Mass Extinctions and Radiations

Learning Objectives

1. Students will identify at least the largest and the most recent mass extinction events in the last 540 million years (the Phanerozoic eon) of Earth history.
2. Students will cite evidence for possible causes of these mass extinctions.
3. Students will explain what causes major radiation events in the history of life on Earth.

Species have come and gone throughout the history of life on Earth. Based on the fossil record, paleontologists estimate that over 99% of all species that ever lived have become extinct. On top of a steady background level of extinction, the fossil record is punctuated with mass extinction events, as shown in the figure below, for the Phanerozoic.

End-Permian Extinction (251-252 mya)

This was the single greatest mass extinction event in the Phanerozoic. About 95% of all species appear to have perished around this time. Global photosynthesis rates declined markedly, atmospheric CO2 increased by 2000 parts per million (ppm) (what is the current atmospheric CO2 level?), and temperature increased by about 10 degrees C.
Possible causes:

• vulcanism – a flood basalt eruption, called the Siberian Traps, one of the largest volanic events in the Phanerozoic, occurred over the last million years of the Permian and covered over 2 million square kilometers with lava
• greenhouse gases and global warming – massive releases of carbon dioxide from volcanic eruptions resulted in warming of the oceans and releases of more gases from warming ocean waters, including carbon dioxide and methane from methane hydrates.
• changes in ocean chemistry – the positive feedback loop of warming oceans resulting in increased release of greenhouse gases amplifies global warming and results in anoxic oceans.
• release of toxic gases? the video below explains one hypothesis about massive growth of sulfate-reducing bacteria releasing toxic hydrogen sulfide gas

End-Cretaceous Extinction

This was the latest mass extinction, associated with the end of dinosaurs as dominant vertebrates on land. This extinction coincides with a bolide (meteor) impact, that created the Chicxulub crater found off the coast of Mexico’s Yucatan peninsula. The impact would have created a huge ejection of dust into the atmosphere, significantly reducing the amount of sunlight that could reach the Earth’s surface, cooling the planet, reducing photosynthesis, and collapse of ecosystems.
The evidence for the bolide impact at the end of the Cretaceous period is compelling. However, other stressors may have pre-dated the impact event and contributed to the failure of dinosaur populations to adapt or recover.
The video below is not very slick, but is relatively short and discusses the extinction from a biologist’s point of view.

 

The Cambrian Explosion and Other Major Evolutionary Radiations

After each mass extinction, life on Earth recovered and diversified over succeeding millions of years, to attain even higher levels of biodiversity than before the extinction. The graph below shows the number of genera during the Phanerozoic:

 
The Cambrian “explosion” refers to an increase in biodiversity of multicellular organisms at the start of the Cambrian, 540 million years ago. Multicellular life appeared only several tens of millions of years before the start of the Cambrian, as enigmatic fossils (Ediacaran biota) exhibiting body plans unlike present-day animals. These largely disappeared and were replaced by Cambrian fauna, whose variety includes all of the body plans found in present-day animal phyla. The appearance of Cambrian fauna span millions of years; they did not all appear simultaneously as the term “explosion” inaccurately implies.

Periods of increasing biodiversity and rapid speciation are called evolutionary radiations. They occur when new ecological niches become available, as after a mass extinction. They may also occur in response to an evolutionary innovation. One example would be the evolution of jaws. The ability to bite gave predators a huge advantage. Prey species adapted in a variety of ways, such as faster speed, body armor, defensive weapons such as horns and stingers, chemical defenses in the form of poison, larger body size, smaller body size, etc.
The Cambrian radiation also coincides with an increase in free oxygen to near-present day levels.

The increase in oxygen enabled the evolution of larger bodies and organs and tissues, such as brains, with high metabolic rates.
The increase in oxygen is a dramatic example of how life can alter the planet. Evolution of oxygenic photosynthesis changed the planet’s atmosphere over billions of years, and in turn caused radical shifts in the biosphere: from an anoxic environment populated by anaerobic, single-celled prokaryotes, to eukaryotes living in a micro-aerophilic (low-oxygen) environment, to multicellular-organisms in an oxygen-rich environment.
Humans are the latest in a succession of evolutionary innovations, who are now altering the planet, with yet undetermined consequences for life on Earth.