Life on Earth, Part 3: Extinction
Life can flourish, and life can fade away.
In thinking about the concept of extinction, I can’t help but fall back on two sayings that I have heard throughout my life.
Life is fragile.
Life finds a way.
Certainly, we all know the fragility of life. It is easy to lose a life, but collectively, life as a whole just keeps going. And throughout Earth’s history, the ebb and flow of life has repeatedly occurred, sometimes with individual extinctions, other times with mass extinctions, and yet still, new life flourishes and evolves into extraordinary species.
I think that Jeff Goldblum, in the blockbuster-movie Jurassic Park by Steven Spielberg, may have summed up the persistence of life as a whole better than anyone:
If there is one thing the history of evolution has taught us, it’s that life will not be contained. Life breaks free, it expands to new territories and crashes through barriers, painfully and maybe even dangerously, but, well, there it is… I am simply saying that life finds a way.
We know from prior Paleoclimate newsletters that life on Earth accelerated dramatically during the Cambrian Explosion, which started about half-a-billion years ago. But since then, there have been five mass extinctions - ‘the big five’ - where life really did encounter major barriers “painfully and maybe even dangerously”, as Goldblum says. These extinctions are illustrated in the following figure.
This figure shows biodiversity (thousands of genera) at any given time. Genera (plural of genus) refers to taxonomic categories for organizing species - domain, kingdom, phylum, class, order, family, genus, and species. ‘The big five’ extinctions are labeled with yellow arrows. The average trend is shown by the red line. The data indicate both extinctions and new originations (new species), with an increase in species moving toward the present (year 0). The letters at the top refer to geologic epochs, for example, Cretaceous (K), Jurassic (J), and Triassic (Tr). The figure is adapted from “Cycles in fossil diversity” in the journal Nature. [Wikipedia, GNU Free Documentation License].
In a classic science paper, “Mass Extinctions in the Marine Fossil Record” in the journal Science, which has similar results as seen in the above figure, I want to highlight two of my favorite quotes from the paper:
A number of mass extinctions have “reset” major parts of the evolutionary system during the Phanerozoic.
Background extinction rates appear to have declined since Cambrian time, which is consistent with the prediction that optimization of fitness should increase through evolutionary time.
Let’s unpack those quotes and dive into evolution a bit before diving back into mass extinction.
The Phanerozoic is geologic-speak for the time since the beginning of the Cambrian period 542 million years ago. In evolutionary-speak, fitness refers to how good a particular type of organism is at leaving offspring in the next generation. Fitness is about success at surviving and reproducing. Then, optimization of fitness means that organisms should get better and better at surviving and reproducing over very long periods of time, since they are constantly evolving better traits for survival. Finally, evolution is the change in heritable characteristics, expressed through genes, which are passed on from parent to offspring during reproduction over successive generations. As an example, sometimes random genetic mutations occur - such as the ability to run a little faster - conferring an evolutionary advantage that might allow a creature to sprint beyond a predator, providing a better chance of survival and reproduction to pass on the sprinting-genetics. Natural selection is the tendency for survival and reproduction of creatures due to differences in traits, like running ability.
I like this story about natural selection a lot, “Pronghorn's Speed May Be Legacy of Past Predators”.
Dr. John A. Byers of the University of Idaho, says the pronghorn runs as fast as it does because it is being chased by ghosts -- the ghosts of predators past.
About ten thousand years ago,
[Pronghorns] had to evade the North American cheetah, the giant short-faced bear, … as well as lions and jaguars, which were even bigger and faster than they are today. The young and the weak faced an even greater array of dangers with saber-toothed cats roaming about as well as numerous types of wolves and plundering dogs.
So, the fastest Pronghorns were naturally selected because they tended to survive when chased by other fast predators, and those sprinting traits persist to this day, even though many of the predators have since perished.
Adult male Pronghorn (Antilocapra americana) in Yellowstone National Park. [Wikipedia, Tobias Klenze, CC BY-SA 4.0].
Another example of natural selection can be found in birds, for example, “Plumage coloration is a sexually selected indicator of male quality”.
In the house finch (Carpodacus mexicanus)… field studies … indicate that females prefer to mate with colourful males and that plumage brightness correlates with a male's capacity for parental care and perhaps its genotypic quality. … Plumage coloration was correlated with nest attentiveness and overwinter survival.
So, if a male, through random genetic mutation, develops brighter plumage, that male has an advantage in mating with a female, meaning through natural selection its’ genetics have a higher chance of being passed on to the next generation.
An adult male House Finch in Madison, Wisconsin. [Wikipedia, John Benson, CC BY 2.0].
Let’s shift back to mass extinctions. The definition of such an event is when species vanish much faster than they are replaced, or more specifically, when 75% of species are lost in a short period of geological time (a few million years or less). So, what caused ‘the big five’ mass extinctions of the past? Some form of climate change, of course. But keep in mind, for each mass extinction, there are many competing hypotheses about the cause. Here is a quick overview:
Ordovician–Silurian (O–S) extinctions, 445–444 Ma (Ma means millions of years ago): This is the second largest extinction event. The causes are debated, but recent research suggests the cause is related to global cooling and glaciation driven by increased delivery of the nutrient phosphorus to the ocean and associated increases in marine productivity. It is a very complicated geochemical story that, honestly, I do not fully understand.
Devonian-Carboniferous (D-C) extinctions, 372–359 Ma: Two extinction events occurred in this interval. The first extinction annihilated coral reefs and numerous tropical benthic (seabed-living) animals. The second mainly wiped out the armored placoderm fish. The causes of these extinctions are unclear. Leading hypotheses include changes in sea level and ocean anoxia (absence of oxygen), possibly triggered by global cooling or oceanic volcanism. The impact of a comet or another extraterrestrial body has also been suggested, such as the Siljan Ring event in Sweden.
Permian-Triassic (P-T) extinction event, 252 Ma: Earth's largest extinction event, where 81% of all marine species and an estimated 70% of land species, including insects, perished. The highly successful marine arthropod, the trilobite, became extinct. The scientific consensus for the cause of the extinction is large amounts of carbon dioxide emitted by volcanic eruptions that created the Siberian Traps, which elevated global temperatures, and in the oceans led to widespread anoxia and acidification.
Triassic-Jurassic (Tr-J) extinction event, 201.3 Ma: This event happened in less than 10,000 years and occurred just before Pangaea, a supercontinent consisting of most land masses, started to break apart. This extinction allowed dinosaurs to become dominant in the Jurassic period. The predominant theory for the cause of the Tr-J extinction is the start of volcanic eruptions in the Central Atlantic Magmatic Province (CAMP), which outputted a high amount of carbon dioxide that induced global warming and ocean acidification.
Cretaceous–Paleogene (K-Pg, formally K-T) extinction event, 66 Ma: The K–Pg extinction was caused by a massive asteroid impact that wiped out the dinosaurs, making way for mammals to dominate. The impact devastated the global environment, causing prolonged winter which halted photosynthesis in plants and plankton.
The K-Pg boundary exposed along Interstate 25 near Raton Pass in southern Colorado. The white layer is the K-Pg ejecta layer. Pollen and spores from Cretaceous plants are found immediately below this layer but not above it. [Credit: Kirk Johnson, Denver Museum of Nature & Science, through the NSF Multimedia Gallery].
Beyond the ‘the big five’ mass extinctions of the past there happens to be a 6th mass extinction, and it is happening right now. During the past 100–200 years, biodiversity loss and species extinction have accelerated, to the point that most conservation biologists now believe that human activity has either produced a period of mass extinction, or is on the cusp of doing so.
Bufo periglenes, the Golden Toad, was last recorded on May 15, 1989. [Wikipedia, Public Domain].
Take this quote from, “The biodiversity of species and their rates of extinction, distribution, and protection” in the journal Science.
Current rates of extinction are about 1000 times the likely background rate of extinction. Future rates depend on many factors and are poised to increase. Although there has been rapid progress in developing protected areas, such efforts are not ecologically representative, nor do they optimally protect biodiversity.
Or another breathtaking quote from, “Introduction to special issue on biodiversity” in the journal Botany.
…biodiversity is diminishing at a rate even faster than the last mass extinction at the end of the Cretaceous Period, 65 million years ago, with possibly two-thirds of existing terrestrial species likely to become extinct by the end of this century—the vast majority of them unknown to science at the time they disappear.
I will just come right out and say it, this is very scary. We need diversity of species for a livable planet, and each time another species goes extinct, a Jenga block is removed from the tree of life. All species depend on other species for survival. Eventually, too many blocks will be removed, and the tree of life will collapse. We shouldn’t chance this one, so I would suggest that any chance you get, please take action or make decisions that you think could help!
This concludes the Paleoclimate: Life on Earth series. In upcoming newsletters, I will definitely continue to discuss past climate, such as The Great Oxygen Catastrophe, but I will also look at modern climate topics and how they relate to the past, like the potential for West Antarctic Ice Sheet (WAIS) collapse, and what that means for coastal cities around the world. Later this year, I will start the Paleoclimate: Ice Core series.
Thanks for reading, and please consider supporting this newsletter with a paid subscription, and as always, share widely!
Sincerely,
TRJ, PaleoClimate Scientist