MASS EXTINCTION: CAUSES AND THE MOST IMPORTANT - BIOLOGY - 2025

Mass extinctions are events characterized by the disappearance of a large number of biological species in a short time. This type of extinction is usually terminal, that is, a species and its relatives disappear without leaving offspring.

Mass extinctions differ from other extinctions, by being abrupt and by eliminating large numbers of species and individuals. In other words, the rate at which species disappear during these events is very high, and its effect is appreciated in a relatively short time.

Figure 1. Hypothesis of the death of dinosaurs due to the effect of toxic gases in the Deccan Stairs. Massive eruptions occurred in south-central India, in one of the largest volcanic formations on earth. Source: nsf.gov

In the context of geological eras (tens or hundreds of millions of years in duration), "short time" can mean a few years (even days), or periods of hundreds of billions of years.

Mass extinctions can have multiple causal agents and consequences. Physical and climatic causes often trigger cascades of effects in food webs or directly on some species. The effects can be "instantaneous," like those that occur after a meteorite hits planet Earth.

Causes of mass extinctions

The causes of mass extinctions could be classified into two main types: biological and environmental.

Biological

Among these are: competition between species for resources available for their survival, predation, epidemics, among others. The biological causes of mass extinctions directly affect a group of species or the entire trophic chain.

Environmental

Among these causes we can mention: increases or decreases in sea level, glaciations, increased volcanism, the effects of nearby stars on planet Earth, effects of comets, asteroid impacts, changes in the Earth's orbit or magnetic field, global warming or cooling, among others.

All of these causes, or a combination of them, could have contributed to a mass extinction at one point.

Multidisciplinary studies of mass extinctions

The ultimate cause of a mass extinction is difficult to establish with absolute certainty, since many events do not leave a detailed record of its onset and development.

For example, we could find a fossil record that evidences the occurrence of an important event of species loss. However, to establish the causes that generated it, we must make correlations with other variables that are registered on the planet.

This type of deep investigation requires the participation of scientists from different areas such as biology, paleontology, geology, geophysics, chemistry, physics, astronomy, among others.

Most important mass extinctions

The following table shows a summary of the most important mass extinctions studied to date, the periods in which they occurred, their age, the duration of each, the estimated percentage of extinct species and their possible cause.

Evolutionary significance of mass extinctions

Reduction of biological diversity

Mass extinctions reduce biological diversity, since complete lineages disappear and, in addition, those that could have arisen from these are dispensed with. Mass extinction could then be compared to pruning the tree of life, in which entire branches are cut off.

Development of pre-existing species and emergence of new species

Mass extinction can also play a "creative" role in evolution, stimulating the development of other pre-existing species or branches, thanks to the disappearance of their main competitors or predators. In addition, the emergence of new species or branches in the tree of life can occur.

The sudden disappearance of plants and animals that occupy specific niches, opens a series of possibilities for the surviving species. We can observe this after several generations of selection, since the surviving lineages and their descendants can come to occupy ecological roles previously performed by disappeared species.

The factors that promote the survival of some species in times of extinction are not necessarily the same that favor survival in times of low intensity of extinctions.

Mass extinctions then allow lineages that were previously a minority to diversify and play important roles in the new post-catastrophe scenario.

The evolution of mammals

A well-known example is that of mammals, which were a minority group for more than 200 million years and only after the Cretaceous-Tertiary mass extinction (in which the dinosaurs disappeared), did they develop and began to play a game. big role.

We can affirm then that the human being could not have appeared, if the mass extinction of the Cretaceous had not occurred.

The KT impact and the Cretaceous-Tertiary mass extinction

Álvarez's hypothesis

Luis Álvarez (1968 Nobel Prize in Physics), together with the geologist Walter Álvarez (his son), Frank Azaro and Helen Michel (nuclear chemists), proposed in 1980 the hypothesis that the Cretaceous-Tertiary (KT) mass extinction was product of the impact of an asteroid 10 ± 4 kilometers in diameter.

This hypothesis arises from the analysis of the so-called KT limit, which is a thin layer of clay rich in iridium, which is found on a planetary scale just on the border that divides the sediments corresponding to the Cretaceous and Tertiary (KT) periods.

Iridium

Iridium (Ir) is the chemical element with atomic number 77 that is located in group 9 of the periodic table. It is a transition metal, from the platinum group.

It is one of the rarest elements on Earth, considered a metal of extraterrestrial origin, as its concentration in meteorites is frequently high compared to concentrations on the ground.

Figure 2. KT or Cretaceous-Paleogene boundary, which marks the end of an era. Anky-man, from Wikimedia Commons

KT limit

Scientists found much higher iridium concentrations in the sediments of this clay layer called the KT boundary than in the preceding strata. In Italy they found an increase of 30 times compared to the previous layers; in Denmark 160 and in New Zealand 20.

Álvarez's hypothesis suggested that the impact of the asteroid darkened the atmosphere, inhibiting photosynthesis and precipitating the death of a large part of the existing flora and fauna.

However, this hypothesis lacked the most important evidence, since they could not locate the place where the asteroid impact had occurred.

Until then, no crater of the expected magnitude had been reported to corroborate that the event had actually occurred.

Chicxulub

Despite not having reported it, geophysicists Antonio Camargo and Glen Penfield (1978) had already discovered the crater as a result of the impact, while they were looking for oil in Yucatán, working for the Mexican state oil company (PEMEX).

Camargo and Penfield achieved an underwater arc of about 180 km wide that continued in the Mexican peninsula of Yucatan, with a center in the town of Chicxulub.

Figure 3. Gravitational map showing the anomaly in the Yucatan peninsula. Source: Computer generated gravity map image of the Chicxulub Crater in México (NASA).

Although these geologists had presented their findings at a conference in 1981, lack of access to the drill cores kept them off the subject.

Finally, in 1990, journalist Carlos Byars contacted Penfield with astrophysicist Alan Hildebrand, who finally gave him access to the drilling cores.

Hildebrand in 1991 published together with Penfield, Camargo and other scientists the discovery of a circular crater in the Yucatan peninsula, Mexico, with a size and shape that reveal anomalies of magnetic and gravitational fields, as a possible impact crater that occurred in the Cretaceous-Tertiary.

Other hypotheses

The Cretaceous-Tertiary mass extinction (and the KT Impact hypothesis) is one of the most studied. However, despite the evidence supporting Álvarez's hypothesis, other different approaches survived.

It has been argued that stratigraphic and micropaleontological data from the Gulf of Mexico and the Chicxulub crater support the hypothesis that this impact preceded the KT boundary by several hundred thousand years and therefore could not have caused the mass extinction that occurred. in the Cretaceous-Tertiary.

It is suggested that other serious environmental effects could be the triggers of the mass extinction at the KT boundary, such as the Deccan volcanic eruptions in India.

Deccan is a large plateau of 800,000 km ² that crosses the south-central territory of India, with traces of lava and huge release of sulfur and carbon dioxide that may have caused the mass extinction at the KT boundary.

Latest evidence

Peter Schulte and a group of 34 researchers in 2010 published, in the prestigious journal Science, a thorough evaluation of the two previous hypotheses.

Schulte et al. Analyzed a synthesis of recent stratigraphic, micropaleontological, petrological, and geochemical data. In addition, they evaluated both extinction mechanisms based on their predicted environmental disturbances and the distribution of life on Earth before and after the KT limit.

They concluded that the Chicxulub impact caused the mass extinction of the KT limit, due to the fact that there is a temporal correspondence between the ejection layer and the onset of extinctions.

Furthermore, ecological patterns in the fossil record and modeled environmental disturbances (such as darkness and cooling) support these conclusions.

References

1. Álvarez, LW, Álvarez, W., Asaro, F., & Michel, HV (1980). Extraterrestrial Cause for the Cretaceous-Tertiary Extinction. Science, 208 (4448), 1095-1108. doi: 10.1126 / science.208.4448.1095
2. Hildebrand, AR, Pilkington, M., Connors, M., Ortiz-Aleman, C., & Chavez, RE (1995). Size and structure of the Chicxulub crater revealed by horizontal gravity gradients and cenotes. Nature, 376 (6539), 415-417. doi: 10.1038 / 376415a0
3. Renne, PR, Deino, AL, Hilgen, FJ, Kuiper, KF, Mark, DF, Mitchell, WS,… Smit, J. (2013). Time Scales of Critical Events Around the Cretaceous-Paleogene Boundary. Science, 339 (6120), 684-687. doi: 10.1126 / science.1230492
4. Schulte, P., Alegret, L., Arenillas, I., Arz, JA, Barton, PJ, Bown, PR,… Willumsen, PS (2010). The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary. Science, 327 (5970), 1214-1218. doi: 10.1126 / science.1177265
5. Pope, KO, Ocampo, AC & Duller, CE (1993) Surficial geology of the Chicxulub impact crater, Yucatan, Mexico. Earth Moon Planets 63, 93–104.
6. Hildebrand, A., Penfield, G., Kring, D., Pilkington, M., Camargo, A., Jacobsen, S. and Boynton, W. (1991). Chicxulub Crater: a possible Cretaceous / Tertiary boundary impact crater on the Yucatán Peninsula, Mexico. Geology. 19 (9): 861-867.