Authors: Robert M. Hazen
As a human being develops from a single fertilized egg, an interesting process occurs in the cells’ DNA. As the cells divide, they
begin to differentiate, so that even after a week or ten days some cells are destined to produce skin, others neurons, others cells in the digestive system, and so on. This process of specialization is accomplished by turning off all the genes in a cell’s DNA except those needed to perform its particular function. Thus, most of the genes in any adult cell are turned off.
For the first half-dozen divisions of the fertilized egg, however, no genes in some of these cells are turned off, so each such cell has the potential to develop into any of the specialized cells eventually present in the adult. These cells are called embryonic stem cells. As the organism develops, a series of intermediate stem cells are formed. The many types of cells that appear in the skin, for example, come from stem cells that can develop into any of these types, but not into muscle or nerve tissue. These cells are called somatic stem cells.
The great hope of a new field called regenerative medicine is that an individual’s stem cells can be used to engineer to create organs that can be transplanted—new heart muscle or nerves, for example. Such organs would contain the individual’s own DNA and would therefore not be rejected by his or her immune system. To realize this dream, however, scientists will need to have access to stem cells in which all of the genes are turned on, so that the cells can be guided into the entire final state.
One way to obtain such cells would be to create a cloned embryo with the patient as the DNA donor and then harvest the stem cells from the embryo. This approach has several problems. For one thing, it would require a large supply of human ova, which would have to be removed surgically from healthy women. For another, in the United States the use of embryonic stem cells is mired in the national debate over abortion and hence encounters political problems.
Fortunately, in 2007 researchers in the United States and Japan
developed ways to produce stem cells without recourse to an embryo. By using viruses to inject genes into the DNA of a normal skin cell, they could reset the switches that turn genes off, just as the egg does in the cloning process. The result is that we can produce stem cells by direct manipulation of a cell’s DNA, without recourse to cloning at all. Thus, there is hope that regenerative medicine can move forward without the restraints of the political and practical problems discussed above.
I
F YOU OR YOUR CHILDREN
went to public school in the United States, chances are that creationism—the biblical account of human origins—wasn’t a part of the science curriculum. This educational decision was not a foregone conclusion. A dedicated group of scientists has been fighting a series of pitched legal battles in courtrooms across the country—in Arkansas, in California, in Pennsylvania—to protect science from what many of us see as one of the greatest threats to the science education of America’s children.
This chapter will offend some people, but that is nothing new. The theory of evolution has been offending people for more than a century. Two strongly held views about the origin of our planet and its life are in severe disagreement. Biblical creationists accept on faith the literal Old Testament account of creation. Their beliefs may include (1) a young Earth, perhaps less than 10,000 years old; (2) catastrophes, especially a worldwide flood, as the origin of Earth’s present form, including mountains, canyons,
oceans, and continents; and (3) miraculous creation of all living things, including humans, in essentially their modern forms. If you are a creationist, the Bible—not nature—dictates what you believe. Creationists subordinate observational evidence to doctrine based on their interpretation of sacred texts. The tenets of biblical creationism are not testable, nor are they subject to dramatic change based on new data. In other words, creationism is a form of religion.
The testimony of nature—evidence that anyone can observe and interpret—belies creationist dogma. If Earth is only 10,000 years old, how could the Grand Canyon have been carved a mile deep in solid rock? How could plate tectonics split apart Europe and North America with spreading rates of only a few inches per year? How could radiometric age dating, based on the steady decay of radioactive elements, give ages of hundreds or thousands of millions of years for most rocks? How could seasonally varying deposits of Mississippi River sediments, coral reefs, and deep ocean deposits contain hundreds of thousands of annual layers, all on top of much older rocks? Nature has much to tell us about our origins, if only we listen without prejudice.
The biblical story of creation has great poetic beauty and metaphorical power. The biblical story of creation (religion) and the theory of evolution (science) are different, complementary ways of answering questions about the origins of life and humans. Because of this fundamental difference, we believe that it is inappropriate to incorporate creationism into any science curriculum.
The scientific theory of evolution has been developed and modified, challenged and tested, over centuries of geological and biological observations. The theory of evolution has led to countless specific predictions regarding location of fossils, age of rock formations, and genetic similarities of different species.
Evolution is testable and, like any scientific theory, subject to change based on new data. The central idea that has emerged from these studies is:
All forms of life evolved by natural selection
.
One must distinguish between the
fact
of evolution and any particular
theory
of evolution, a distinction that will be clear if you think about gravity. There have been many theories of gravity, from Newton to Einstein to (perhaps) a fully unified field theory Any one of these theories may be wrong, incomplete, or incorporated into another. But if you drop an object, it falls, regardless of which theory you believe. That is the fact of gravity.
In the same way, the fossil record, molecular biology, and geological research all buttress the notion that modern complex life on Earth evolved out of earlier, simpler forms. This is the fact of evolution. As with gravity, there are different theories of evolution that purport to describe this process. Any of them, starting with Darwin’s, may be wrong or incomplete. The correctness or incorrectness of any particular theory, however, doesn’t change the fact of evolution, any more than one can question the fact of gravity.
Most scientists agree about one aspect of evolution. Life seems to have arisen in a two-step process. The first stage—chemical evolution—encompasses the origin of life from nonlife. Once life appeared, the second stage—biological evolution—took over.
On a clear winter’s night, gazing into the cold depth of the sky, you can face the brute fact that the universe is a cold, hostile, forbidding
place, almost completely devoid of havens for living things. That life should evolve at all is a remarkable thing, requiring just the right temperature, pressure, and chemical elements, as well as a source of energy to combine those elements. The early Earth provided all those conditions.
The first requirement for the evolution of life as we know it is an ocean, the mixing bowl for the chemicals of life. The early Earth had both an abundance of water and temperatures that remained within the rather narrow range of freezing and boiling water. Within a few million years of Earth’s solidification, water covered most of the globe’s surface.
Life’s origins also required an abundant supply of at least four key elements: carbon, hydrogen, nitrogen, and oxygen. All of these components were present in the early atmosphere, which was very different from the air we now breathe. The gases that came from volcanoes to form the first atmosphere were a mixture of nitrogen (N
2
), carbon dioxide (CO
2
), and water (H
2
O), perhaps with a bit of hydrogen (H
2
), methane (CH
4
), and ammonia (NH
3
) tossed in. These gases mixed with the wave-tossed surface layers of the early ocean, which thus contained all the essential elements of life.
It is a big step from an ocean with a few essential chemical elements to a living organism. In 1953 Stanley Miller and Harold Urey at the University of Chicago designed an experiment to find out what natural process might have formed the complex molecules necessary for life.
Miller and Urey tried to reproduce Earth’s early environment in a jar. Into the glassware they poured water and created an atmosphere
of ammonia, methane, water, and hydrogen gases. They continually heated and mixed the gases and water while electric sparks, simulating lightning, added energy. The results were amazing. Within a few days the water turned brown, and chemical analysis revealed amino acids—the building blocks of proteins.
Subsequent experiments using other combinations of gases or ultraviolet radiation yielded similar results. Amino acids, sugars, and other essential molecules of life formed in every case. The longer the experiment lasted, the more diverse and concentrated the molecular broth. For a time the public feared that some new and dangerous form of life might actually arise from the test tube. In fact the Miller-Urey molecules were several steps removed from life, but they demonstrated that under the right conditions the molecules of life will form in abundance.
Today, there are laboratories all around the world devoted to “origins of life” studies—laboratories where high-tech descendants of the Miller-Urey experiment probe the ability of the early Earth to produce ever larger and more complex molecules. This research demonstrates that there is no problem making extremely complex molecules in conditions like those in the atmosphere or oceans of the primitive Earth.
Atmospheric processes are by no means the only way to make life’s essential molecules on the early Earth. Energetic volcanic vents on the deep ocean floor, where carbon dioxide and other gases interact with minerals, have been shown to generate a rich variety of organic molecules—amino acids, lipids, carbohydrates, and more. Yet another source of complex molecules on the early Earth was the meteorites that were still falling. We know that modern meteorites contain organic molecules like amino acids, and these molecules would have added to those being produced by Miller-Urey processes. A minority of scientists
argue that life actually began with spaceborne debris, a proposition most view with skepticism.
Given the large quantities of organic molecules produced at Earth’s surface, in deep volcanic zones, and in space, where and how did life begin? One theory involves the creation of the appropriately named primordial soup. This theory holds that radiation from the sun and lightning from the sky provided the energy required to combine simple gases into complex carbon-based molecules, which must have crowded the early oceans. For hundreds of millions of years life’s chemicals were created and concentrated in the ocean’s upper layers. There may have been scores of different amino acids, linking together to make primitive proteins. Lipid molecules clumped together to form membrane-like sheets and spheres. And DNA-like strands of sugars and bases also may have been present from time to time in that pre-life soup.
Other researchers invoke mineral surfaces, perhaps in a shallow tidal pool or along cracks and fissures in rocks far below the ocean floor, where life’s molecules were selected and concentrated in an orderly way. Still other scientists suggest that a primitive oil slick on the ocean’s surface provided an ideal place for life to begin. As with all scientific hypotheses, further experimental tests will help us to learn whether we’re on the right track.
In any event, we can be sure that life was well established on Earth almost 4 billion years ago. Earth’s oldest known sedimentary rocks in Isua, Greenland, more than 3.8 billion years old, show tantalizing evidence of cellular life. Suddenly there was a whole new ball game on Earth.
The first living cell was not threatened by predators and it lived in an ocean filled with nutritious molecules. It had no competition from any other life-form. It may have taken hundreds of millions of years to create the first cell, but within a relatively short time, perhaps only a few years, that cell’s offspring probably filled the world’s oceans, consuming much of the organic raw materials and greatly reducing the chance that any other type of cell would spontaneously arise. In essence, the first cell, once it appeared, preempted other possibilities of life.
All the diversity of life on Earth—trees, mushrooms, amoebas, and humans—evolved from the first cell by the process of natural selection. By the mid-nineteenth century most geologists and paleontologists had accepted the fact that no species lasts forever; new species appear and old species become extinct. Still, the mechanism of these changes was a mystery. Charles Darwin, a meticulous British naturalist, proposed a solution brilliant in both its power and simplicity. Twenty years of research culminated in the 1859 publication of
On the Origin of Species by Means of Natural Selection
, which remains one of the pivotal events in the history of science.
Darwin studied individual variations in domesticated and wild animals and arrived at three major conclusions. First, every species exhibits variations: size, strength, coloration, and hundreds of other traits vary from individual to individual. Second, many traits are passed on from parents to children: taller parents tend to produce taller children and so on. Both of these
ideas were second nature to readers in Darwin’s native England, where animal breeders had artificially selected desirable traits for centuries.