CONCEPTS IN BIOLOGY
PART V. THE ORIGIN AND CLASSIFICATION OF LIFE
19. The Origin of Life and the Evolution of Cells
Scientists Create Virus from Simple Chemicals
Could this be a step toward greating life in a test tube?
In 2004, scientists used off-the-shelf chemicals to synthesize the poliovirus. They had to use several highly technical procedures, but they were able to create a poliovirus from scratch. The process involved several steps. The genetic code of the poliovirus is in the form of RNA and the sequence of RNA nucleotides is known. The first step involved using this known RNA sequence to assemble a DNA molecule from individual DNA nucleotides. RNA viruses, like the poliovirus, do not make copies of their RNA from RNA but use the enzymes and other structures of the host cell to make a DNA copy of its RNA. The DNA copy is then transcribed in the host cell to produce multiple copies of the poliovirus RNA.
Because polioviruses are normally parasites in human cells, the researchers then placed the newly synthesized RNA into a culture made from human cells. The cells were ground up and large structures, such as nuclei, were removed. However, the ribosomes, enzymes, amino acids, and other materials necessary for protein synthesis were still present. When the newly synthesized viral RNA was placed in this juice, it was able to direct the synthesis of the protein coat that surrounds the RNA of the virus. To show that the poliovirus they produced was functional, they infected mice with the virus and the mice developed the disease.
The scientists started with individual chemical units they obtained from chemical suppliers and produced a functional poliovirus. Although viruses are not cells, they show some similarities to living things and may be similar to some of the earliest forms of life.
• What conditions on Earth billions of years ago could have allowed for the creation of the first living thing?
• What additional steps would be necessary to get cells from simple chemicals?
• Do you think it is ethical for scientists to go one step further and try to create living cells from scratch?
ü Background Check
Concepts you should already know to get the most out of this chapter:
• Carbohydrates, lipids, proteins, and nucleic acids are components of living things (chapter 3)
• Chemical reactions are either exothermic or endothermic (chapter 2)
• Basic cell structure of both prokaryotic and eukaryotic cells (chapter 4)
19.1. Early Thoughts About the Origin of Life
For centuries, people have asked the question “How did life originate?” Scientists are still trying to answer that question today. One hypothesis for the origin of life is spontaneous generation. Spontaneous generation is the concept that living things arise from nonliving material. Aristotle (384-322 B.c.) proposed this concept and it was widely accepted until the seventeenth century. People believed that maggots arose from decaying meat; mice developed from wheat stored in dark, damp places; lice formed from sweat; and frogs originated from damp mud. However, as time passed, people began to question this long-held belief and proposed an alternative idea, called biogenesis. Biogenesis is the concept that living things develop only from other living things and not from nonliving matter. For several hundred years, proponents of these two alternative concepts about how life originated argued and performed scientific experiments to test their ideas.
In 1668, Francesco Redi, an Italian physician, performed an experiment that challenged the concept of spontaneous generation. He set up a controlled experiment to test the hypothesis that maggots arose spontaneously from rotting meat (figure 19.1). He used two sets of jars that were identical except for one aspect. Both sets of jars contained decaying meat, and both were exposed to the atmosphere. However, one set of jars was covered by gauze while the other was uncovered. Redi observed that flies settled on the meat in the open jars, but the gauze blocked their access to the meat in the covered jars. When maggots appeared on the meat in the uncovered jars but not on the meat in the covered ones, Redi concluded that the maggots arose from the eggs of the flies (biogenesis), not from spontaneous generation in the meat.
FIGURE 19.1. Redi's Experiment
Francesco Redi performed an experiment in which he prepared two sets of jars that were identical in every way except one. One set of jars had a gauze covering. The uncovered set was the control group; the covered set was the experimental group. Any differences seen between the control and the experimental groups were the result of a single variable—being covered by gauze. In this manner, Redi concluded that the presence of maggots in meat was due to flies laying their eggs on the meat and not spontaneous generation.
However, Redi’s experiment did not settle the question. Eighty years later, in 1748, John T. Needham, an English priest, performed an experiment that led him to conclude that spontaneous generation did happen. He placed a solution of boiled mutton broth in containers, which he sealed with corks. Needham reasoned that boiling would kill any organisms in the broth and that the corks would prevent any living thing from entering. Thus, if the broth was found to contain living things, it must be the result of spontaneous generation. Conversely, if no life appeared, the concept of biogenesis would be supported. When, after several days, the broth became cloudy with a large population of microorganisms, he concluded that life in the broth was the result of spontaneous generation.
A short time later, in 1767, another Italian scientist and Catholic priest, Abb’e Lazzaro Spallanzani, challenged Needham’s findings. He thought that Needham’s experimental design may have allowed something to enter the broth and devised a slightly different experiment that made certain that nothing accidentally entered the broth. Spallanzani boiled a meat and vegetable broth, placed this mixture in clean glass containers, and sealed the openings by melting the glass over a flame. This would prevent anything from entering the flask. He placed the sealed containers in boiling water to kill any living things that might have been in the broth. As a control, he set up the same conditions but did not seal the necks, allowing air to enter the flasks (figure 19.2). Two days later, the open containers had a large population of microorganisms, but there were none in the sealed containers. He concluded that spontaneous generation did not occur and that something had entered the unsealed flasks from the air that caused the growth in the broth.
FIGURE 19.2. Spallanzani's Experiment
Spallanzani boiled a meat and vegetable broth and placed this mixture into clean flasks. He sealed one and put it in boiling water. As a control, he subjected another flask to the same conditions, except he left it open. Within 2 days, the open flask had a population of microorganisms. Spallanzani maintained that this demonstrated that spontaneous generation did not occur and that something from the air was responsible for the growth in the broth.
Spallanzani’s experiment did not completely disprove spontaneous generation to everyone’s satisfaction, however. The supporters of spontaneous generation attacked Spallanzani by stating that he excluded air from his sealed flasks, a factor believed necessary for spontaneous generation. Supporters of spontaneous generation also argued that boiling had destroyed a “vital element.” In 1774, when Joseph Priestly discovered oxygen, the proponents of spontaneous generation claimed that oxygen was the “vital element” that Spallanzani had excluded from his sealed containers.
In 1861, French chemist Louis Pasteur convinced most scientists that spontaneous generation could not occur. He designed an experiment that would allow air to enter a flask but would prevent tiny living things from entering. He placed a fermentable sugar solution and yeast mixture in a flask that had a long swan neck. The mixture and the flask were boiled for a long time. The flask was left open to allow oxygen, the “vital element,” to enter, but no organisms developed in the mixture. Organisms did not enter the part of the flask with the sugar mixture because they settled on the bottom of the curved portion of the neck and could not reach the sugar- water mixture. As a control, he cut off the swan neck (figure 19.3). This allowed microorganisms from the air to fall into the flask, and within 2 days the fermentable solution was supporting a population of microorganisms. In his address to the French Academy, Pasteur stated, “Never will the doctrine of spontaneous generation arise from this mortal blow.”
FIGURE 19.3. Pasteur's Experiment
Louis Pasteur conducted an experiment designed to test the idea that a “vital element” (oxygen) from the air was necessary to produce life. He boiled a mixture of sugar and yeast in swan-neck flasks that allowed oxygen, but not airborne organisms, to enter them. He left some flasks intact (the experimental group) and broke the neck off others (the control group). Within 2 days, there was growth in the flasks that had their swan necks removed but none in the intact flasks. Thus, Pasteur concluded that it was not oxygen in the air that caused growth in the flasks but that the growth resulted from living things, which entered the flask when the swan neck was broken off. This supported the concept of biogenesis and argued against the concept of spontaneous generation.
19.1. CONCEPT REVIEW
1. What is meant by the term spontaneous generation?
2. What is meant by the term biogenesis?
3. Describe the contribution of each of the following scientists to the biogenesis/spontaneous generation debate: Redi, Needham, Spallanzani, and Pasteur.