Evolution

D

Molecular Similarities

With advances in molecular biology in the last few decades, researchers seek evolutionary clues at the smallest level: within the molecules of living organisms. Despite the enormous variety of form and function seen in living things, the underlying genetic code—the molecular building material of life—displays a striking uniformity. Almost all living organisms have DNA, and in each case it consists of different pairings of the same building blocks: four nucleotide bases called adenine, thymine, guanine, and cytosine. Using different combinations of these bases, DNA directs the assembly of amino acids into functional proteins. The same uniform code operates within all living things.

These molecules contain more than the master plan for living organisms—each is a record of an organism’s evolutionary history. By examining the makeup of such molecules, scientists gain insights into how different species are related. For example, scientists compare the protein cytochrome c from different species. In closely related species, the proteins have amino-acid sequences that are very similar, perhaps varying by one or a few amino acids. More distantly related organisms generally have proteins with fewer similarities. The more distant the relationship, the less alike the proteins.

The idea that species become genetically more different as they diverge from a common ancestor laid the groundwork for the concept of the molecular clock. Scientists know that, statistically, neutral mutations tend to accumulate at a regular rate, like ticks of a clock. Therefore, the number of molecular differences in a shared molecule is proportional to the amount of time that has elapsed since the species shared a common ancestor. This calculation has provided new knowledge of the evolutionary relationship between modern apes and modern humans. The molecular clock concept is controversial, however, and has caused much disagreement between evolutionary scientists who study molecules and those who study fossils. This disagreement arises particularly when the molecular clock time estimates do not agree with the estimates derived from studying the fossil record.

E

Direct Observation

Information about evolutionary processes is also obtained by direct observation of species that undergo rapid modification in only a few generations. One of the most powerful tools in the study of evolutionary mechanisms is also one of the tiniest—the common fruit fly. These insects have short life spans and, therefore, short generations. This enables researchers to observe and manipulate fruit fly reproduction in the laboratory and learn about evolutionary change in the process.

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Scientists also study organisms in their natural environments to learn about evolutionary processes—for example, how insects develop genetic resistance to human-made pesticides, such as DDT. While pesticides are often initially effective in killing crop-destroying pests, sometimes the insect populations bounce back. In every insect population there are a few individual insects that are not affected by the pesticide. The pesticide wipes out most of the population, leaving only the genetically resistant individuals to multiply and flourish. Gradually, resistant individuals predominate in the population, and the pesticide loses its effectiveness. The same phenomenon has been observed in strains of disease-causing bacteria that have become resistant to even the most powerful antibiotics. Bacterial resistance forces scientists to continuously develop new antibacterial compounds. Scientists have hoped that curbing overuse of antibiotics might cause the drugs to become effective again. Recent research, however, suggests that bacteria may retain their resistance to antibiotics over many generations, even if they have not been exposed to the agent.

F

Determining Life’s Origins

In addition to studying how life changes and diversifies over time, some evolutionary biologists are trying to understand how life originated on Earth. This too requires the careful examination and interpretation of many indirect clues. In one well-known series of experiments in 1953, American chemists Stanley L. Miller and Harold C. Urey attempted to reproduce the atmosphere of the primitive Earth nearly 4 billion years ago. They circulated a mixture of gases believed to have been present at the time (hydrogen, methane, ammonia, and water vapor) over water in a sterile glass container. They then subjected the gases to the energy of electrical sparks, simulating the action of lightning on the primitive Earth. After about a week, the fluid turned brown and was found to contain amino acids—the building blocks of proteins. Subsequent work by these scientists and others also succeeded in producing nucleotides, the building blocks of DNA and other nucleic acids. While the artificial generation of these molecules in laboratories did not produce a living organism, this research offers some support that the first building blocks of life could have arisen from raw materials that were present in the environment of the primitive Earth.

Other theories regarding the origin of life on Earth point to outer space. Molecules formerly believed to be produced only by living systems have been found to spontaneously form in great abundance in space. Some scientists speculate that the building blocks of early life might have reached the primitive Earth on meteorites or from the dust of a comet tail.

Once all the raw materials were in place—nucleic acids, proteins, and the other components of simple cells—it is not clear how the first self-replicating life forms actually came about. Recent theories center on the role of a particular nucleic acid—ribonucleic acid (RNA), which, in modern cells, carries out the task of translating the instructions coded in DNA for the assembling of proteins. RNA also acts as a catalyst—that is, to cause other chemical reactions—and perhaps most significantly, to make copies of itself. Some scientists believe that the first self-replicating organisms were based on RNA.

According to the fossil record, the first single-celled bacteria appeared some 3.5 billion to 3.9 billion years ago. These microscopic creatures lived in the water, converting the Sun’s light into chemical energy. This metabolic process, called photosynthesis, released oxygen gas as a byproduct. Photosynthesis slowly changed the composition of the early atmosphere, adding more oxygen to what scientists believe was a mixture of sulfur and carbon gases and water vapor. Perhaps 2 billion years ago, more-complex cells appeared. These were the first eukaryotic cells, containing a nucleus and other organized internal structures. At around the same time, the level of oxygen in Earth’s atmosphere increased to nearly what it is today—another step that was crucial to the development of early life. Around 1 billion years ago, the first multicellular life forms began to appear. The beginning of the Cambrian Period (around 540 million years ago), known as the Cambrian explosion, marked an enormous expansion in the diversity and complexity of life. Subsequent to this great diversification, plant life found its way to land, while the first fishes evolved, ultimately giving rise to amphibians. Later came reptiles and, later still, mammals. The tumult of evolution was in full swing, as it remains today.

VIII

Development of Evolutionary Theory

The origins of life on Earth have been a source of speculation among philosophers, religious thinkers, and scientists for thousands of years. Many human civilizations used rich and complex creation stories and myths to explain the presence of living organisms. Ancient Greek philosophers and scientists were among the earliest to apply the principles of modern science to the mysterious complexity and variety of life around them. During early Christian times, ancient Greek ideas gave way to Creationism, the view that a single God created the universe, the world, and all life on Earth. For the next 1,500 years, evolutionary science was at a standstill. The dawn of the Renaissance in the early 14th century brought a renewed interest in science and medicine. Advances in anatomy highlighted physical similarities in the features of widely different organisms. Fossils provided evidence that life on this planet was vastly different millions of years ago. With each new development came new ideas and theories about the nature of life.

A

Ancient Views

The Greek philosopher Anaximander, who lived in the 500s bc, is generally credited as the earliest evolutionist. Anaximander believed that the Earth first existed in a liquid state. Further, he believed that humans evolved from fishlike aquatic beings who left the water once they had developed sufficiently to survive on land. Greek scientist Empedocles speculated in the 400s bc that plant life arose first on Earth, followed by animals. Empedocles proposed that humans and animals arose not as complete individuals but as various body parts that joined together randomly to form strange, fantastic creatures. Some of these creatures, being unable to reproduce, became extinct, while others thrived. Outlandish as his ideas seem today, Empedocles’ thinking anticipates the fundamental principles of natural selection.

The Greek philosopher and scientist Aristotle, who lived in the 300s bc, referred to a “ladder of nature”—a progression of life forms from lower to higher—but his ladder was a static hierarchy of levels of perfection, not an evolutionary concept. Each rung on this ladder was occupied by organisms of higher complexity than the rung before it, with humans occupying the top rung. Aristotle acknowledged that some organisms are incapable of meeting the challenges of nature and so cease to exist. As he saw it, successful creatures possessed a gift, or perfecting principle, that enabled them to rise to meet the demands of their world. Creatures without the perfecting principle died out. In Aristotle’s view it was this principle—not evolution—that accounted for the progression of forms in nature.