By Robert Hazen, George Mason University
One of the key ideas of Darwin’s theory of evolution by natural selection is that new species have to replace old species. By some estimates, as much as 99.9 percent of all the species that ever lived on Earth are now extinct. This process of extinction, and how it occurs, is one of the hottest topics in evolutionary research today.
Fossil Records of Mass Extinctions
Fossil records reveal at least five distinct mass extinction events: this is when a significant fraction of all the species became extinct, in essentially a geological instant.
In a brief geological interval—certainly less than a million years, maybe much shorter than that—as much as 96 percent of all living species vanished. We can find out about these extinctions by carefully looking at rock layer after rock layer, and the organisms that are contained therein. This also shows us that life on Earth is very resilient, because each time there’s been a mass extinction, it’s come back with a vengeance, with whole new varieties of organisms.
How a Comet Caused an Extinction
The causes of mass extinctions are a matter of much debate. One clear contributing cause is death by a large asteroid or comet impact. Worldwide evidence suggested this happened with the demise of the dinosaurs, 65 million years ago.
A giant asteroid, perhaps 20 miles in diameter, slammed into the Yucatan Peninsula. Huge amounts of dust and smoke from that collision, ensuing fires all over the world, would have blanketed the Earth with a thick coating of dust and debris and blocked the sunlight; it would have killed off many of the plants and caused a prolonged, premature winter that many larger organisms couldn’t have survived.
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The Permian Extinction
The great Permian extinction, 245 million years ago, is a different matter. There’s no clear evidence for any large impact in that period; instead, this episode seems to have spanned about a million years of extreme environmental stress.
By the end of the Permian period, all of the Earth’s major continents had merged into one supercontinent. This caused drastic changes in global climate: it caused changes in the amount of coastline; global sea levels dropped, probably by about 300 feet, perhaps as a result of the increase of the volume of the ocean basin when the continents all came together.
Other researchers point to the possibility of sudden poisoning of the ocean by a rapid increase in carbon dioxide—or even an overturning of the ocean, in a very unusual kind of event; this is evidenced from massive limestone deposits from right around that period. Added to these dramatic changes, there was a million year period of intense volcanism in Siberia. That’s an episode that may have spewed out a cubic mile of lava every year, while it polluted the atmosphere and darkened the skies with various volcanic exhalations.
Extinction by Supernova Explosion
Astronomers have come up with at least one other plausible scenario for mass extinction. That would be a supernova—any star much larger than the Sun ends its life in a supernova explosion that instantaneously unleashes frightening amounts of intense radiation for a short time period.
If one of the stars underwent a supernova explosion, our planet would’ve been bathed in killer radiation, and the effects on the planet’s biosphere might have been severe. There’s no evidence that such an extinction has ever occurred, but the possibility still remains.
Man-induced Extinction
Fossil bones indicate that approximately two-thirds of the large mammal species in North America and South America died out about 11,000 years ago. This mass extinction differs from others in two key respects:
- Smaller life forms don’t appear to have been affected at all by this mass extinction.
- Human stone weapons are often found among the scavenged bones.
So, to the list of potential causes of mass extinction we have to add human activities. The growing evidence for a human-induced mass extinction is absolutely staggering. In Australia, which was the home of 50 species of large mammals just 50,000 years ago, there are now only 4 species of that size. Hawaii has lost more than half of its approximately 100 species of native birds, and it’s lost perhaps 99 percent of the estimated 1,000 species of land snails. These are brilliantly colored snails, avidly sought by collectors. Collectors would go to Hawaii, wiping out an entire species, through this collecting.
Overfishing has drastically reduced populations of wild salmon in the Northwest, of blue crabs and oysters in the Chesapeake Bay, of schools of fish in the northern oceans, which are dwindling drastically now.
Biologists fear that the planet’s extinction rate will be in the order of one species going extinct every few minutes. As the human population increases virtually without control, wilderness areas, especially the dwindling tracts of species-rich tropical rainforest, are being gobbled up. To these immediate causes of extinction, you have to add the long-term effects of global warming, of ozone depletion, of atmospheric pollution—all of which lead to environmental stress.
Charles Darwin’s theory of evolution by natural selection continues to receive much study and much research, as scientists attempt to understand the rates of evolutionary change, as well as the causes of extinction.
Common Questions about Causes of Mass Extinction
The fossil records reveal at least five distinct mass extinction events: this is when a significant fraction of all the species became extinct, in essentially a geological instant. By carefully looking at the fossils, we can see that in a brief geological interval—certainly less than a million years, maybe much shorter than that—as much as 96 percent of all living species vanished.
Astronomers believe that if any star much larger than the Sun ends its life in a supernova explosion, then it instantaneously unleashes frightening amounts of intense radiation for a short time period. If one of the stars underwent a supernova explosion, our planet would’ve been bathed in killer radiation, and the effects on the planet’s biosphere might have been severe.
As the human population increases virtually without control, wilderness areas, especially the dwindling tracts of species-rich tropical rainforest, are being gobbled up. Moreover, overfishing has drastically reduced populations of wild salmon in the Northwest, and of blue crabs and oysters in the Chesapeake Bay.
