants such as the clash 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 genes of the organisms.

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

Molecular clocks have shown that radiation from the three bilateral animal clades during the early Cambrian period it happened after the diversification of the Hox genes, so unfortunately this theory has run out of steam quite a bit in recent years. time. If the Hox genes diversified before the clades, this would not be an adequate explanation for Cambrian radiation.

• You may be interested in: "What is a codon? Features and functions"

Darwin's dilemma

Perhaps we can understand the puzzle that this rapid appearance of new species and phyla, since not even today a completely irrefutable answer can be given regarding the issue.

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

As indicated by professionals in the field who have studied this phenomenon over the last 20 years, "a moderately accelerated and sustained evolution for a few tens of millions of years could give rise to an evolutionary explosion”, a fact that, to a certain extent, agrees with the initial postulations of Darwin.

Also is true that colonization of new environments may promote a much faster, skewed rate of evolution, since a taxon that acquires an innovative characteristic could require an evolutionary explosion to occupy new niches. For example, if a primary adaptation were the acquisition of legs and a grounding, it is to some extent logical to think that the mechanisms evolutionary would speed up for a while in the legged group, as it would find itself with thousands of empty niches to exploit after a single evolutionary history in water.

Summary

If we wanted to show one thing with all this terminological conglomerate, ramblings and hypotheses, it is that, unfortunately, there are questions that surely can never be fully answered. This makes phylogenetics and paleontology sciences that are as exciting as they are frustrating, since the Debate and cavil reign in a world where we try to guess what happened 530 million years ago. years. The Cambrian Explosion, as impressive as it is, remains without a definitive answer.

Bibliographic references:

• Cambrian Explosion, wikipedia.org. Collected on November 15 in https://es.wikipedia.org/wiki/Explosi%C3%B3n_c%C3%A1mbrica
• The Cambrian Explosion, Understandingevolution. Collected on November 15 in 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. Collected on November 15 in https://www.nationalgeographic.es/historia/periodo-cambrico
• red.escubre, Scientific and cultural news bulletin, Complutense University of Madrid (UCM). Collected on November 15 in 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.