We looked at asteroid impacts and mass extinctions last week, focusing on the K/T “dinosaur-killing” extinction.
In this second of a three-part series, we’ll try to look more closely at other mass extinctions. It isn’t easy. Except for the K/T, none of the major events seems to have involved impacts.
Why should we care about them, if they didn’t involve sudden death from the skies?
Well, the Permian extinction wiped out over 90% of the species living on Earth . We really need to know why such a large die-off happened.
And, too, in the aftermath of all extinctions (including the K/T), life has rebounded. How does it find a way?
Another very good reason involves dollars and cents.
Mass extinction is an expensive topic
Through taxes, university fees, business transactions, and private donations, we fund most scientific research. Among the conflicting, often very dramatic claims about how mass extinctions work, how can we judge the worth of what we’re paying for?
There is no easy way to answer that question.
However, mass extinction is everybody’s business. And, fortunately, a lot of it involves basic geology, especially fossils.
How do we know about mass extinctions?
Earth’s fossil record is the key to understanding the geological record and how it all fits together.
In the 19th century, scientists knew that sometimes fossils change dramatically between rock layers that were deposited at different times. In between those layers are zones, representing many thousands or even millions of years, when little or no sign of life can be found.
To explain these changes and the rather lifeless zones, French naturalist Georges Cuvier reasoned that Earth is very old and has always been pretty much the same as it is now. There have been what he termed “revolutions” to account for those zones.
During these “revolutions,” great numbers of species are wiped out through natural causes – perhaps a succession of worldwide floods like Noah’s. Certain species die off, and new ones are introduced or created.
A few earth scientists, among them Charles Darwin, suggested instead that little things, like weathering and the gradual rise and fall of the land, have big geologic consequences, given enough time.
Uniformitarians could explain the zones of little or no life by some long-term process like changing climate. The dinosaurs, for example, died out because they couldn’t adapt to a changing world.
Here is a modern video, from the lay viewpoint, that nicely sums up the gist of that conflict between catastrophism and gradualism.
If you feel a little tense watching that (I did), you might now start to understand why it is so difficult to write about mass extinction today – well, considering that we now believe an impact was involved in the K/T extinction but not the other major die-offs, more difficult to write about.
There are a variety of competing interests invested in non-impact-related causes for mass extinction, even though there isn’t much that can be undeniably proven one way or the other.
Human nature being what it is, when we’re not sure, we argue.
I like what the guy in that video says: Whatever you think, that’s beautiful.
How many extinctions were there?
Five mass extinctions are definitely known to have happened. In each one, over 70% of the world’s species may have vanished:
- The Ordovician (Ash., in the graph below), some 440 million years ago
- The Devonian (F/F in the graph below), about 375-360 million years ago
- The Permian (P/T), about 250 million years ago
- The Triassic (T/J), 200 million years ago
- The Cretaceous (K/T), some about 66 million years ago
Here is a list of things that one group of scientists believes might have caused each of the Big Five. Note that there are other explanations out there, and no particular explanation has gained credence over any others.
There have probably been many other extinctions besides those five, for example, all the smaller ones shown on that graph by Keller et al. that we saw last week:
However, no one can agree on definitions for these lesser mass extinctions, so I’m not going to go into detail on them here.
There does seem to be a consensus on a few general points regarding mass extinction:
- Whatever the cause, extinction seems to be a thing only among complex organisms. Blue-green algae, for example, have been around for over 3 billion years.
- It seems to come in pulses, though no clear-cut periodicity has been identified.
- Species diversity increases after each event. Right now we are living in the most diverse age ever for life on Earth.
The devil is in the details.
So, are we all going to die?
The UK’s Natural History Museum quite nicely sums up what I have also found written about the Earth’s extinctions:
The evidence suggests there was no single cause for any of the mass extinctions in Earth’s history. Instead, a combination of factors created environmental changes that were so extreme they spelt disaster for all but a small number of lucky survivors.
Massive volcanic eruptions, shifting tectonic plates and asteroid impacts are leading culprits for many of the mass extinctions. They can create huge changes to climate, sea levels and the amount of oxygen in the oceans.
That’s wonderfully vague. Extinction after an asteroid impact happens quickly, for example, while tectonic plates move incredibly slowly and the effects of these motions also take time.
However, I also think that is as precise a description of mankind’s understanding of mass extinction as we’re going to get today.
When you get right down to it, apart from the evidence of impact associated with the K/T extinction, the only information we have about any of these events is just a collection of indirect data that can be interpreted in many ways.
The why of it may be up in the air, but what answers are there to the other questions we laypeople have:
- What do we need to do, if anything?
- How will the next extinction happen?
- Is it happening now?
- Which species are most vulnerable?
- Where should we focus our research?
As Mark Twain might say, there is a whole lot of conjecture about those matters, but very few known facts.
On Friday, we’ll try to get a useful perspective on all this.
- Fossils. Wikipedia.
- Barnosky et al., Table 1: The ‘Big Five’ mass extinction events. Nature 471, 51–57 (03 March 2011). doi:10.1038/nature09678
- Surviving Mass Extinction. Natural History Museum.
- Georges Cuvier. University of California Museum of Paleontology.
- Peter Ward. Under a Green Sky. HarperCollins. New York, 2007