A new review in Nature argues that the earth’s sixth “mass extinction event” may have already started. Bummer.
As their name implies, “mass extinction events” are marked by the death of at least 75% of living species over a relatively short period of geological time. This has happened five times in the history of life on earth. Modern fossil, ecological, and genetic evidence shows current trends that are leading us down the ominous path towards mass extinction. Changes in climate are the ultimate cause of mass extinctions; of course, these changes have never been brought about by a single species. Nice one, humans.
Here’s a quick rundown of the mass extinction events of yester-millionsofyears that left in their destructive wake the phylogenetic equivalent of a handful of cockroaches, some moss, and Charlie Sheen.
The Cretaceous –Tertiary Event
65.5 million years ago; 76% of species die
Read: dinosaurs getting murdered by a giant asteroid. This oft-cited mass extinction event (let’s call them MEEs now) is the most recent. But it wasn’t just the dinos that got wiped out by the asteroid and its ruinous global-cooling and sun-blocking after-effects; species of microbiota, plants, insects, and tons of marine life died out in the Cretaceous MEE as well. And the one class, the mammals, that somehow made it through without losing a single lineage, now houses the species likely responsible for the next MEE. Go figure.
The Triassic-Jurassic Extinction Event
200 million years ago; 80% of species die
This MEE is characterized by a sharp loss in marine life, though every ecosystem took a hit. It was marked by the death of an entire class – the conodonts – who were slithery big-eyed eel-like creatures that populated the ocean. This MEE is thought to be the result of global warming caused by massive C02-releasing volcanic eruptions. It is not known if Lord Xenu / atomic weapons had any involvement.
The Permian–Triassic Extinction Event
250 million years ago; 96% of species die
This huge MEE is aptly titled “The Great Dying.” Scientists think this event was likely caused by a giant clusterfuck of environmental disasters all happening at the same time. Eat your heart out, God; this hell-on-earth makes locusts and floods look like ladybugs and tiny puddles. I’ll turn to the concise words of the paleontologists behind the aforementioned Nature review for this one:
Siberian volcanism. Global warming. Spread of deep marine anoxic waters. Elevated H2S and CO2 concentrations in both marine and terrestrial realms. Ocean acidification. Evidence for a bolide [asteroid] impact still debated.
The Late Devonian Extinction
360 million years ago; 75% of species die
The world was one big continent called Pangaea at the time of the late Devonian MEE. There are some interesting ideas about the causes of this one. In most cases, an increase in the number of species (speciation) is followed by a decrease as new invades the niches of old. In this MEE, a vast diversification of land-based plants led to a drop in global C02 levels which led to global cooling. Global cooling led to sea level changes, and lot’s of marine species went extinct. So, plants are to the late Devonian MEE as humans are to the impending MEE, minus the conscience.
The Ordovician Extinction
443 million years ago; 86% of species die
All life was marine during the Ordovician period, existing mostly in the shallow seas surrounding Pangaea. As the mega-continent drifted south and a severe ice age reared its head, temperatures plummeted, C02 concentrations dropped, and sea-levels bobbed up and down. Most species living in the fragile Ordovician ecosystems couldn’t handle the rapid changes and bit the bullet. Furthermore, the Appalachian Mountains – which at the time were on their way to becoming as tall as the Himalayas are today – were going through puberty and messing with Pangaea’s chemistry. Luckily, some species made it through.
So: are we having an MEE now? Elizabeth Ferrer and her colleagues seem to think we’re either in one now or on the way:
Increasingly, scientists are recognizing modern extinctions of species and populations. Documented numbers are likely to be serious underestimates, because most species have not yet been formally described. Such observations suggest that humans are now causing the sixth mass extinction, through co-opting resources, fragmenting habitats, introducing non-native species, spreading pathogens, killing species directly, and changing global climate. If so, recovery of biodiversity will not occur on any timeframe meaningful to people: evolution of new species typically takes at least hundreds of thousands of years and recovery from mass extinction episodes probably occurs on timescales encompassing millions of years.
The current mass extinction could end up with a boring name like the “Quaternary extinction event,” but that’s assuming our scrappy little species lives long enough to name it (or decides to move on to mass-extincting some Martians). Maybe we should just call it the “Grand Finale!”