This activity will remind you that our understanding of why something occurred sometimes changes over time based on new evidence and new discoveries. At one point in history, spontaneous generation was a reasonable explanation for the emergence of life due to the evidence (or lack of it) for how life developed.

How did life begin? For centuries many people believed in an idea called spontaneous generation, the idea that life could “spontaneously” appear from nothing. Why did many people believe this explanation for the origin of life? Read the following statements:

• “If a soiled shirt is placed in the opening of a vessel containing grains of wheat, the reaction of the leaven in the shirt with fumes from the wheat will, after approximately twenty-one days, transform the wheat into mice.”
  
  Source of Louis Pasteur’s description of an experiment of Jan Baptista van Helmont.: An Address delivered by Louis Pasteur at the "Sorbonne Scientific Soirée" on April 7, 1864. Accessed 15 May 2013. https://eee.uci.edu/clients/bjbecker/NatureandArtifice/week7f.html.
• “Pieces of cheese and bread wrapped in rags and left in a dark corner…produce mice… because after several weeks, there were mice in the rags.”
  
  Source: “Spontaneous Generation. Dictionary.com. Encyclopedia Britannica, Inc. Accessed: May 15, 2013. http://dictionary.reference.com/browse/spontaneous generation.
• “Mold seems to grow spontaneously on bread, maggots appear as if by magic in old meat, and every spring new plants sprout and grow….”
  
  Source: Robert Hazen, Genesis (Washington, DC: Joseph Henry Press, 2005) 83.

Think about these examples of spontaneous generation and respond to the following questions on your worksheet:

1. Why did the scientists who studied these cases think that life had “spontaneously” appeared?
2. Why did these scientists think their conclusions were based on sound scientific methods?
3. What was wrong with the scientific methods they used?
4. What do you think the scientific explanation for the appearance of life in each of these cases was?

Choose one of the claims above and use the Claim Testing Worksheet to more thoroughly evaluate the claim, based on what you now know. Be prepared to talk about how you tested your claim. In the second half of the course, you’ll be doing a lot of research on your own and you’ll need to have claim testing down pat.

This repeated activity should help you become familiar with a process for understanding unfamiliar words anytime you encounter them in the course.

Take out your vocab tracker and be sure to record new and unfamiliar words on it according to your teacher’s instructions.

Life changed slowly but dramatically over the course of many millions of years. The appearance of complex multicelled organisms did not happen quickly.

This video will remind you of what thresholds of increasing complexity are, and will explain the Goldilocks Conditions that were needed for life to emerge on Earth.

Preview

In the opening of this lesson of this lesson, you looked at the idea that life could spontaneously appear, seemingly from nowhere. Biologists have disproven the idea of spontaneous generation, but what idea has replaced it to explain the appearance of—and changes in—life?

Key Ideas – Factual

Think about the following questions as you watch the video:

1. What are the Goldilocks Conditions for life?
2. What makes rocky planets ideal locations for the appearance of life?
3. Why do many scientists believe that life may have originated deep in the oceans around oceanic vents?

Thinking Conceptually

Think about whether it’s possible that at some point scientists will change their minds about life originating at the bottom of the oceans. How might a change in this type of thinking occur?

You will need a copy of the BHP Thresholds Graphic you worked on in earlier Narrative and Thresholds activities. If you skipped those activities, you can use the Thresholds Graphic included in the PDF for this activity.

You'll also need a copy of the brief Big History story you wrote for the Narrative and Thresholds – The Stars Light Up activity in Unit 3.

This activity will build on all the work you’ve done in the Narrative and Thresholds progression. So far, you’ve practiced synthesizing, image analysis, and working with the BHP narrative. The purpose of this activity is to use the skills that you’ve been developing in the course to build your own BHP story up to this threshold. By doing so, you will be deepening your understanding of each individual threshold and the larger story. Additionally, this activity will set you up to reflect on the plot points and narrative as you continue in the course.

Reading, writing, causation
The eight thresholds of increasing complexity are staples in the Big History Project. As you work through the course, it is important to incorporate summarizing and synthesizing activities so you are able to discern the larger themes of Big History. In this activity, you’ll have the opportunity to reflect on and connect Thresholds 1 through 5 through graphical analysis, writing your own BHP narrative, and reflecting on your earlier threshold summaries. You will connect your observations to the BHP Thresholds Graphic and the Threshold 5 video to explain in your own words the through lines of the thresholds.

Take out your BHP Thresholds Graphic and take a minute to review the images and your writing from the previous thresholds. After watching the Threshold 5: Life video, take what you learned and write a concise and grammatically correct sentence that describes the main idea of Threshold 5.

Now that you’re an expert on much of the BHP narrative, you’re going to write the history of the Universe in your own words. Think about the following questions as you write, and be ready to discuss your answers with the class:

1. How do the thresholds of increasing complexity help you construct your narrative?
2. How has your story changed since the first time you wrote it?
• Which of your original ideas were supported by what you’ve learned?
• Which were extended by what you learned?
• Which of your ideas were challenged and subsequently changed by what you learned?
3. What is the biggest change in your understanding so far?

Scientists have many questions about how life first appeared, but they’ve developed much evidence that billions of years passed between the appearance of the first, single-celled organisms and the appearance of complex, multicelled organisms like humans. As we continue this lesson, we’ll look further into the changes in living things that led to the appearance of humans.

The development and evolution of life on Earth was a long and complicated process. Therefore, David Christian breaks this process into mini-thresholds as a means to explain the major turning points in the development of life. This video should assist in your understanding of this evolution.

Preview

This video is jam-packed: Six mini-thresholds are covered here, each important to the story of life changing over time. You might want to watch this video more than once to help you remember all the content that’s covered.

Key Ideas – Factual

Think about the following questions as you watch the video:

1. What makes the emergence of photosynthesizing cells a mini-threshold?
2. What makes the emergence of eukaryotes a mini-threshold?
3. What makes the emergence of multicelled organisms a mini-threshold?
4. What makes the emergence of brains a mini-threshold?
5. What makes the move of sea creatures to land a mini-threshold?
6. What makes the emergence of mammals a mini-threshold?

Thinking Conceptually

Compare and contrast the mini-thresholds of life to the larger thresholds of increasing complexity that define Big History. What makes two types of thresholds similar and what makes them different?

This activity is intended to help you explore thresholds and how we can use the idea of thresholds to break other historical phenomena into logical pieces. By looking at mini-thresholds in comparison to our larger thresholds, you can contrast the two and understand the details of each a little better.

Scale
In this activity, you will learn more about what it means to periodize history and about the ways that periodizing impacts how historical accounts are both constructed and interpreted. History can be broken up, or periodized, in multiple ways, including by time and space (temporal and spatial scales), and theme. Understanding how a historian has chosen to periodize history will help you analyze historical accounts, because it provides you with important background knowledge—context—that would have affected how the account was created.

Working in groups, you’ll create a mini-threshold of life that you think is important enough to include in the list David Christian gives, or one that might be important enough to replace one of them. Be sure you can answer these questions about the new mini-threshold you come up with:

• What were the Goldilocks Conditions that allowed for this mini-threshold to occur?
• What were the ingredients?
• Does this mini-threshold represent a change in complexity that could not easily be undone?
• How do the elements of the mini-threshold compare to one of the Big History thresholds of increasing complexity? Do they have the same underlying features?
• Based on these answers, do you think this threshold is the right threshold?

After your group has finished, create a threshold card for your new mini-threshold of life. Your group should be prepared to present your new mini-threshold to the class.

Our ideas about the number of species on the Earth have changed pretty dramatically over time. Three hundred years ago, Carl Linnaeus estimated that there were 4,000 species. Today, some scientists estimate there are as many as 8.7 million species, and this doesn’t even include bacteria!

This video introduces you to the diversity of life that has developed on Earth, a great illustration of how simple things can become incredibly complex over time.

Preview

Although the early history of life is characterized by a lack of diversity, tremendous diversity has characterized the recent history of living things. The biosphere is home to a huge number of species. Scientists are constantly revising their estimates of the total number of species on Earth based on studies of the number of species in small segments of the biosphere.

Key Ideas – Factual

Think about the following questions as you watch the video:

1. How have our ideas about the number of species on Earth changed over time?
2. How do scientists determine the number of species there are on Earth?

Thinking Conceptually

Brainstorm some ideas regarding how we determine the number of species on the planet. There are so many new technologies available to us today; do you think there might be a better method for counting species besides cordoning off areas and counting?

As the daughter of a Mexican diplomat, Ynés Mexía (1870–1938) moved around often. Her early life was full of struggle and disappointment, so Mexía moved to San Francisco to seek mental health care. She fell in love with the wilderness and joined the environmentalist movement there. At the age of fifty-one, she went back to school to study botany. During her short, thirteen-year career, she ventured into many wild areas across the Americas, collecting a stunning number of plant samples. She was the first to catalog hundreds of new species, many of which were later named after her.

In this unit, we discover life! Well, life has been around a while; we didn’t discover it. But it’s thanks to the work of people like Ynés Mexía that we understand everything we do today about plants, the natural world, and the evolution of life. These brave souls ventured out into the wilderness, often in difficult or dangerous conditions, and searched for new life. This biography will provide you with new evidence as you learn how we came to know what we know about life on this planet.

Read 1: Observe

As you read this graphic biography for the first time, review the Read 1: Observe section of your Three Close Reads for Graphic Bios Tool. Be sure to record one question in the thought bubble on the top-right. You don’t need to write anything else down. However, if you’d like to record your observations, feel free to do so on scrap paper.

Read 2: Understand

On the tool, summarize the main idea of the comic and provide two pieces of evidence that helped you understand the creator’s main idea. You can do this only in writing or you can get creative with some art. Some of the evidence you find may come in the form of text (words). But other evidence will come in the form of art (images). You should read the text looking for unfamiliar vocabulary words, the main idea, and key supporting details. You should also spend some time looking at the images and the way in which the page is designed. By the end of the second close read, you should be able to answer the following questions:

1. Why does the comic call Mexía a “late bloomer,” and why is this important?
2. Why was Mexía’s career significant to the study of life?
3. What do the quotes written by Mexía tell you about sexism in the field of botany and how she responded to it?
4. How has the artist designed the page, text, and illustrations to tell you about Mexía?

Read 3: Connect

In this read, you should use the graphic biography as evidence to support, extend, or challenge claims made in this unit of the course. On the bottom of the tool, record what you learned about this person’s life and how it relates to what you’re learning.

1. How does this biography support, extend, or challenge what you have learned about the types of people who study the development of life and do fieldwork in the sciences? Had you ever heard the name Ynés Mexía before?

To Be Continued…

On the second page of the tool, your teacher might ask you to extend the graphic biography to a second page. This is where you can draw and write what you think might come next. Here, you can become a co-creator of this graphic biography!

This activity will give you practice with interpreting infographics and will also support your understanding of the similarities and differences between humans and other species.

Locate the species listed below on the infographic, and write down the domain to which each belongs:

• brown rat
• zebrafish
• Giardia lamblia
• Salmonella typhi
• Campylobacter jejuni
• Streptomyces coelicolor
• Streptococcus mutans
• Methanosarcina acetivorans

Answer the following questions:

1. What is the importance of each of the species listed above?
2. Which domain is probably the largest?
3. What are the major differences between each of the domains?
4. What is LUCA?
5. What species is our closest relative?
6. Did we branch off the taxonomic tree from our closest relative or did we develop separately from each other? What does this tell you about our relationship?
7. Homo sapiens and Pan troglodytes both branched off from what type of animal?
8. What are the taxonomic similarities between the following species: Pan troglodytes, Homo sapiens, Mus musculus, Rattus norvegicus?

Be prepared to discuss your answers with the class.

Humans have only existed for a tiny sliver of time compared to the Earth’s age. The idea that a sixth mass extinction could be brought on so quickly due to the unprecedented success of our species is a downer, but not a surprise given that we are life forms doing what life always does—taking what we need to survive. The good news is that we have the ability recognize this future danger, and we can adapt our behavior based on what we have learned from the previous five mass extinctions.

This video will give you a better understanding of the five mass extinctions that have occurred in the four billion years since life on Earth began. Although the evolutionary epic of the past is one we cannot change, it matters a great deal if we are to learn what can be done to slow down the sixth extinction (which includes humans!) that is already underway.

Preview

Whether it’s caused by sun-blocking dust kicked up by volcanoes and asteroids, or too many plants sucking up the CO₂, or weather extremes, life on Earth has a habit of dying out and starting over. The sudden acceleration of human development indicates a sixth mass extinction is in our future, but one thing we have that the dinosaurs didn’t: videos of coming attractions—and ideas for what to do about it.

Key Ideas – Factual

Think about the following questions as you watch the video:

1. What, overall, were the various causes of the first five mass extinctions?
2. Evolutionarily speaking, what do single cell bacteria (pre-Cambrian) and the human species have in common?
3. What are some of the reasons the year 1500 marks the beginning of the “sixth extinction”?
4. What are some of the most significant consequences of an increased global temperature?
5. What is the difference between humans and all the other things (volcanoes, asteroids, for example) that has caused mass extinctions?

Thinking Conceptually

The video reminds us that it is humans’ natural behavior, not evil, that put the sixth extinction in motion centuries ago. Why it is that today many suggest we have a moral responsibility to preserve the environment? When did we start to care, and why? Do different people have different reasons for wanting to stop (or reverse) the effects humans have on other species?