Mass Extinctions - 3

Put the words or phrases used in the text into the available blanks to complete the sentences.

---

Mass Extinctions

Most people are familiar with the term ‘mass extinction’ due to what is believed to have happened to the dinosaurs 65 million years ago when an asteroid hit the Yucatan Peninsula. Besides the impact of the asteroid itself, which caused massive fires and floods, the Earth’s atmosphere was filled with dust which blocked the sunlight and lowered temperatures dramatically, leading to the demise of the dinosaurs as a species. Others have cast doubt on this theory citing that dinosaur remains have been found that post-date the meteor strike and still others believe that the massive volcanic activity going on in India at that time also contributed to the darkening of the skies and the consequent fall in temperature. What is unquestionably true nevertheless is that besides the dinosaurs, a total of 76% of life disappeared in the End-Cretaceous event. The extinction of the dinosaurs, however, was good news for mammals because they flourished. In a way, mankind should thank his lucky stars for the asteroid that struck the Earth 65 million years ago because if the dinosaurs were still around, we wouldn’t exist.

This event, however, was not the only time when there were mass extinctions. The first mass extinction, called End-Ordovician,  happened 443 million years ago due to a change in ocean currents. This was initiated by a build-up in ice sheets which made warm currents flowing in the tropical and mid-latitude regions to become more frigid. This change in climate caused many forms of life to perish. Following this, when the ice sheets melted, there was still another change in currents and the remaining life forms that adjusted to the previous change were not able to deal with this one. All these changes resulted in 86% of all life forms on Earth becoming extinct.

The mass extinction that followed was between 359 to 380 years ago and was called End-Devonian. The reason behind this mass extinction was volcanic activity in Siberia which caused variations in climate, a lowering of oxygen levels in oceans and changes in the natural habitats that many species were not able to tolerate. 75% of life vanished during this time.

The greatest mass extinction that the planet experienced was 251million years ago in which 96% of all species ceased to exist. Again, volcanic activity in Siberia was the reason behind it and the event is referred to as End-Permian. Volcanic activity again was the cause of the following mass extinction  50 million years later, the End-Triassic, but this time Siberia was not to blame for the volcanic activity but instead was found in an area that is now covered by the Atlantic Ocean. This mass extinction event wiped out 80% of whatever life there was on the planet then. It did, however, clear the way for dinosaurs to dominate the Earth for millions of years.

As the text shows, mass extinctions are not a one off event. In fact, of all the species that have existed on the planet Earth, less than 1% are still with us now. Furthermore, some scientists believe that we are in the middle of a mass extinction event now. Life forms are becoming extinct at a rate which is over 100 times faster than what is natural. Unlike all the previous mass extinction events previously mentioned, this one is not due to natural causes but is caused by a life form that is on the planet now: man.