Standards: SC.912.N.1.3, SC.912.N.1.4, SC.912.N.2.1, SC.912.L.15.8, SC.912.L.18.1

Textbook: Chapter 20.3

Origins of Life

Essential Content

A. Origins of Life (15.8)

1 Scientific Explanations

a. Law vs. Theories in Science

b. Development of Theory of Origin of Life

2. Earth’s Early Atmosphere

3. Conditions allowing for life on Earth

4. Organic Molecules/ Eukaryotes/ Chemical Evolution (18.1)

B. Endosymbiotic Theory (15.8)

1. Evolution of Eukaryotes from Prokaryotes

2. Evidence Supporting Theory

Objectives

· Apply knowledge of the development of a scientific theory and/or recognize the differences between theories and laws. (ALD)

· Summarize scientific explanations of the origin of life on Earth. (ALD)

· Identify situations or conditions contributing to the origin of life on Earth.

· Compare different ideas about how organic molecules and cells first came about.

· Summarize contributions of scientists (Pasteur, Oparin, Miller and Urey, Margulis, and Fox) and how their discoveries aided in the development of scientific explanations of the origin of life. (ONLY need to know CONTRIBUTION, not scientist responsible)

· Describe the experiments that try to prove the origin of life on Earth.

· Explain how the organic molecules that define life were created and assembled into macromolecules.

Vocabulary:

biogenesis, abiogenesis, spontaneous generation, endosymbiotic theory, prokaryote, organelle, chloroplast, mitochondria, eukaryote, organic molecule, amino acid, protein

Endosymbiosis and the Evolution of Eukaryotes

Eukaryotes may have been a product of one cell engulfing another and evolving over time until the separate cells became a single organism.

• Endosymbiosis is the concept of one cell engulfing another and both cells benefiting from the relationship.

• Endosymbiosis was originally considered after the observation of the similarity between plant chloroplasts and free-living cyanobacteria.

• Peroxisomes may have been the first endosymbionts, caused by the increasing amount of atmospheric oxygen at that point in geological time.

• Over time, endosymbionts may have transferred some of their DNA to the host nucleus, thus becoming dependent on the host for survival and completing full integration into a single organism.

• Mitochondria are energy-producing organelles that are thought to have once been a type of free-living alpha-proteobacterium.

• Eukaryotic cells contain varying amounts of mitochondria, depending on the cells’ energy needs.

• Mitochondria have many features that suggest they were formerly independent organisms, including their own DNA, cell-independent division, and physical characteristics similar to alpha-proteobacteria.

• Some mitochondrial genes transferred to the nuclear genome over time, yet mitochondria retained some genetic material for reasons not completely understood.

• The hypothesized transfer of genes from mitochondria to the host cell’s nucleus likely explains why mitochondria are not able to survive outside the host cell.

Endosymbiotic Theory

The endosymbiotic theory was first articulated by the Russian botanist Konstantin Mereschkowski in 1905. Mereschkowski was familiar with work by botanist Andreas Schimper, who had observed in 1883 that the division of chloroplasts in green plants closely resembled that of free-living cyanobacteria. Schimper had tentatively proposed that green plants arose from a symbiotic union of two organisms. Ivan Wallin extended the idea of an endosymbiotic origin to mitochondria in the 1920s. These theories were initially dismissed or ignored. More detailed electron microscopic comparisons between cyanobacteria and chloroplasts combined with the discovery that plastids ( organelles associated with photosynthesis) and mitochondria contain their own DNA led to a resurrection of the idea in the 1960s. The endosymbiotic theory was advanced and substantiated with microbiological evidence by Lynn Margulis in 1967.

Chloroplasts in plants: A eukaryote with mitochondria engulfed a cyanobacterium in an event of serial primary endosymbiosis, creating a lineage of cells with both organelles. These cyanobacteria have become chloroplasts in modern plant cells. The cyanobacterial endosymbiont already had a double membrane.

In 1981 she argued that eukaryotic cells originated as communities of interacting entities, including endosymbiotic spirochetes that developed into eukaryotic flagella and cilia. This last idea has not received much acceptance because flagella lack DNA and do not show ultrastructural similarities to bacteria or archaea. According to Margulis and Dorion Sagan, “Life did not take over the globe by combat, but by networking” (i.e., by cooperation). The possibility that the peroxisome organelles may have an endosymbiotic origin has also been considered, although they lack DNA. Christian de Duve proposed that they may have been the first endosymbionts, allowing cells to withstand growing amounts of free molecular oxygen in the earth’s atmosphere. However, it now appears that they may be formed de novo, contradicting the idea that they have a symbiotic origin.

It is believed that over millennia these endosymbionts transferred some of their own DNA to the host cell’s nucleus during the evolutionary transition from a symbiotic community to an instituted eukaryotic cell (called “serial endosymbiosis”). This hypothesis is thought to be possible because it is known today from scientific observation that transfer of DNA occurs between bacteria species, even if they are not closely related. Bacteria can take up DNA from their surroundings and have a limited ability to incorporate it into their own genome.

Adapted from Lumen Learning

Review of Scientific Inquiry and Scientific Method

• A hypothesis is a statement/prediction that can be tested by experimentation.

• A theory is an explanation for a set of observations or phenomena that is supported by extensive research and that can be used as the basis for further research; a well-substantiated explanation of some aspect of the natural world based on knowledge that has been repeatedly confirmed through observation and experimentation

• Inductive reasoning draws on observations to infer logical conclusions based on the evidence.

• Deductive reasoning is hypothesis-based logical reasoning that deduces conclusions from test results.

• scientific method: a way of discovering knowledge based on making falsifiable predictions (hypotheses), testing them, and developing theories based on collected data

• control group: a group that contains every feature of the experimental group except it is not given the manipulation that is hypothesized

• peer review: The scholarly process whereby manuscripts intended to be published in an academic journal are reviewed by independent researchers to evaluate the contribution, importance, and accuracy of the manuscript’s contents.

  • The body of scientific knowledge is recorded in peer-reviewed science journals which allow other scientists to determine what has been done previously and where their own research fits in the larger field of study.

  • A scientific article generally follows the steps of the scientific method: introduction (background, observations, question), materials and methods (hypothesis and experimental plan), results (analysis of collected data), and discussion (conclusions drawn from analysis).

  • Peer reviewers are other researchers in that field of study who carefully dissect, analyze, and critique a research article submitted for publication.

  • Review articles (summaries and commentaries on prior research in a field of study) also go through the peer-review process.

20.3 Origins of Life

Scientist Contributions

The Origins of Cell Theory

Robert Hooke first used the term “cells” in 1665 to describe the small chambers within cork that he observed under a microscope of his own design. To Hooke, thin sections of cork resembled “Honey-comb,” or “small Boxes or Bladders of Air.” He noted that each “Cavern, Bubble, or Cell” was distinct from the others

1838, Matthias Schleiden (1804–1881), a German botanist who made extensive microscopic observations of plant tissues, described them as being composed of cells. Visualizing plant cells was relatively easy because plant cells are clearly separated by their thick cell walls. Schleiden believed that cells formed through crystallization, rather than cell division.

Theodor Schwann (1810–1882), a noted German physiologist, made similar microscopic observations of animal tissue. In 1839, after a conversation with Schleiden, Schwann realized that similarities existed between plant and animal tissues. This laid the foundation for the idea that cells are the fundamental components of plants and animals.

Rudolf Virchow (1821–1902), a well-respected pathologist, published an editorial essay entitled “Cellular Pathology,” which popularized the concept of cell theory using the Latin phrase omnis cellula a cellula (“all cells arise from cells”), which is essentially the second tenet of modern cell theory

The endosymbiotic theory, which is now defined as the theory that mitochondria and chloroplasts arose as a result of prokaryotic cells establishing a symbiotic relationship within a eukaryotic host. Genetic sequencing and phylogenetic analysis show that mitochondrial DNA and chloroplast DNA are highly related to their bacterial counterparts, both in DNA sequence and chromosome structure. However, mitochondrial DNA and chloroplast DNA are reduced compared with nuclear DNA because many of the genes have moved from the organelles into the host cell’s nucleus. Additionally, mitochondrial and chloroplast ribosomes are structurally similar to bacterial ribosomes, rather than to the eukaryotic ribosomes of their hosts. Last, the binary fission of these organelles strongly resembles the binary fission of bacteria, as compared with mitosis performed by eukaryotic cells.

Works of Louis Pasteur, Robert Koch, and Joseph Lister would further substantiate the germ theory of disease.

While studying the causes of beer and wine spoilage in 1856, Pasteur discovered properties of fermentation by microorganisms. He had demonstrated with his swan-neck flask experiments that airborne microbes, not spontaneous generation, were the cause of food spoilage, and he suggested that if microbes were responsible for food spoilage and fermentation, they could also be responsible for causing infection. This was the foundation for the germ theory of disease.