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.
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 tend to trigger cascades of effects on 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.
Among these are: competition between species for the 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.
Among these causes we can cite: 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 research 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 promote 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 begin 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
Luis Álvarez (Nobel Prize in Physics 1968), 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 (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.
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 stated 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 that moment, no crater of the expected magnitude had been reported to corroborate that the event had actually occurred.
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 Yucatán, with a center in the town of Chicxulub.
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 the journalist Carlos Byars contacted Penfield with the 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 .
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 for 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 could have caused the mass extinction at the KT boundary.
Most recent 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. Furthermore, 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.
- 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
- 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
- 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
- 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
- Pope, KO, Ocampo, AC & Duller, CE (1993) Surficial geology of the Chicxulub impact crater, Yucatan, Mexico. Earth Moon Planets 63, 93–104.
- 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.