SCIENCE22 - Mass Extinctions Accelerated the Evolution of Life on Earth

I have long been fascinated by so called “mass extinction” events that killed off so many species over the course of the development of life on Earth.  Recently, I became aware that these events, while devastating, may have provided opportunities for accelerated evolution and development of certain animal species, including humans.  So that is the subject of this blog:  the role of mass extinction events in the evolution of life on Earth.

 

In a short introduction, I will discuss the mechanisms, or processes, that directly affect evolution.  Then I will discuss the currently-accepted timeline of the development of life on Earth.  Next, I will talk about the five major extinction events that had such a profound effect on development of life, noting when they occurred in the timeline, and their dramatic effect on evolution.  Then I will discuss how these devastating events resulted in opportunities for new species to replace the ones that had died off and, in some cases, accelerated the evolution of certain species, including humans. 

I will list my principal sources at the end.

 

Introduction

The mechanisms, or processes, that directly affect evolution are:

Natural Selection:  The process by which organisms with traits better suited to their environment tend to survive (via natural selection, according to Charles Darwin) and reproduce more successfully, leading to the evolution of populations over time. 

Genetic Variation:  Random mutation in the genes of living organisms is essential for the evolution of life on Earth.  It is the primary source of genetic variation, providing the raw material (new traits) for natural selection of organisms whose mutations make them better able to survive long enough to reproduce and thus pass on the same genes to their offspring.  Without gene mutation, there would be no new genetic variations to select for, and evolution would not be possible.  Random gene mutation occurs spontaneously, relatively frequently, and acts as building blocks for evolution.

Periodic Natural Extinction: Extinctions are a normal part of evolution: they occur naturally and periodically over time.  When groups of organisms are no longer able to adapt to changing environmental conditions or compete with more successful groups of organisms, they become extinct.  There’s a natural background rate to the timing and frequency of these extinctions: 10% of species are lost every million years, 30% every 10 million years, and 65% every 100 million years. 

Mass Extinctions:  Mass extinctions are periods with much higher extinction rates than normal. They are defined by both magnitude and rate.  Magnitude is the percentage of species that are lost.  Rate is how quickly this happens.  In a mass extinction, at least 75% of species go extinct within a relatively (by geological standard) short period of time, typically less than two million years.

Here’s some eye-opening data about extinction.

 

Adaptive Radiation:  Not radiation in the physics sense.  Evolutionary radiation is the process, typically following a mass extinction event, where a surviving species rapidly adapts and diversifies into multiple species that can occupy different ecological space, and essentially fill the evolutionary niche left open by the mass extinction.  This diversification occurs due to natural selection, where individuals with traits better suited for specific environments are more likely to survive and reproduce, leading to the evolution of new species. 

Timeline of Evolution of Life on Earth  

The evolution of life on Earth was/is a long and complex process.  Life is thought to have originated on Earth around 3.5 to 4 billion years ago, with microscopic organisms.  Over time, these organisms evolved, leading to the development of simple cells, then multicellular life, and eventually an explosion of biodiversity. 

Perhaps the most remarkable geobiological (yes, this is a word) event in Earth’s history, aside from the origin of life itself, was the so-called Cambrian Explosion.  Beginning about 540 million years ago, and lasting 55 million years or so, this (geologically) rapid development of new types of organisms from a common ancestor via adaptive radiation had an extraordinary effect on future evolution, as the fossil record shows.  Pretty much all the ancestors of life as we know it were created during this period.

Before the Cambrian Explosion, life existed on Earth only in its oceans, as primitive life forms, without hard body parts.  The Cambrian Explosion was a time of rapid diversification of life on Earth, characterized by a sudden appearance of diverse and complex life forms, seeing a rapid change in animal body plans, animal size, and the adaptation of shells and exoskeletons.

While the exact causes of the Cambrian Explosion are still debated, theories include: a rise in oxygen levels in the Earth's atmosphere, the evolution of key animal traits like vision, and a combination of environmental and developmental factors. 

The Cambrian Explosion was a period of rapid development of new life forms.

After the Cambrian Period, the Earth’s ecosystem changed dramatically as animals that developed vision, legs, and predator-prey relationships started to evolve.  The first animals to appear on land were arthropods, specifically millipedes, around 428 million years ago.

The figure below shows the timeline of the development of life on Earth from the Cambrian Period through today.  The timeline is broken into eras and periods, that were named according to major new species developments.

Timeline of evolution of life on Earth.  Note the red stars identifying the occurrence of mass extinction events.

 

In five cases, the boundaries of these geologic time periods were determined by mass extinctions of truly global magnitude, marking the dramatic sudden demise, or extinction, of a particular group of organisms, followed by a rapid (geologically) rise, or radiation, of a new group of organisms, stimulated by the availability of new evolutionary niches after a mass extinction eliminated established, highly competitive groups of organisms.

 

Mass Extinction Events   

There have been five mass extinction events in Earth’s history, since the Cambrian Period.  We know very little about extinction events before that.  All of the "Big Five" were caused by some combination of rapid and dramatic changes in climate, combined with significant changes in the composition of environments on land or the ocean (such as ocean acidification or acid rain from intense volcanic activity).

The table below lists the five mass extinction events, including when they occurred, the percentage of species lost, and the probable cause.

 

Extinction Event	Age (million years ago)	Percentage of species lost	Cause of extinction
End Ordovician	444	86%	Intense glacial and interglacial periods created large sea-level swings and moved shorelines dramatically.  The tectonic uplift of the Appalachian Mountains created lots of weathering, sequestration of CO2, and with it, changes in climate and ocean chemistry.
Late Devonian	360	75%	Rapid growth and diversification of land plants generated rapid and severe global cooling.
End Permian	250	96%	Intense volcanic activity in Siberia caused global warming. Elevated CO2 and sulfur (H2S) levels from volcanoes caused ocean acidification, acid rain, and other changes in ocean and land chemistry.
End Triassic	200	80%	Underwater volcanic activity caused global warming and a dramatic change in the chemical composition of the oceans.
End Cretaceous	65	76%	Asteroid impact in Yucatán, Mexico. This caused a global cataclysm and rapid cooling.  This is the event that killed off the non-avian dinosaurs.  Some changes may have already pre-dated this asteroid, with intense volcanic activity and tectonic uplift.

 

A possible sixth mass extinction may be underway today, largely due to disruptive human activities causing accelerated biodiversity loss, primarily driven by deforestation, pollution, overexploitation of resources, and climate change. 

 

Radiation Opportunities from Mass Extinction Events

In the past half-billion years, Earth has been hit five times by mass extinctions, wiping out most species on the planet.  And every time, life recovered and ultimately went on to increase in diversity.

Mass extinctions have sometimes accelerated the evolution of life on Earth.  When dominance of particular ecological niches passes from one group of organisms to another, it is rarely because the new dominant group is "superior" to the old, but usually because an extinction event eliminates the old dominant group and makes way for the new one.

The End-Cretaceous Extinction, famously known for wiping out the non-avian dinosaurs, played a crucial role in shaping the evolution of humans and other mammals.  The worldwide cloud of debris and dust from the asteroid impact shut down photosynthesis for years and caused decades of global cooling.  Anything that couldn’t shelter from the cold, or find food in the darkness - which was most species - perished. 

Mammals existed throughout the long reign of the dinosaurs, but could not compete for the large terrestrial vertebrate niches which dinosaurs monopolized. Ninety percent of mammal species were eliminated by the asteroid, but the remaining mammal species soon diversified and expanded into the large terrestrial vertebrate niches, going on to rapidly evolve into horses, whales, bats, and our primate ancestors. 

Birds and fish experienced similarly rapid recovery and radiation.  And many other organisms - snakes, tuna and swordfish, butterflies and ants, grasses, orchids, and asters - evolved or diversified at the same time.

In essence the End-Cretaceous Extinction event, significantly influenced human evolution by creating new opportunities for placental mammals, including our ancestors, to diversify and evolve more rapidly.  

This pattern of recovery and diversification happened after every mass extinction.

Looking back to the timeline of evolution of life on Earth, above, after reptiles suffered during the End-Triassic Event, the surviving dinosaurs diversified and took over the planet.  Although a mass extinction ended the dinosaurs, they only evolved in the first place because of mass extinction.

Despite this chaos, life slowly diversified over the past 500 million years. 

In fact, several things hint that extinction drives this increased diversity.  For one, the most rapid periods of diversity increase occur immediately after mass extinctions.  In a recovery, animals exploit new habitats, new foods, new means of locomotion.  For example, our fish-like ancestors first crawled onto land after the End-Devonian Extinction.

Recovery may increase the number of species that can coexist because it allows species to move into new niches, instead of fighting over the old ones.  Fish crawling onto land didn’t compete with fish in the seas.  Bats hunting at night with sonar didn’t compete with birds that were active during the day.  Species can diversify without driving others extinct.  In the lulls after an extinction, evolution may be able to experiment with designs that are initially poorly adapted, but with long-term potential. 

Evolution suggests that there’s a kind of creative destruction, where the destruction of the old system in mass extinctions creates a vacuum, and actually drives the creation of something new and often better. 

I was going to close this blog with details of the evolution of our human species, homo sapiens, from a number of competing lineages, but I think I covered that subject adequately in an earlier blog, https://bobringreflections.blogspot.com/2022/02/science-history-of-human-evolution.html. The point with this blog was to establish the influence of mass extinctions. 

 

"Extinction is the rule. Survival is the exception." - Carl Sagan 

 

 

Sources

My principal sources include: “Extinction Event,” enwikipedia.org; “There have been five mass extinction events in Earth’s history,” ourworldindata.org; “Mass Extinctions,” courses.lumenlearning.com; “Mass extinctions made life on Earth more diverse - and might again,” theconversation.com; plus, numerous other online sources.  I particularly want to recognize the use of online artificial intelligence ChatGPT summaries on many subjects related to this blog.