Ideas for teaching about early life

Are stromatolites biogenic?

Submitted by Susan Childers, Bauer, Mark Leckie, Shoichi Kiyokawa, Bruce Oldfield, Huiming Bao

Goals:

1. evaluating hypotheses
2. demonstrating scientific method

Hypothesis:

How do you know stromatolites are biogenic?

Activities/Data:

Collect examples of modern stromatolites and inorganic structures; compare to Archaean stromatolites and discuss what is biogenic, how do you know?

Using photos of:

Organic: mats, varnish Inorganic: travertines, speleothems, laminated sediments (varves, black shales), soft sediment deformation, metamorphic rocks with folds, agates, flow banding in rhyolites, nodules

Analysis/Discussion:

Apply the credibility scale to ancient forms

• Discuss levels of credibility (what makes something credible)
• Conclusion/wrap up

Where do we go from here?

Begin to discuss biosignatures; biomarkers; isotopes; bring in discussions of 'life' in meteorites, astrobiology.

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Early Life Brainstorming

Submitted by Julie Baldwin and others

Learning Goals

Dealing with uncertain science
Personal Bias
Reasonable Doubt

Analogy—Trial involves uncertainty
establish procedure for dealing with uncertainty

Question:

When did life originate on Earth?

Objective:

How do you deal with uncertainties in science?

Convey that uncertainty is a necessary and acceptable part of doing science

Introduction - What is Life?

• Break into small groups and come up with their own definitions of life
• Lab—how would you classify fossils/pseudofossils—alive or dead?
• Darwinian definition of life—mutation, replication, natural selection
• Discussion of viruses?

Mock Trial

• instructor acts as lawyer—presents evidence
• Schopf vs. Brasier
• debate on fossil evidence for life
• abiotitic vs. biotic stromatolite images
• explain Grotzinger & Rothman hypothesis
• 12 non-experts does not make for good scientific judgment
• there will always be reasonable doubt—convey to students that that is ok in science
• differences between science and law—appeals ad nauseum in science
• discussions and replies in science vs. finite appeals in law

Wrap-up - When did Life arise?

• use Stan's "credibility meter" to classify based on level of uncertainty
• show images (similar to those in Stan Awramik's talk)
• have students classify how certain they are a particular example is bioitic using PRS or hand votes

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The Big Question: Are Archean stromatolites biogenic?

Submitted by Mark Skidmore, Stan Awramik, Susan Conrad, Darrell Henry, Aaron Cavosie, Dyanna Czeck

Learning Goals

Enforce concept of scientific method
Evaluate the abiogenic vs. biogenic origin

A non-genetic definition of stromatolite:

"an attached, laminated, lithified sedimentary growth structure, accretionary away from a point or limited surface of initiation." Semikhatov et al. (1979)

A genetic definition:

"laminated organo-sedimentary structures formed by the trapping and binding, and/or precipitation of minerals by microorganisms" Walter (1976)

Activities:

A Mars Return Mission: A laminated sedimentary rock is returned from Mars and provided to the students (use your favorite local laminated sediment for proxy).

Question to pose to students:
How does one determine life is recorded in the rock?

To address morphology issues:

(a) Provide students with solid examples of inorganic forms of layered rocks malachite, geodes, stromatic migmatites, sheath folds

(b) Provide students with solid examples of organic forms stromatopaloid (Paleozoic sponge), corals, colonial bryozoans

(c) Hybrid examples: travertine

Expectations of students

Upper level students: Write an outline of a proposal to NASA to allow you to work on the rocks because you have determined how to determine evidence of life Lower level students: Write a press-release for the local newspaper that describes aspects of the rock

Aspects to ponder:

Are stromatolite forms scale independent?
observations in thin section (~100 um) to outcrop (~5 m scale)

References:

Awramik and Grey (2005) SPIE

Grotzinger and Knoll (1999) Annual Reviews of Earth and Planetary Sciences

Hoffmann et al. (1999) GSA Bulletin

McKay et al. (1996) Science

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Life exercises

Submitted by Linda Sohl, Joe Hill, Paul Mueller, Alisa Hylton, Carol Frost, Mike Willimas, Bosiljka Glumac, Mike Phillips

Question 1:

How do you explain the origin of conical laminated structures in the geologic record? Explain in terms of (A) making the structure and (B) preserving the structure.

Ref: Hoffman et al., GSA Bulletin, 1999.

Activity:

1. Provide students with pictures and thin sections of conical stromatolites, and various sizes of sands (wet & dry), and a 'biofilm' (e.g. hairspray or similar).
2. Have students try to replicate the conical structures using the provided material in various combinations (size, wet vs dry, with or w/o 'biofilm')
3. Measure angle of repose (goiniometer or protractor) and record results of various experimental runs.
4. Discuss experimental results.
5. Provide examples and discussion of the 3-dimensional aspects of actual stromatilites.
6. Provide examples and discussion of other non-biogenic structures (e.g. buckle folds)

Question 2:

At what point is there convincing evidence of life?

Activity:

Present students with Awramik meter of certainty and present students with synopses of key bits of evidence along a timeline.

Ask students to place an arrow at the point on the timeline where they are comfortable with clear evidence for life.

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What is the evidence for Pre-Phanerozoic life, and does it extend back into the Archaean?

Submitted by Albert Patino Douce, John Zawiskie, Joe Reese, Wally Borowski, Michelle Markley, Lucinda Shellito

Build a large time line on the scale of a lab table, with ages and photos of classic localities at progressively increasing magnifications: organo-sedimentary structures and thin sections. A world map with geographic location of localities and outcrop photos will also be included. Two versions could be possible: at an introductory level, the table will be given to them, together with some background information on e.g., prokaryote vs. eukaryote cells, and have them draw their own conclusions from the information on the table. At a more advanced level, students will be given some key references and ask them to build the time line themselves. The goal is to show them how to integrate information and draw their own conclusions, and to visualize the relative magnitudes of information and gaps. Once they have mastered this stage, students would be guided in how some of the gaps can be addressed with biomarkers, isotopic and other chemical information.

Define unequivocal evidence for Archaean life. Base on assigned reading of the controversial papers by Schopf and Brasier, having half the class take each side and try to build a heated argument.