Species Knockdowns by Damned Extinctions

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As species, we, Sapiens, are the product of biological evolution. Well, it is not only that. That evolution is subject to other forces, like climate changes, our Sun and Earth movements, and so on. 

Biological evolution itself has many components. The output of that evolution for species is either a split into new species, continuation of existence, or extinction. How much do we know about the species’ extinctions?  Are those extinctions only a part of biological evolution or are they also a subject of other global forces? Let us look into it.

Happy One Percent Club

A biological evolution is a big game with severe competition for anybody involved. The environment is changing over time. But one thing is certain – not all species will survive. Some of them with become extinct. What is extinction?

“Extinction is the death of all members of a species of plants, animals, or other organisms.”.

Most of extinctions are a normal part of evolution. That part is called background extinction. There are several ways to measure background extinctions. One method is to count how many years species or genera typically exist before going extinct. The rate varies for different organisms. For mammals, the average lifespan of species is around one million years. 

There is a famous term “Survival of the fittest“, suggested by Herbert Spenser and Charles Darwin. The term means “that organisms best adjusted to their environment are the most successful in surviving and reproducing”. Other species might go extinct. Well, … that works for background extinctions. During mass extinctions, even the fittest organisms could go extinct.

It is estimated that more than 99% of all species that ever lived got extinct. We do not know how long we, Homo sapiens, will last. But for now, we, Sapiens, are in a very happy one percent club.

Big Five Mass Extinctions

There were times in Earth history when the extinction rate was much higher than an average background rate. Scientists are typically talking about Big Five mass extinctions, which happen from around 500 million years ago till Homo sapiens emergence. Every such an extinction wiped out over 50 % of existent species.

More Than Five For Sure

Scientists knew about times in history when there were big gaps in the amount of found fossil records. The initial opinion was that it shows that fossil artifacts are an unreliable source of knowledge. That was a wrong assumption. It was proved later that at those times fossil records were rare due to happening at the time extinctions. This is a good example to show how much our understanding of events and artifacts could shift over time.

Then, in 1982, Jack Sepkoski and David M. Raup identified five mass extinctions. Typically, mass extinction events are defined as events when 50 or more percent of all species die.

Mass Extinction Event	When, Million years ago	Duration, Million years	% of Species died	% of genera died	Reference
	“Big Five”	Mass	Extinction	Events
Permian-Triassic	252	?	90 – 96	83	[1]
Cretaceous-Paleogene	66	?	75	50	[2]
Triassic-Jurassic	201.3	?	70 – 75	48	[3]
Late Devonian	375 – 360	15	>= 70	50	[4]
Ortovician-Silurian	450 – 440	10	60 – 70	57	[5]
	Other	Mass	Extinction	Events
Great Oxygenation	2,400 – 2,000	?	80 – 99.5	?	[6]
End-Botomian	513 – 509	4	50 – 80	57 % of marina genera died	[7]

From this table, we could see that the statement that there were just five mass extinction events is incorrect. For example, Great Oxygenation and End-Botomian extinction events were among the biggest extinction events ever. That tells us that sometimes names or short descriptions of events could be misleading.

In most cases an extinction events are identified by fossil records. The discussion of how many extinctions have been caused by biological evolution and how many by environmental catastrophes is still ongoing. The picture is much clearer for mass extinction events. Researchers found that most of them were caused due to global catastrophes in the environment on Earth.

When It Rains It Pours

Yet, for us, they look just like dry numbers that do not evoke emotions. Could we even remotely imagine the scale of those perturbations?

Yes, we could. We all heard a powerful story about the biblical flood. “And the rain fell upon the earth for forty days and forty nights“. It was re-told and illustrated by artists numerous times. This story is, probably, vivid for you.

Well, researchers found that 234 – 232 million years ago there was a mass extinction event on Earth. The name for it is the Carnian, Late Triassic event. It was global, and around 33% of marine genera disappeared. One of the causes was a catastrophic global rain. It lasted longer than a famous biblical rain. Could you guess how long that rain lasted? And, . . .  the answer is one million years. Of course, at that time there were no humans on Earth. Moreover, there were no mammals on Earth at that time. Compare this one-million-year-long rain to a 40-days biblical rain!

Yet, this one-million-year-long rain was not big enough to be included by scholars in the famous “five mass extinction events”!

Mass extinction events greatly impact biological evolution and biological diversity. We could look into it in further posts. Go to Comments on this blog post.

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