Summary of major eras. Note the large amount of time before the Paleozoic, or era having first complex life forms.
History of life
The earliest morphological evidence for life is 3.5 billion years old, fossils of stromatolites (colonies of cyanobacteria) and single, undifferentiated cells, or Prokaryotes.
For 1.6 billion years these simple cells were the only kind of living organism, until the arrival of Eukaryotes, or single cells with differentiated nuclei and cell organelles. Although representing a large leap in complexity, the Eukaryotes were still only single cells or cell aggregates.
It would be another 1.4 billion years, before complex, multicellular life made an appearance in the form of the Ediacaran faunas followed by all the variety of the true Cambrian animals about 550 million years ago.
Summary of major eras. Note the large amount of time before the Paleozoic, or era having first complex life forms.
Early life
Ediacaran or Vendian fauna
Animals first appear in the fossil record around 580 million years ago as frond-like forms, jellyfish-like imprints, and trace fossils. These fossils appear simultaneously on all continents, except Antarctica, and each assemblage contains roughly the same kinds.
Cambrian
First diversity of animals that resemble modern forms occurs in the Cambrian "explosion". Almost all major groups of animals can be traced back to fossils from this time period. Compare the diagrams of the vendian animals on your left with that of Cambrian reconstructions on your right. Those on your right are recognizable as relative of arthropods, snails, and various "worm" groups.
Vendian >>>>>>>>>>>>>>>>>>>>>>>>>Cambrian
In fact, it is at the base of the Cambrian period, about 545 million years ago, that most modern animal clades at the traditional level of phyla appear. Many also radiate during this period. This event is sometimes called the "Cambrian Explosion", because of the relatively short time over which this diversity of forms appears. We know have several sites where organisms were preserved because they were buried rapidly in a mud slide. Burgess Shale type assemblages show that this is a radiation of soft-bodied as well as skeletonized organisms or body plans in general. Between 60% and 80% of the fossil remains are those of soft organisms.
http://park.org/Canada/Museum/burgessshale/faunaandflora.html
Most explanations for this radiation involve properties of animals themselves (intrinsic causes), but the coincident events in algae and protozoans suggest perhaps a more ubiquitous, ecologic trigger (extrinsic causes). One possibility is oceanographic changes that increased nutrient supplies to the shallow waters, hence a radiation of trophic links in the food chains of that time. Oxygen levels were increasing for the same reason. After the glacial period, temperatures rose until by the time of the Cambrian itself the climate was warmer than it is today. We also do not know why the innovation stopped. There have been extinctions since the Cambrian but an increase in very different body has not been part of any following radiation of forms.
Now we have evidence from two other localities that are older than the Burgess Shale or the first site yielding a number of Cambrian fossils. Chengiang adds many new species to a Burgess shale type setting, so sets the time for the Cambrian explosion back about 10-15 million years. Interestingly enough there are many more chordates and ancestors in this biota.
http://en.wikipedia.org/wiki/Vetulicolia
The Sirius Passet biota are also about 15 million years older but hint at simpler forms (perhaps living deeper?), but also give more evidence for more worm like and mollusk type of fossils.
The role of chance at the macro level or extinction in shaping evolutionary history.
Some Generalities About Extinctions
The fossil record of marine animal species reveals that over the last 550 million years the average species lived for only about 4 million years. So most of the animals living on the earth have gone extinct. The record of extinction is broken up into background extinction and major extinctions.
Background extinction?
http://evolution.berkeley.edu/evolibrary/news/090901_extinctionrisk
The most changes in flora and fauna, however, have been associated with mass extinctions. These have undeniably reset evolutionary history .
Mass extinctions
What is a mass extinction? A highly elevated rate of extinction of species, extending over an interval that is relatively short on a geological time scale Note short geologically may be 100,000 years or more..
Most texts recognize at least five major extinctions based on the number of species involved or the changes caused by disappearance of groups on diversity.How to recognize a mass extinction from the fossil record.
Examine the diagram below. A period of diversity in several groups ends abruptly and is followed by a period of little diversity. Finally we can see radiation occurring often in unrelated groups. For example, the extinction of the dinosaurs at the end of the cretaceous is followed after a time by mammal radiation.
What factors contribute to major extinctions.
Continental drift.
One of the contributing factors to both radiation and extinctions is continental drift. Compare the diagram below of positions of the continents 1 billion years ago to their present distributions.
1 billion years ago
As continents move, bump and collide with one another, volcanoes and volcanic belts form. Ocean expansion by molten material moving upward toward the crust also leads to upheaval.
https://www.youtube.com/watch?v=JmC-vjQGSNM
Another viewpoint meant for a younger audience but extremely graphic with regard to plate movement. https://www.youtube.com/watch?v=zbtAXW-2nz0
When the effects of drifting continents contribute to an extinction is chance. Eventually the bumping and also moving of plates eventually results in volcanic activity that leads to major changes in undersea levels, ocean circulation, warning of waters and land and also changes in atmospheric gases. Sometimes these effects can occur in a short geological period of time
Global changes in earth's temperature.
Another contribution to extinction is the reported rises and falls in temperature that gave rise to ice ages followed by periods of relatively mild conditions. These can be tied to variations in earth orbit as well as continental drift.
(For more information if you are interested. You will not be tested on the information on these websites.
https://www.livescience.com/58407-how-often-do-ice-ages-happen.html
http://www.pbs.org/wgbh/nova/earth/cause-ice-age.html)
Other extrinsic (not part of the planet-moon-sun dynamic interaction) forces such as meteorites have been implicated.
The best data to date implicating meteorites exist for the extinction at the end of the Cretaceous.
https://www.youtube.com/watch?v=tRPu5u_Pizk
http://phys.org/news/2014-12-asteroid-dinosaurs-mammals.html#nRlv
The greatest extinction was at the end of the Permian when some scientists claim about 90% of life forms disappeared.
This film clip summarizes the major changes occurring and contributing factors to the Permian extinction.
After viewing the film clip, view the first 45 minutes of the youtube film below. This film give a more detailed explanation of how carbon dioxide as well as methane gas contributed to this major extinction.
So who lives through a mass extinction?
Plants are better at it than animals http://phys.org/news/2015-02-survive-mass-extinctions-animals.html
Other than widespread distribution, animals were on the smallish size, had generalized diet, and exhibited broad ecological tolerance. A burrowing habit, ability to go dormant, and fondness for seeds and dried material are considered good assets. Nothing on land larger than a house cat is reported to have survived the KT extinction event. But the first hits are random For example, how far away you are from an erupting volcano or falling meteor will certainly affect your ability to survive. So it pays to also be lucky.
Extinction: Study guide for quiz.
1. In one short paragraph, compare Vendian fauna to Cambrian fauna.
2. What is the difference between background and mass extinctions?
3. In a few sentences describe how do you recognize a mass extinction in the fossil record? .
4. Describe two lines of evidence that a large meteorite contributed to the extinction at the end of the Cretaceous.
5. Describe the three phases of the Permian extinction. Include the role of carbon dioxide anmethane gas
