Credit: Ken Doud

The Precambrian-Cambrian transition is an enigmatic time in Earth history that is currently receiving much focused attention from the scientific community. Part of this is the endeavor to better understand our evolutionary roots, while another part of this is to better understand the perturbations that cause permanent changes in the way Earth’s spheres interact. One point in time that is receiving much scrutiny is the Cambrian ‘Explosion’ and the events that led up to it. The term ‘Cambrian Explosion’ refers to the interval where life began a massive radiation that gave rise to new phyla, and some of the most mysterious organisms in the fossil record. This page is setup to walk you through the possible ’causes’ or ‘triggers’ for this famous radiation event, and then walk you through the Cambrian Explosion. There are four main triggers that have been proposed, each operating over different temporal ranges. We will explore these triggers below. Break this assignment up, and don’t try to complete it in one sitting. Within this page is a vast amount of information, so give yourself time to pace yourself and not get burned out.

For each of the readings on this page, you will need to complete a 3-2-1 assignment. To do this, you will identify the 3 most important concepts of the reading, and justify each of these choices. Then, give 2 explanations of how these concepts come together to form the hypothesis for each of the triggers described. Finish it up by asking one question of some aspect of the paper that you are still unsure of.

Lastly, you will write a short reading response that outlines each of the triggers discussed on this page, and how they come together in terms of the Cambrian Explosion. Use your already constructed 3-2-1 assessments to guide this reading response. Additional instructions for this can be found at the end of of this module.

The rise of atmospheric oxygen

We’ll begin by exploring the first potential trigger: oxygen in the atmosphere. Early earth, such as during the Archean, is hypothesized to have not had as much available oxygen in the atmosphere, due to a low abundance and diversity of photosynthetic organisms. Through the years there have been multiple hypotheses regarding the rise of atmospheric oxygen, with many hypotheses centered on a two-phase addition of oxygen. Read Lyons et al.’s (2014) paper below to get a better idea of what early Earth’s atmosphere was like, what drivers pushed oxygen into the atmosphere, and how the Earth was forever changed.

 

 

As early photosynthetic life was exporting oxygen into the atmosphere, it was pulling down carbon dioxide from the atmosphere as well. Lower carbon dioxide levels are usually associated with lower temperatures. This massive drawdown has been hypothesized to have setup the Earth for another event that would have played a role in Earth’s life system: Snowball Earth. Here are two quick readings about Snowball Earth and how Earth could have been placed in these dramatic conditions.

Snowball Earth Review

 

So…given what you have learned in your readings above, how did the Earth get out of Snowball Earth? AND, how would this have influenced life, in terms of being able to undergo the evolutionary transitions required to produce complex organisms closer to what we have today?

Eustatic sea level change centered around the SAUK transgression

Another possible trigger that has been hypothesized to lead to the diversification of life is the SAUK transgression, which is the first cratonic sequence of the Phanerozoic. Read the following paper which will outline how this transgression could have potentially fed the radiation of life.

 

True polar wander 

One of the more unique hypotheses that has been proposed as a trigger for the Cambrian Explosion is True Polar Wander. Here is an article that outlines this hypothesis and will help provide insight for how this might be a trigger. You don’t need to read the whole thing, but skim through it and make sure that you understand how this would work and its implications.

 

Oxygen and ecology

One of the more recent ideas that has come out points to a combination of factors that may have triggered that radiation of life. Read the article below focusing on the Eltonian Pyramid.

 

Now that you have gone through all of that, check out this cool reconstruction created by the Royal Ontario Museum. This is an idea of what all of these hypotheses lead up to!

 

 

For each of the readings above, complete a 3-2-1 reading assessment. Identify the 3 most important concepts of the reading, and justify each of these choices. Give 2 explanations of how these concepts come together to form the hypothesis for each of these triggers. Finish it up by asking one question of something that you’re still unsure on.

Lastly, write a reading response that chronologically outlines the above triggers and how they come together to explain the Cambrian Explosion. Use your 3-2-1 reading assessments as guides to accomplish this. Your final reading response should be about 1.5 pages and should being by summarizing the different hypotheses, and end by justifying which possible trigger you think provides the best explanation/most evidence for creating the Cambrian Explosion.

 

 

 

For more information on this dynamic and pivotal transition in life, the Royal Ontario Museum has a series of excellent pages covering the progression of life during this time. For ore information on Snowball Earth, checkout the official Snowball Earth webpage.

References
Ken Doud at http://www.videoscapes.net

External sources accessed on Nov. 2016