VII.

How Scientists Study Evolution

Species do not change overnight, or even in the course of one lifetime. Rather, evolutionary change usually occurs in tiny, almost imperceptible increments over the course of thousands of generations—periods that range from decades to millions of years. To study the evolutionary relationships among organisms, scientists must perform complex detective work, deriving indirect clues from the fossil record, patterns of animal distribution, comparative anatomy, molecular biology, and finally, direct observation in laboratories and the natural environment.

A.

Fossils

One way biologists learn about the evolutionary relationships between species is by examining fossils. These ancient remains of living things are created when a dead plant or animal is buried under layers of mud or sand that gradually turn into stone. Over time, the organism remains themselves may turn to stone, becoming preserved within the rock layer in which they came to rest. By measuring radioactivity in the rock in which a fossil is embedded, paleontologists (scientists who study the fossil record) can determine the age of a fossil (see Dating Methods).

Fossils present a vivid record of the earliest life on Earth, and of a progression over time from simple to more-complex life forms. The earliest fossils, for example, are those of primitive bacteria some 3.5 billion years old. In more recent layers of rock, the first animal fossils appear—primitive jellyfish that date from 680 million years ago. Still more-complex forms, such as the first vertebrates (animals with backbones), are documented by fossils some 570 million years old. Fossils also indicate that the first mammals appeared roughly 200 million years ago.

Although these ancient forms of life have not existed on Earth for millions of years, scientists have been able, in many instances, to show a clear evolutionary line between extinct animals and their modern descendants. The horse’s lineage, for example, can be traced back about 50 million years to a four-toed animal about the size of a dog. Fossils provide evidence of several different transitional forms between this ancient horselike animal and the modern species. In another example, the extinct, winged creature Archaeopteryx lived about 145 million years ago. Its fossil shows the skeleton of a dinosaur and the feathers of a bird. Many paleontologists consider this creature an intermediate step in the evolution of reptilian dinosaurs into modern birds. Fossils show clear evidence that the earliest human species had many apelike features. These features included large, strong jaws and teeth; short stature, long, curved fingers; and faces that protruded outward from the forehead. Later species evolved progressively more humanlike features.

B.

Distribution of Species

Scientists also learn about evolution by studying how different species of plants and animals are geographically distributed in nature, and how they relate to their environment and to each other. In particular, populations that exist on islands provide living clues of patterns of evolution. The study of these evolutionary relationships, known as island biogeography, has its roots in Darwin’s observations of the adaptive radiation of the Galapagos finches. The Hawaiian Islands provide similar examples, particularly in the species of birds known as honeycreepers. Like the Galapagos finches, the honeycreepers of Hawaii evolved from a common ancestor and branched into several species, showing a striking variety of beak shapes adapted for obtaining different food sources in their various niches.

C.

Anatomical Similarities

Detailed study of the internal and external features of different living things, a discipline known as comparative anatomy, also provides a wealth of information about evolution. The arm of a human, the flipper of a whale, the foreleg of a horse, and the wing of a bird have different forms and are adapted to different functions. Yet they correspond in some way, and this correspondence extends to many details. In the case of the arm, flipper, foreleg, and wing, for example, each appendage shows a similar bone structure. The study of comparative anatomy has revealed many instances of correspondence within various groups of organisms and these bodily structures are said to be homologous. Evolutionary biologists suggest that such homologous structures originated in a common ancestor. The differences arose as each group diverged from the common ancestor and adapted to different ways of life. The more recent the common ancestor, the more similar the species.

The skeletons of humans, for instance, retain evidence of a tail-like structure that is probably a relic from previous mammalian ancestors. This feature, called the coccyx, or more commonly, the tailbone, has little apparent function in modern humans. Relic features such as the coccyx are called vestigial organs. Another vestigial organ in humans is the appendix, a narrow tube attached to the large intestine. In some plant-eating mammals, the appendix is a functioning organ that helps to digest plant material. In humans, however, the organ lacks this purpose and is considerably reduced in size, serving only as a minor source of certain white blood cells that guard against infection.

The field of embryology, the study of how organisms develop from a fertilized egg until they are ready for birth or hatching, also provides evolutionary clues. Scientists have noted that in the earliest stages of development, the embryos of organisms that share a recent common ancestor are very similar in appearance. As the embryos develop, they grow less similar. For example, the embryos of dogs and cats, both members of the mammal order Carnivora, are more similar in the early stages of development than just before birth. The same is true of human and ape embryos.

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.

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.