RAVEN 9/e

CHAPTER 26: The Tree of Life

WHERE DOES IT ALL FIT IN?

Chapter 26 blends genetics and taxonomy to build a picture of the origin of life on Earth. A familiarization with the properties of life and cell structure is essential for completely understanding the information in this chapter. All of the information on organismic diversity and adaptations covered later in the book relies on Chapter 26. The taxonomic schemes expounded in this chapter will be carried throughout the book starting with Chapter 27 as the diversity of life is explained.

SYNOPSIS

Many of the properties that we associate with life do not sufficiently describe only life. The seven properties that best do so are: cellular organization, sensitivity, growth, development, reproduction, regulation, and homeostasis. Finally, all organisms possess heredity, a mechanism for the preservation of improvement and the driving force of evolution. There are three probable explanations for the presence of life on earth. Panspermia and spontaneous origin are the only testable hypotheses currently available.

There is great scientific controversy as to the conditions on the early earth and the likely locations that spurred the origin of life on earth. The early earth was physically quite different from the world of today, perhaps possessing an atmosphere with some of the same gases, but notably devoid of oxygen. That, as well as the presence of various hydrogen compounds, constitutes its description as a reducing atmosphere. The reactions that occurred billions of years ago could not occur today partly because of the lack of that unique atmosphere. Experiments simulating these early conditions produced various organic compounds ultimately including the amino acids and nucleic acids required for life. The first protocells possessed elements of the properties of life and became true cells with the development of heredity. The current explanations of the origin of cells derive from Oparin’s bubble theory.

The earliest fossil cells closely resemble present day eubacteria. A few unique archaebacteria have survived unchanged through billions of years and are relics of those very diverse first life forms. Archaebacteria are prokaryotic in structure, but are significantly different from eubacteria in their cell wall and membrane structure. The membrane structures of modern eukaryotes probably evolved from invaginations of the membranes of early prokaryotes. The theory of endosymbiosis explains how certain organelles of eukaryotes likely evolved from a variety of prokaryotes. The first evidence of eukaryotes, cells with distinct nuclei, appeared after 1.5 billion years. All living organisms other than bacteria evolved from these forms.

The existence of life on other worlds is a mathematical certainty. Recent examination of Mars meteorites provides supporting evidence for the existence of life there. Future investigation of other celestial objects is necessary to substantiate the presence of life elsewhere.

LEARNING OUTCOMES

26.1 Origins of Life

  • Explain what qualifies something as “living.”
  • Describe different proposals for the origin of life on Earth.

26.2 Classification of Organisms

  • Explain how taxonomists name and group organisms.
  • Evaluate the usefulness of taxonomic hierarchies inanswering evolutionary questions.

26.3 Grouping Organisms

  • List examples showing that the three domains of life aremonophyletic, but the six kingdoms are not.
  • Distinguish among the characteristics of Eukarya, Archae,and Bacteria.
  • Explain why biologists do not include viruses in the treeof life.

26.4 Making Sense of the Protists

  • Describe the relationships among groups of protists.

26.5 Origin of Plants

  • Describe the evolutionary relationship between algaeand plants.
  • Explain how moss genes have come to be found in thegenome of a flowering plant.

26.6 Sorting out the Animals

  • Describe current ideas regarding the origin ofsegmentation in animals.
  • Explain why insects are termed “flying crustaceans.”
  • State the evidence for a common ancestor between whalesand hippos.

COMMON STUDENT MISCONCEPTIONS

There is ample evidence in the educational literature that student misconceptions of information will inhibit the learning of concepts related to the misinformation. The following concepts covered in Chapter 26 are commonly the subject of student misconceptions. This information on “bioliteracy” was collected from faculty and the science education literature.

  • Students believe that all only DNA can be the genetic information
  • Students believe that DNA is needed to carry out metabolism
  • Students do not understand that genes are conserved in distantly related organisms
  • Students believe that DNA variation between different organisms if always very high
  • Students believe that all evolutionary changes are gradual
  • Students believe that organic evolution could not have taken place on Earth
  • Students believe that the origins of life started out as random atoms coming together to form molecules
  • Student believe that the early oceans were salty
  • Students believe the early atmosphere contained oxygen
  • Students believe that species are genetically distinct and fixed

INSTRUCTIONAL STRATEGY PRESENTATION ASSISTANCE

This is one of those chapters that could be difficult if the students don’t have their fact/belief concepts straight. A short story by Isaac Asimov, The Last Question (in Nine Tomorrows), treats the combination of science and religion quite interestingly; it has to be read to be appreciated. As a last resort, you are teaching a science class and only those concepts that can be tested by experiment and observation are fair game for discussion.

One of the more difficult presentations concerns the definition of life. Students want specific, clear-cut characteristics. Many of the things we associate with life can individually be applied to inanimate objects. Recent technological advances in computers and biotechnology make the delineation even more difficult. After all, most biologists do not consider viruses to be examples of life.

Stress that the conditions on the primordial earth were significantly different from those that exist today and that one of the most important life-sustaining characteristics, free oxygen gas, is a by-product of the metabolism of primitive living organisms. Consider the likelihood of life continuing as the environment changes, especially with regard to the ozone hole and the greenhouse effect.

HIGHER LEVEL ASSESSMENT

Higher level assessment measures a student’s ability to use terms and concepts learned from the lecture and the textbook. A complete understanding of biology content provides students with the tools to synthesize new hypotheses and knowledge using the facts they have learned. The following table provides examples of assessing a student’s ability to apply, analyze, synthesize, and evaluate information from Chapter 26.

Application /
  • Have students explain the role of duplicated genes in determining specialization of segments.
  • Have students describe the role of hox genes in segment specialization.
  • Ask students to explain the role of horizontal gene transfer in plant evolution.

Analysis /
  • Have students explain how segmentation is evident in the human body.
  • Ask students to why organic evolution as shown in the Miller and Urey experiment is not prevalent today.
  • Ask students discuss how to classify an organism that has characteristics of both the eubacteria and archeae.

Synthesis /
  • Ask students design a Miller and Urey type of setup that would be effective the study if life can originate on Mars.
  • Have the students hypothesize about medical applications of the Miller and Urey experiment.
  • Ask the students to design a way that horizontal gene transfer can be eliminated between genetically modified crop plants and wild plants.

Evaluation /
  • Ask students evaluate the impacts of horizontal gene transfer on the release of genetically modified organisms into the environment
  • Ask students to evaluate the possible uses of a genetic manipulation procedure that alters segmentation and subsequently can increase the number vertebrae in an animal.
  • Ask students debate the evidence that life on Earth could have started on another planet.

VISUAL RESOURCES

Pass around the vial of primordial soup that you cooked up in the lab the day before. Add a spark from your always-handy lightening generator and voila — life in the classroom! (Probably the first sign of it if your lecture is at 8:00 am.) Include something that can be divided into smaller imitations of itself (simulating reproduction), something mechanical (simulating metabolism), something dead (but still made up of cells), and some kind of computer storage device (simulating heredity).

A science-fiction aficionado could have a lot of fun with the possibility of life elsewhere in the universe. How many different episodes of each of the versions of Star Trek have addressed this? Students should realize that it is unlikely that other planets will have life as it appears on earth. Even a carbon-based world is full of physical differences; look at the diversity of life on this planet. It is just as likely that another life form will look like a centipede as it is that it will look like a human, and probably more likely that it will look like neither. This makes our definition of life even more difficult to conceptualize.

IN-CLASS CONCEPTUAL DEMONSTRATIONS

A. Miller and Urey Experiment

Introduction

A wonderful and effective way to reinforce a lecture on the origins of life is by demonstrating the actual Miller and Urey experiment. This activity supplements a lecture on the classical experiment and show students the excitement of seeing immediate results.

Materials

  • Computer with live access to Internet
  • LCD projector attached to computer
  • Web browser bookmarked to the Miller-Urey Experiment website at

Procedure & Inquiry

  1. Review the concept of organic evolution with the class.
  2. Tell the class they are going to see the actual Miller and Urey experiment.
  3. Play the first video and ask the students to describe the conditions of the experiment.
  4. Then show the second video and ask the class to explain what happened.
  5. Ask the class to explain what factors led to the evolution of the jumping trait.
  6. Scroll down to show the diversity of ant jaws.
  7. Have the students describe the conserved and variable features of the jaws.

LABORATORY IDEAS

This activity engages students in studying variations in segmentation between and within taxonomic groups. In this laboratory session students are asked questions to hypothesize about the types of mutations leading to the segmentation variation seen in worms, arthropods and mollusks.

  1. Tell students that they will be investigating the variation in a variety of specimens.
  2. Students should be provided with the following materials to perform this open-ended inquiry.
  3. Preserved specimens
  4. Nematode
  5. Earthworm
  6. Millipede
  7. Grasshopper
  8. Chiton
  9. Snail
  10. Fish
  11. Dissecting equipment
  12. Dissecting trays
  13. Dissecting microscopes
  14. Tell students to record any evidence of segmentation in the specimens.
  15. Then have the students investigate the degree of segment specialization.
  16. Have the students hypothesize about the mutations and selective mechanisms leading to the different degrees of segmentation and the specialization of segments.
  17. Also, have the students explain why segmentation is not obvious in certain organisms.

LEARNING THROUGH SERVICE

Service learning is a strategy of teaching, learning and reflective assessment that merges the academic curriculum with meaningful community service. As a teaching methodology, it falls under the category of experiential education. It is a way students can carry out volunteer projects in the community for public agencies, nonprofit agencies, civic groups, charitable organizations, and governmental organizations. It encourages critical thinking and reinforces many of the concepts learned in a course.

  1. Have students produce a PowerPoint presentation about the Miller and Urey experiment for high school teachers.
  2. Have students tutor high school students covering evolution in a biology class.
  3. Have students work with area environmental groups on biodiversity issues.
  4. Have students do sponsor a science fair project on biodiversity.