One of the most amazing things about Earth is the record of past events we find in the rocky layers of Earth's crust. As scientists study this record, global patterns emerge and we see that at least five times in Earth's history there were major periods of extinction. During these periods of time, large numbers of species simply disappear from the geological record to be replaced by other, newer species. Glacial cooling, oxygen reduction, ocean level reduction, meteorite impacts, volcanic eruptions, and major changes in the shapes of continents are blamed for most of these mass-extinctions.

Evidence for past mass-extinctions comes almost entirely from the fossil record. It is a great challenge for scientists to interpret the fossil record and be certain that their interpretations are correct. The events mentioned above represent the best explanations scientists have up to this point. You may wonder why it is important for people to know about things that happend so far in the past. One reason is that biologists are studying and documenting the extinction of many species happening today. It may be possible to add humans to the list of agents that might cause mass-extinctions, but scientists are not certain how much damage humans are doing. Comparing events from the fossil record to things we can see today may help us understand the role humans have in the biosphere. For humans to interact with the biosphere requires that we develop this type of understanding.

Some scientists believe that Earth can take care of itself and, if humans misbehave, we will simply be wiped out and replaced by other species. Other scientists and activists believe that humans are causing the extinction of so many species that we are damaging the biosphere beyond repair. If you think about it, these points of view may lead to the same conclusion.

Major extinctions today are caused by human activities such as habitat descruction, introduction of non-native species, overharvesting and pollution. Scientists agree that we need to study these human activities and learn to manage them where possible. It is also true that some extinctions are caused by natural events that have nothing to do with human activities.

Extinction Project:

1. Gather information about the evidence for an ancient mass extinction.
   • Internet search (use "mass extinction")
   • Textbooks
   • Library
   Gather information about the evidence for an current extinction.
   • Internet search (use "current extinction")
   • Textbooks
   • Library
2. Prepare a report comparing the reasons for ancient and current extinctions. Your report may be any of the following:
   • Written essay
   • Multimedia Presentation
   • Video Newscast
   • Magazine article
3. Share your presentation with your teacher, a parent or guardian, friends, and/or classmates.