Aim: How did Life Begin ?(Early Hypotheses) By the end of this lesson, SWBAT Describe the evidence for spontaneous generation Explain how Redi used controlled experiments to disprove the widely accepted hypothesis of the spontaneous generation of maggots. Explain how Spallanzani’s and then Pasteur’s experiments finally disproved the hypothesis of the spontaneous generation of microorganisms.

Do you believe that the living mold comes from the nonliving bread or from other living mold? Why? Sample 1 Sample 2

Define Spontaneous Generation or Abiogenesis and describe beliefs that support the hypothesis.

The Debate over Spontaneous Generation Spontaneous generation: The hypothesis that living organisms arise from nonliving matter; a “vital force” forms life Biogenesis: The hypothesis that the living organisms arise from preexisting life

Early Hypothesis: Ancient Greek Aristotle:325 BC thought that life arose spontaneously

Spontaneous Generation A. Ancient Egyptians- felt that eels and frogs came from the mud of the Nile. Aristotle- “active principle”, was responsible for life fleas and lice came from sweat mice came from garbage flies and maggots came from dead and decaying meat

Early Hypothesis: Van Helmont Van Helmont:1577-1644  formula for mice: dirty rags, wheat bran, place in dark open barrel, 2 weeks - 1 month would have mice. i.e., Maggots arose "spontaneously" from decaying meat.

Early Hypothesis: Francesco Redi 1668: Italian, first true scientific experiment, tested spontaneous generation hypothesis: sealed three jars tightly with meat, left three jars unsealed. Open jars produced maggots, sealed did not. But critics thought that fresh air was required. He repeated experiment so that air could get in, using gauze to keep out flies, deduced that maggots appeared only when flies preceded.

Anton van Leewenhoek

The First Observations 1673-1723, Antoni van Leeuwenhoek described live microorganisms that he observed in teeth scrapings, rain water, and peppercorn infusions. Figure 1.2b

ANTONY VAN LEEUWENHOEK

Early Hypothesis: John Needham 1745 : He heated infusion of chicken broth and corn, poured into covered “clean” flasks. Soon contaminated (turbid: Latin, confused, disordered, crowd). Said could only be due to spontaneous generation.

Early Hypothesis: Spallanzani 1765: He modified Needham’s experiment: the fluid was sealed in the flasks, and then boiled. noted that they did not show contamination if sterilized in the sealed flask.

Louis Pasteur: Disproved Spontaneous Generation 1861: He filled long-necked flasks with beef broth. Bent necks of some into S shape, other straight. Reasoned that S trapped airborne contamination. Boiled to sterilize. Deduced that micro organisms ubiquitous, can be destroyed by heating. Blocking access to medium will prevent growth.

The Theory of Biogenesis Pasteur’s S-shaped flask kept microbes out but let air in Figure 1.3

How Did Life Begin? (Modern Hypotheses) Do Now What are models? Give two examples of models in your science classroom. Why do scientists use models? How does a single-celled organism differ from a multicellular organism?

Answers to the Do Now 1. Models are tentative schemes or structures that correspond to real structures, events or events . 2. Cell model and Human body model. 3. Models help scientists learn how things work. 4. A living thing composed of one cell; A living thing composed of more than one cell.

ORIGIN & EVOLUTION OF LIFE Origin of Earth: 4.6 billion years ago Life appeared on Earth about 3.8 billion years ago as simple, single-celled organisms known as prokaryotes. About a billion years ago, increasingly complex multicellular organisms began to evolve.

Haldene-Oparin or Heterotroph Hypothesis or Spontaneous origin

Spontaneous origin is a hypothesis about the process through which life may have developed during the first billion years of Earth's history.

Primordial Soup Model of Spontaneous Origin In 1923, Alexander Oparin proposed the primordial soup model of spontaneous origin. This model states that the Earth's early atmosphere contained inorganic gases that reacted to form simple organic compounds. These compounds condensed into the oceans, where they reacted further, fueled by energy from the sun, volcanoes, and lightning, to become the building blocks of the first cells.

Miller and Urey

The Basic Chemicals of Life The Miller-Urey Experiment In this experiment, Miller placed hydrogen-containing gases into a device. He used electrical sparks to simulate lightning. After a few days, Miller found organic molecules in his device, including some of life’s basic building blocks: amino acids, fatty acids, and other hydrocarbons (molecules made of carbon and hydrogen).

The Basic Chemicals of Life The Miller-Urey experiment showed that, under certain conditions, organic molecules could form from inorganic molecules. We now know that the molecules used in Miller’s experiment could not have existed in abundance on early Earth. Four billion years ago, shortly after it formed, Earth did not have a protective ozone layer.

The Basic Chemicals of Life Ultraviolet radiation from the sun would have destroyed any methane and ammonia in the atmosphere. When ammonia and methane gases were absent from the Miller-Urey experiment, key biological molecules were not made. However, the experiment shows that complex biological compounds can form from simple building blocks.

Life’s Building Blocks One hypothesis that addresses the origin of life states that early biological molecules formed close to hydrothermal vents. The heat from hydrothermal vents could have provided energy for chemical reactions. Within the sea, biological molecules would have been protected from harmful solar radiation.

Hydrothermal vents are cracks in the ocean floor that emit jets of hot water loaded with minerals and bacteria.

Life’s Building Blocks Another hypothesis that addresses the origin of life states that organic molecules arrived on Earth in meteorites or comets. Some meteorites contain amino acids. Such molecules could have arrived on early Earth, when frequent meteorite impacts were common.

Panspermia Hypothesis Panspermia is the hypothesis that "seeds" of life exist already all over the Universe, that life on Earth may have originated through these "seeds", and that they may deliver or have delivered life to other habitable bodies.

The First Cells Research continues to discover how biological molecules grouped together to form cells. Scientists must figure out how amino acids first linked together to form proteins. Forming a Cell Scientists have conducted research to determine how molecules became packaged together inside a cell membrane.

The First Cells When studying the behavior of organic molecules in water, scientists have observed that lipids tend to combine in water. Certain lipids, when combined with other molecules, can form a tiny droplet that has a surface that resembles a cell membrane. Further research has shown that short chains of amino acids can form tiny spherical structures called microspheres.

Microsphere

Cell Membrane

Endosymbiosis One species spends life cycle inside another species, an interaction that benefits both.

The First Cells Many scientists think that the formation of microspheres may have been the first step toward cellular organization. Microspheres could not be considered cells unless they had the characteristic of heredity. Origins of Heredity Scientists have studied the origins of heredity by studying the formation of proteins.

The First Cells, continued In the laboratory, scientists have not been able to make proteins or DNA form spontaneously in water. They have been able to form short chains of RNA, the nucleic acid that helps to carry out the instructions of DNA. RNA molecules may have been the first self-replicating molecule.

The First Cells, continued In the 1980s, American scientists Thomas Cech and Sidney Altman found a certain type of RNA molecule, called a ribozyme, that can act like an enzyme. They also showed that RNA can form spontaneously in water, without DNA. They hypothesized that RNA was the first self-replicating molecule that stored information and catalyzed the formation of the first proteins.

The First Cells, continued One idea of how RNA could have been involved in protein synthesis is shown here.

The First Cells, continued Cech and Altman further hypothesized that RNA could have changed—evolved—from one generation to the next. Scientists hypothesize that DNA and proteins eventually took over these roles in the cell.

Summary The Miller-Urey experiment showed that, under certain circumstances, organic compounds could form from inorganic molecules. Among the scientific theories that address the origin of life, one suggests that life began close to hydrothermal vents, and another proposes that organic molecules arrived on early Earth from a meteorite. The formation of microspheres might have been the first step toward cellular organization.