There can be a lot of controversy about what life was like on the early Earth. I feel it is important that students explore the evidence that scientists use to construct their theories. Once students know the evidence and reasoning, they can choose to accept or reject those theories based on evidence. Here is an overview of what students will learn today.
Students need to check the bacterial lawns that they made yesterday for growth. In their lab notebook (or using the student handout), they will need to make a sketch of the surface of the plate.
Possible answers: A lawn of bacteria needs to be made so that the entire plate is covered with bacteria. That way you can tell if the bacteria are not resistant to antibiotics. If antibiotics keep bacteria from growing then no bacteria will be present in a circle around the antibiotic disk. If the bacteria are resistant to the antibiotics, then bacteria will grow up to the edge of the the disk or a couple of millimeters away from the disk.
Possible answers: A streak plate could be used because the presence or absence of bacteria would also show if a bacteria was resistance to antibiotics.
Ask the students how they define life? What criteria would you use to explain whether something is alive?
Have students list indicators of life in their lab notebook.
The purpose of this station is to provide an understanding of the Hadean period of the Earth's prehistory. Students will analyze several studies that explain how scientists explain Earth's prehistory. Specifically, they will look at the Late Heavy Bombardment. By using data collected from the Moon, scientists model what the early geology of the Earth was like. This is because unlike the Earth, the Moon does not experience erosion. Secondly, students will explore computer simulations that model asteroid strikes on Earth and that there would still have been areas that could have supported life.
(Note: I provide images of the surface of the Moon for my students and graph and raw data from the scientific studies. Students also read the following news article from Purdue University concerning rocks affected by asteroid impacts. Using their computer, they also stream parts of the computer simulation and answer targeted questions. If you have a large class, you can have students read the 2005 Scientific American article, A Cool Early Earth while they wait for the next station to become available.)
(Note: While explanations of early geology of the Moon were based on evidence collected from the Apollo missions, recently there have been other unmanned probes that have visited the Moon. It is this evidence as well as computer simulations which are the basis of this exploratorium station, If, like my students, your students want to know more, you can refer them to these more recent missions: LCROSS, Chandrayaan-1, and GRAIL).
First watch this video that describes and explains the great oxidation event.
Then students should consider the data scientists are using to explain the existence of the great oxidation event. The graphs from the several peer-reviewed studies for them to considered. Students also analyze three carboys. One carboy contains growing blue-green algae that may be similar to determine how much oxygen it releases. This carboy has one sensor monitoring atmospheric carbon dioxide and another sensor monitoring atmospheric oxygen. Another carboy contains sterile distilled water and has oxygen and carbon dioxide sensors. The third carboy contains a bacterial culture connected to an oxygen sensor and a carbon dioxide sensor.
Use this student handout with this station.
First view the video clip that explains what a stromatolite is
what a stromatolite fossil looks like.
Using the stromatolite information given in the data table and the application Google Earth, fly to the following sites. Next mark the location of the stromatolites on the World Map provided and place the completed World Map in the lab notebook or use the student handout provided.
Upload the images of the stromatolites (Stromatolite images from Schopf (2006)) into LoggerPro. Next, using the ruler, rock hammer, and scales on the pictures and the measure tool within LoggerPro, determine the size of the stromatolites in the images. Record that information in your lab notebook or on the data table provided in the student handout.
Provide a brief description of the possible ecosystem where these fossils may have lived.
Next look at the microfossils data table and using the application Google Earth, fly to the sites listed. Mark the location of the microfossils on the World Map provided. Place the completed World Map in the lab notebook.
Upload the images of the microfossils (Evidence of microfossils from Schopf (2006), Evidence for microfossils #2 from Schopf (2006), and Evidence for microfossils #3 from Schopf (2006)) into LoggerPro. Next, using the scales in the pictures and the measure tool within LoggerPro, determine the size of the stromatolites in the images. Record that information in your lab notebook or on the data table provided in the student handout.
Finally, look at the data for geochemical data and describe how prevalent molecular biomarkers and carbon/carbonate deposits are.
(Note: The information from this station primarily comes from a 2006 survey article compiled by William Schopf for the Royal Society B outlining the evidence for the existence of life during the Archaean segment of Earth's history. 48 Archaean deposits were reported to contain biogenic stromatolites, 14 deposits contained microfossils, and 13 deposits contained organic geochemical data that included molecular biomarkers and carbon and sulfur isotopic data.
For more information: Schopf, J. William. 2006. “Fossil evidence of Archaean life.” Philosophical Transactions of the Royal Society B/Biological Sci. DOI: 10.1098/rstb.2006.1834.)
At this station, students will consider how ancient living things self-replicated by considering if it was a DNA world, a RNA world, or a protein world. First, students will play the role of scientist and brainstorm the evidence that would be seen if it were a DNA world, RNA world, or protein world.
Next, students will read several articles about the three conflicting theories of what was the early replicator of life. Using this student handout and suggested resources sheet, students should practice their critical reading by classifying the evidence supporting a DNA, RNA, or protein world. Then students will revisit the evidence they listed and highlight all evidence based on experimentation with a pink highlighter, all evidence based on environmental observation with a blue highlighter, all the evidence based on computer modeling with a green highlighter, and all evidence based on deductive reasoning with a yellow highlighter.
Then, students will look at the environmental conditions necessary for the formation of not only the self-replicating molecules, but also the molecules necessary for metabolism. Students will consider how the raw products arrived on Earth by looking at areas around deep sea ocean vents, the Urey-Miller experiments and the new analysis that has been done on the products of that experiment, and the reanalysis of the Murchison meteorite as well as a meteorite of the same composition that landed at Sutter's Mill, CA.
(Note: it is important to allow students to explore all of these scenarios without giving them the "right" answer. By allowing students to weigh the credibility of the evidence, they can determine which scenario to be the most widely accepted at this time. As students learn more about the inner workings of the cell throughout the year, you can revisit these scenario and determine if students have changed their mind as they have gained a greater understanding about cell structure and cellular processes.)
Students look at the work of Jack Szostak with protocells. After familiarizing themselves with his data, students design their own "protocell" albeit only on paper. Other resources that students can use at this station are Jack Szostak's iBiology lecture, Protocell Membranes (note: the first 10 minutes will be the most helpful) and Building a Protocell.
Have students revisit the criteria they listed for life at the beginning of the lesson. Ask them after having looked at the studies they did, how would they say that scientists might be redefining life as they search for it throughout the universe?
Based on what they have learned in this exploratorium, students should find that scientists (especially organic chemists) are looking at early forms of life or protocells from a thermodynamics perspective. The thermodynamics view of life is based on five criteria:
Give each student a piece of white legal paper and have them design and draw a mural that best reflects what they think the early Earth was like. Students should cite the evidence that they use to support their artistic representation of the early Earth. Students should place their mural in their lab notebooks.
Homework: Students will view a iBiology lecture by Jack Szostak(Jack Szostak (Harvard/HHMI) Part 1: The Origin of Cellular Life on Earth) and complete a current events summary sheet.