Print

24.4Theories on the Origins of Life

The classic Japanese literature Tsurezuregusa (Essays in Idleness) contains a famous saying: “What is Buddha composed of?*12 The same question can be raised about the origins of life. In ancient times, people believed that life was created naturally from dirt or mud referring to how bacteria and insects rose out of putrid matter. The theory of spontaneous generation, which is based on the belief that organisms are born from the life force in air, remained strongly rooted until the 19th century. In those times, Pasteur conducted his famous experiment in which he added nutrient broth to two flasks and bent the necks of the flasks. The results of this experiment completely refuted the theory of spontaneous generation.

*12 Essays in Idleness Section 243: When I was eight, I asked my father, “What is Buddha?” My father said, “A man who was enlightened.” Against I asked, “How can a man become Buddha?” He said, “By practicing the teachings of Buddhahood.” To that I asked, “If Buddha teaches Buddhahood, then who did Buddha learn from?” He said, “The Buddha before him.” Again I questioned, “Who taught the very first Buddha then, father?” He said, “He may have come down from the heavens or come out of the ground,” and laughed. He told this story to everyone he met later with amusement that he had run out of ideas for answering my incessant questions.

Today, there is nothing to doubt about Pasteurs experiments, but the new question arises, then what is the real origin of life? Various theories on this exist, and some of these theories are within the realm of imagination, e.g., life on earth came from space. Be it space or earth, the question remains: “How did early life forms start?” respectively.

Top of Page

24.4.1

History of earth and life

For life forms to live, the environment that surrounds them is very important. When discussing the birth and evolution of life, the earth’s environment for fostering life forms must be understood.

Point mutations due to mutagens such as ultraviolet light and incorporation errors that occur during genetic replication are also examples of genomic changes. Especially in organisms that carry out sexual reproduction, gene duplication, which occurs during meiosis of gametes, and genetic recombination such as crossing over and inversion promote genomic changes in vertical transmission. For example, it is considered that chromosome 2 of humans was created by fragmentary linkage of chromosomes 12 and 13 of chimpanzees*9. This suggests that the genomic changes such as recombination of chromosomes are associated with the evolution of species (Figure 24-4).

■Formation of primitive earth
According to the calculation of the Big Bang Theory, the universe is thought to have been created some 13.7 billion years ago and the earth some 4.6 billion years ago. The signs of vital actions can be seen from rocks existing from 3.8 billion years ago, and the oldest cell fossil is said to be from about 3.5 billion years ago (Figure 24-7). On the primitive earth, asteroids and meteors repeatedly collided on earth, and the collision energy is thought to have caused a state resembling very hot magma on earth. The air covering earth then was similar to volcanic gas today, comprised of components such as vapor, CO2, and N2 gas. However, as the earth started to become colder, the vapor in the air condensed to form seas, and CO2 dissolved into the seas to form primitive seas. It is known from topographical surveys that seas existed from about 3.8–4.0 billion years ago. However, it is anticipated that O2 levels were extremely low for 2 billion years after the birth of the earth. It can be easily imagined that the earth’s environment in such ancient times contributed enormously to the birth of primitive life*13 *10 .

*13 Some say that 3.5 billion years ago, the concentration of oxygen in the earth’s atmosphere was higher than generally considered, even comparable to the levels today. There are many theories on how primitive earth was formed, a process yet to be fully understood. If the earth environment at the time primitive life was born differs from what we imagine today, then naturally, the life-generation process would be different as well.

Top of Page

24.4.2

Changes in the earth’s environment and chemical evolution

Assuming that life was indeed created on earth, there must have been at least one major turning point at which “life forms” emerged from inorganic non-living matter on earth. During the transient period, there may have been a time when organic states existed that cannot be called life forms. The turning point must have occurred between the 0.8 billion years from 4.6 billion years ago when the earth was born to 3.8 billion years ago, from which time signs of life forms have been discovered.

Figure 24-7 History of the Earth’s environment and life

■Oparin and Miller: Theory of chemical evolution
The most famous theory and experiments on the origin of life are the coacervate theory proposed by Aleksandr Oparin and experiments conducted by Stanley Miller on the basis of this theory. Oparin’s hypothesis is a pioneering chemical evolution theory, proposing that at first, chemical reactions occured between non-living matters in the earth’s inorganic environment, which produced organic matter, which in turn went through complex reactions to create complex organic substances. These complex organic substances then gathered and separated as small drops, and finally, life forms were formed. Meanwhile, Miller assumed that the air of the primitive earth was composed of methane, vapor, ammonia, and hydrogen gases. In his experiments, he placed these gases in a flask, repeated electrical discharge resembling lightning, and discovered that organic substances such as amino acids are formed. As discussed earlier, it is very likely that the atmosphere of the primitive earth was not like the composition assumed in Oparin’s predictions and Miller’s experiments. If the initial conditions in the flask were different, it would be easy to imagine that the types of organic substances obtained and the synthesis efficiency are different*14 . However, what is important here is the fact that organic matter and even life could potentially evolve from inorganic matter in the natural world. Huge contributions have been made by Oparin, who introduced the concept of chemical evolution, and Miller, who showed there is a strong possibility that organic substances could be synthesized in the environment of the natural world.

*14 Some say that organic substances required for chemical evolution are made somewhere in space, and brought to earth by comets and meteors. If so, it will overcome the contradiction of Miller’s experiment. Though there is no proper evidence as of now, recent space resesarch may make new discoveries.

■RNA world
The RNA world hypothesis, which suggests that primitive life forms originated from a simple self-replicating RNA molecule, was proposed by Walter Gilbert*15 in 1986*16. Some of the reasons given to support this theory include: deoxyribonucleotides are biosynthesized from ribonucleotides (in other words, ribonucleotides must have appeared before deoxyribonucleotides did); ribonucleotides are often included in active substances inside the cell (e.g., ATP and NAD); RNA viruses store genetic information in RNA; and the most decisive reason being RNA itself has the activity to catalyze biochemical reactions (called ribozyme). There was a period when RNA was responsible for both functions: accumulation of genetic information and catalyst activities as a function molecule. Later, DNA replaced RNA to store genetic information because of its feature being more stable than RNA, whereas proteins (amino acids) replaced RNA as the function molecule because proteins are more diverse and are more functional than RNA. This RNA world hypothesis, as outlined above, has become the mainstream theory today due to the large body of biochemical evidence present.

The RNA world is not a thing of the past. In recent years, it has been found that short untranslated RNAs*17 transcribed from the genome act on other transcribed RNA involved in coding genes, to regulate gene expressions. This is a good example suggesting that organisms, including humans, use RNA as a functional molecule. On the other hand, although the theory of chemical evolution depicts how organic substances were synthesized under natural conditions, it does not explain how such synthesis of organic substances led to the creation of the RNA world. In fact, there are no persuading hypotheses available today that sufficiently explains this transition, and hence, future study outcomes must be expected.

*15 This is the same Gilbert in the footnote onColumn Selection 1 of Chapter 24.
*16 It is said that this concept existed before his proposal.
*17 Called microRNA (miRNA).

Top of Page

next

prev