One fundamental question that scientists have asked is:
how did life on Earth begin?
Many theories have been suggested in answer to this important question; we will present two of them. First, however, we’ll describe the most ancient fossils discovered so far, and significant laboratory experiments that have been performed to determine how life might have originated from nonliving materials.
I. Evidence of Early Life
One goal of geologists has been to identify the most ancient fossils. The first step in that process is to find the most ancient rocks and to examine them for evidence of once-living organisms. After many decades of research—analyzing radioactive isotopes in samples from around the world—geologists identified rocks that were formed about 3.9 billion years ago. While it is likely that some rocks formed even earlier, they were probably melted by volcanic action or the bombardment of asteroids and comets from space.
Some evidence of once-living material was found in rocks that are 3.8 billion years old, but these chemical traces are not a clear indicator of early life. Much better evidence of the first living organisms was found in rocks that date from about 3.5 billion years ago. These dome-shaped structures, which are called stromatolites, range in size from a small pebble to a mound that is a meter or more in diameter.
Stromatolites are composed of thin layers of calcium carbonate, created by colonies of various microorganisms. Among these are single-celled cyanobacteria, which are sometimes referred to as blue-green algae. When the ancient stromatolites were carefully investigated with microscopes, geologists discovered structures embedded in the rocks that looked exactly like several species of blue-green algae that survive today in the shallow waters of the Gulf of California and in western Australia. Like most plants, living algae absorb carbon dioxide from water and produce oxygen.
Finding the ancient fossils was one of the most important discoveries in unraveling the puzzle of how life evolved on our planet. But like many scientific discoveries, it raises a more profound question: how did life get started in the first place?
II. The Tide Pool Theory of the Origin of Life
The earliest scientific theory about the origin of life on Earth was proposed by Charles Darwin almost 150 years ago. Darwin imagined that raw materials may have combined into lifelike organisms by chance. A likely place for this to have occurred was at the edge of the ocean, in the intertidal zone, between high and low tide marks. That is where we find tide pools—natural basins formed by rocks. Occurring in many areas of the world, today’s tide pools are rich repositories of marine life. Starfish, urchins, anemones, seaweed, crabs, and tiny fish abound in these little pools, and are protected from large predators and the pounding surf.
Darwin imagined what tide pools might have been like before the first one-celled animals. When the tide came in, salt water rushed into the pool, carrying with it minerals and complex molecules that may have formed in the sea. When the tide went out, the water caught in the pool began to evaporate, which left the minerals and complex molecules to cluster closer together. As a result, they could easily join into more complex groupings. Energy to assemble the first living organisms and to support their growth and reproduction was provided by the warmth of the Sun, or lightning from storms. Any primitive organisms that formed would have been refreshed with water and raw materials as the next tide washed into the pool.
III. Creating the Building Blocks of Life
The first laboratory experiment to test Darwin’s theory of the origin of life was by a young graduate student, Stanley Miller, who worked and studied in the laboratory of the distinguished chemist, Harold Urey, at the University of Chicago. In 1953, Miller put water in the bottom of a glass flask to simulate the ocean or a pond. An electric heater kept the water warm and allowed some of it to evaporate. With a glass tube, he connected the flask representing the ocean to another flask representing the atmosphere. He filled the second flask with a mixture of gases thought to be like Earth’s early atmosphere. A spark gap in the “atmosphere” modeled the action of lightning. To complete the water cycle, he connected another glass tube to the “atmosphere,” and cooled the gases in the tube so that the water would condense and “rain” into the “ocean.” Water continued to circulate through the “ocean” and “atmosphere” for several days.
Eventually, a reddish brown sludge formed on the bottom of the flask representing the “ocean.” When the sludge was analyzed, it was found to contain amino acids. Amino acids are complex molecules that combine to form proteins, which are essential components of plant and animal tissues. Although amino acids are not “alive,” they are the basic building blocks of life on Earth.
Other experimenters have since replicated and modified Miller’s work. In many of these experiments amino acids were formed. Unfortunately, no blue-green algae or other simple life forms were created.
Recently, amino acids have been found in meteorites that are 4.5 billion years old! However, at the time this chapter is written, the missing process or ingredient that would allow living tissue to form from raw materials and energy remains a mystery.
Even though Stanley Miller’s experiment did not establish that living organisms can arise from nonliving material, the production of the building blocks of life provided some support to Darwin’s tide pool theory. Other evidence also supports Darwin’s theory. Scientists are now aware that when Earth’s crust was young, it was covered by a great ocean with rocky continents, providing many locations for tide pools. In those days, Earth turned faster than it does now, so tides were more frequent. Also, the moon was closer to Earth than it is now, so that high tides were higher, and low tides were lower, making an even larger intertidal zone. As Miller showed, the raw materials necessary for life could have organized naturally and could have been available in those tide pools.
IV. Meet Kathleen Crane
The tide pool theory is one of several plausible theories being considered by scientists today to describe how life may have originated from nonliving materials. A different theory is that life originated at the bottom of the ocean, where volcanic action provides heat and minerals. A proponent of this theory is oceanographer Kathleen Crane.
Dr. Kathleen Crane is an oceanographer whose research has taken her all over the world—from the frozen reaches of the Arctic Ocean to coral reefs on the Virgin Islands. In this interview, she answers such questions as: Where do you think life began? What happened to the carbon dioxide in Earth’s early atmosphere? Where did the oxygen come from?
Interviewer: How did you get interested in science?
Crane: When I was about 10 years old, I went to a lecture by Jacques Cousteau. That’s what got me interested in becoming an oceanographer. I always liked the outdoors and wanted to do something exploration-oriented. That meant being either an astronaut or an oceanographer.
Interviewer: How did you prepare for your current job?
Crane: I took a lot of science in high school—every kind I could. Then I decided that even though my parents wanted me to go to an ivy league school, I would go to Oregon State because I wanted to be out in the wilderness. I could not major in oceanography as an undergraduate, so I considered biology. However, I thought that if I majored in biology I would have to spend too much time in the laboratory, so I took geology, where I would be out in the field most of the time. I also studied German, which helps me a lot because I do a lot of work with people from other countries.
Interviewer: What did you do after college?
Crane: I went to Germany for a while to improve my ability to speak German, and then studied earth science in the Alps. But I kept my interest in oceanography and eventually chose to study at Scripps Institution because they had the most ships that went to the South Pacific, and I really wanted to meet people of different cultures and see the world! In the early 1970s oceanography was a brand new field, so people interested in it were not the typical lab types. Oceanography was a precursor to today’s global science studies because it really got people from different fields to work together. Later I worked at the Lamont-Doherty Earth Observatory, and now I’m a marine geologist at Hunter College in New York.
Interviewer: (Shows photograph.) Where were you in this photograph? It looks like it was really cold.
Crane: I was on a Swedish ice breaker expedition to the Arctic Ocean to celebrate a Swedish explorer named Nordenskjold. The King of Sweden was on board, as were about 100 scientists from 12 countries, conducting a wide variety of research projects.
Interviewer: What were you doing on the expedition?
Crane: We took measurements showing how carbon dioxide is absorbed into the ocean. The reason that our atmosphere on Earth doesn’t have as much carbon dioxide as Venus is because of the ocean. The ocean is like seltzer water. Cold water in polar oceans holds more carbon dioxide. Marine plants and animals remove carbon dioxide from the water. The animals incorporate carbon from the carbon dioxide into their shells. When they die, their shells sink to the bottom and becomes calcium carbonate, also known as chalk or limestone. The carbon remains on the ocean floor, while the oxygen is released into the water.
Interviewer: What other observations did you make?
Crane: We also measured gravity. The amount of gravity in a particular spot tells you about Earth’s crust on the ocean floor, whether there’s a volcano underneath or less dense continental material, like granite. On the day this picture was taken, I was with two other scientists. One of us had a shotgun for protection against polar bears, while the other two took gravity measurements.
Interviewer: What is your most memorable experience?
Crane: It was when my colleagues and I discovered hot vents near the Galapagos Islands. When I was at Scripps, I decided to study the midocean ridge. There was a big mystery at the time about why the heat flow measurements near the ridge were so low. Since there were volcanoes, the low temperatures were very surprising.
I thought it was possible that the temperature was being measured incorrectly, and the area should be thoroughly explored. Eventually, we received funds for an expedition. We used the “Deep Tow,” which is a remote controlled underwater probe that maps the ocean floor with sonar and cameras, and measures the temperature with thermometers. With all that equipment we must have looked like the SWAT team of oceanography!
The thermometers told us that, as I suspected, previous temperature measurements were too low. The temperatures at the Galapagos vent site ranged from 12°C to 25°C. A couple of years later, black smoker vents were discovered on the East Pacific Rise pouring hot water out of the ground like fire hoses—with temperatures above 350°C!
Everywhere we pointed the cameras near the vent we saw huge clam shells. There was enough interest about our discovery to obtain funds for a more detailed expedition in the submarine Alvin. I was the navigator. When we neared the vents, we discovered entirely new species of animals, including giant tube worms up to five feet long! Articles were published in National Geographic and other places, showing these newly discovered life forms.
Interviewer: What are some of the implications of your discoveries?
Crane: Deep sea vents may play a major role in driving deep ocean currents, which in turn affect world climates. For example, the El Niño current is an upwelling of huge amounts of warm water in the Pacific that occurs about every five years, and has a major influence on the weather. It may even be possible that El Niño and other deep sea currents are driven by the heat from deep sea vents.
Another interesting possibility is that life on Earth may have started in deep sea hot vents near the midocean ridge. All of the amino acids are right there, along with methane, ammonia, water vapor, and carbon dioxide from the volcanoes. The hot vents also provide heat energy for chemical reactions. In addition, there is a theory that sheets of clay may have assisted in the development of life by providing structures to help the organic molecules organize themselves. Sheets of clay of this sort can be found near the hot vents, formed by the hot spring waters.
Interviewer: Where do you like to work most of all?
Crane: I like working in the Arctic, even though I don’t like really cold weather! I have never seen an environment more beautiful and pristine, with wonderful animals like polar bears. It is also one of the more delicate ecosystems, and has such an important effect on world climate that it is very important to preserve it in its natural state.
V. Where Do You Think Life Originated?
Look back over the interview and summarize Kathleen Crane’s views about these questions:
- What theory does she suggest for where life might have begun?
- Crane compared Earth and Venus. Earth has an ocean and Venus does not. What was the effect of this difference?
- What ingredients for life are found near deep sea vents?
Use Crane’s information and the previous tide pool origin-of-life point of view to write a short essay in which you present your argument about where you think life might have begun.
VI. The Importance of Scientific Arguments
Competing theories of the origin of life have provoked arguments in the scientific community, because at present there is no strong evidence that favors one theory over another. These arguments are essential to the nature of science. It’s important to point out, however, that the term “argument” has a different meaning in science than it does in everyday language.
In everyday language, the word “argument” refers to an exchange of ideas in which each person states an opinion as strongly as possible. An argument may reduce to a shouting match in which each side declares its position is right and, without examining the other point of view, maintains that the other side is completely wrong. This kind of argument is not scientific.
In making a scientific argument, a person defines all the terms, assembles as many pertinent observations as possible, and presents them in a logical fashion so that the listener will be persuaded that a certain theory is correct and should be adopted. The listener, if not persuaded, may cite other observations or analyze the matter differently. While the process of scientific argumentation may not result in agreement, it usually produces a clearer understanding of the evidence and ideas for further research.
The discovery of ancient fossils indicates that 3.5 billion years ago living organisms—probably blue-green algae—were growing and multiplying in shallow seas. At present, there is no satisfactory explanation for how these organisms arose from nonliving material. Laboratory experiments have shown that amino acids, the building blocks of life, could have arisen by chance chemical interactions, but no one knows for certain how the amino acids combined to form living organisms. There are, however, plausible scientific theories to explain this, such as the warm tide pool theory and the possibility that life arose near deep-ocean vents.
In Chapter 5 we will explore the next major development in the evolution of life, the establishment of an atmosphere rich in oxygen.