Extinction History

Woolly Mammoth ExtinctionExtinction is a fairly new concept.  In 1796, Georges Cuvier at the Paris Museum of Natural History suggested that some animal species that had existed on Earth in the past, had subsequently gone extinct.  He had received the fossilized remains of a Woolly Mammoth from Ohio and determined that they were not the bones of any existing elephant.  Prior to 1796, it was assumed that creatures that didn’t live in an area where their bones were found had simply moved on to a different habitat.  Many species were quickly added to the extinct species list.

 

Normal Extinction Causes

Species are good at finding ecological niches where conditions are right for finding nutrients, reproducing, and finding shelter.  As conditions in the niche change, organisms that are mobile can move to a better location.  Species that are good at adapting to change will stay put and evolve over time to the new conditions.  Species that can neither move nor adapt, may die out completely – become extinct.

Habitat change may occur because of changes in water availability or purity and weather or temperature changes.  Invasive species may arrive to compete for nutrients, prey on local species, or take over nesting areas.  Occasionally a large disruptive force such as a volcanic eruption or asteroid impact will change habitats abruptly with devastating results.

As the science of Geology developed, fossilized remains of plants and animals were discovered in different sediment layers.  It has been determined that there have been five mass extinction events over the last 450 million years.  Although extinction is a normal part of life with a background rate of one species going extinct each year for every million species on Earth, these mass events saw huge spikes in the numbers of extinctions.

Ordovician-Silurian Extinction – 443 million years ago

At this time, most life on Earth lived in the water with a few animals and plants that had moved onto the shorelines.  This event was triggered by climate swings associated with glaciation and melting cycles which also affected the water level and pH.  This event lasted for 4.4 million years and wiped out 85% of all marine life.

Late Devonian Extinction – 359 million years ago

The causes of this event are not well understood, but could have been up to seven separate events occurring over a 25 million year period.  75% of all life was wiped out.  Corals were especially hard hit and didn’t recover for 100 million years when new corals evolved.

Permian Extinction – 244 million years ago

This is the monster of the five mass extinctions.  Magma started flowing in the Siberian Steppes and eventually covered an area the size of the United States to a depth of four miles!  This created huge amounts of carbon dioxide and methane gases which caused the planet to heat up.  In addition, the CO2 mixed with water at the surface of the oceans to increase its acidity.  There were also lethal acid rains and as the vegetation died, flash floods became common.  This event which lasted for 60,000 years killed off 96% of all life on earth including almost all the insects.

Triassic – Jurassic Extinction – 200 million years ago

This is another extinction that was probably caused by multiple events – climate change, eruptions, and possible asteroid collisions.  Over an 18 million year window, 76% of life was extinguished – mostly animals.

Cretaceous – Paleogene Extinction – 65 million years ago

After the Triassic-Jurassic Extinction, certain animals thrived including the dinosaurs which became dominant.  This extinction event may have been building for a couple of million years, but reached a climax when a 7.5 mile wide asteroid struck near the Yucatan Peninsula at 45,000 miles per hour.  The shock wave killed animals as far away as Canada a couple of minutes after the impact.  Besides a huge tidal wave that circled the world, massive amounts of dust were kicked up into the atmosphere that blocked the sun for years.  The plants died off.  That lead to the demise of the herbivores which led to the demise of the carnivores including all of the land dinosaurs.  This extinction wiped out 75% of all life.

Biodiversity Today

There are about 2 million named species.  There are an estimated 5 – 30 million species on Earth which means that there are still a lot left to be discovered.  As Edward O. Wilson says, “We live on a little known planet.”  Each time there was a mass extinction event, opportunities were created for the surviving species to thrive without as much competition.  The background speciation rate has been greater than the background extinction rate so we currently have the most species of all time.