The Miller-Urey Experiment
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.
The Primordial Soup and Mineral Surfaces
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.
BIOLOGICAL EVOLUTION
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.
Natural Selection
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.
Darwin’s great contribution was the recognition that variations and inheritance of traits also influence survival and breeding in a natural, wild setting. Each offspring, Darwin realized, inherits traits from both parents, yet no child is exactly like either parent. Most of the variations within a species have little to do with survival—green-eyed flies probably do as well as red-eyed flies—but once in a while a trait matters. If an individual is better able to survive and attract a mate, by virtue of stronger muscles or better camouflage or fancier feathers, chances are that it will pass those traits on to more offspring than its rivals, and eventually the entire population will share them. Darwin called this process “natural selection.”
Variation in a common species of British insect, the peppered moth, provided a striking example of Darwin’s view of life. Early in the 1800s light-colored lichens covered many British trees. Light-colored, mottled peppered moths blended in well with this background and so avoided birds and other predators. The industrial revolution, powered by giant coal-burning engines, blackened the skies and tree trunks over much of Britain, eliminating the light peppered moth’s protection. As trees became sooty in industrial areas, natural selection favored the dark individuals, who were always in the population, albeit in small numbers up to that point. With the change in bark color, more dark moths survived to produce more dark offspring. By the late 1800s British peppered moths were predominantly dark. The species survived by adapting to the changing environment. Today, as environmental measures have reduced sooty output, the tree trunks have become lighter and so too has the population of peppered moths.
Life is a contest for limited resources. An individual can survive only if it is able to beat the competition and withstand the vagaries of floods, droughts, hot spells, and ice ages. Life uses many different strategies to survive. Most insects like mosquitoes and cockroaches produce vast numbers of young, of which just a few live long enough to reproduce. Most large animals like us, on the other hand, devote a great deal of energy into nurturing just a few offspring. Some species develop remarkable camouflage that mimics bark or foliage or snow, while others sport flamboyant coloration to warn potential predators of poison.
The phrase most often associated with Darwin, “the survival of the fittest,” has from time to time been misused to rationalize political or economic actions by a powerful elite. Darwin defined “fit” in a very restricted sense: the most fit individual is the one that passes his or her genes to offspring that can themselves produce yet more offspring. Number of descend
ants is the full measure of biological success.
The Mechanism for Change
The great gap in Darwin’s thesis, and an obvious target for his critics, was the absence of any known mechanism to introduce and pass on new traits and variation. Mendel and subsequent geneticists learned part of the story, but not until the structure and function of DNA were determined did the way variations are produced become clear. No matter how reliable the duplication of DNA might be, mistakes do happen. Damage from X-rays, ultraviolet radiation, heat, or certain chemicals in creases the rate of errors. Over time many small changes, called mutations, creep into a gene. Some errors are unimportant and get passed on to offspring without any discernible effect. Some errors are disastrous and destroy any chance for viable offspring. Some errors lead to genetic diseases, by which some small but critical part of the body’s chemical machinery fails to operate properly. And once in a very great while a chance error results in a new, desirable trait that confers an advantage on offspring, and natural selection takes over to spread that trait through subsequent generations. Over the span of millions of years such small changes accumulate to become major differences.
One of Darwin’s most profound insights is that life evolves because it is so competitive. Random variations and chance mutations occasionally lead to advantages, which are preserved as non-random evolution. Giraffes did not evolve long necks by stretching to reach the highest branches. Rather, natural selection favored the animals that by chance happen to be slightly taller. Individual traits vary at random, but nature selects traits by circumstance.
The random mutations acted on by natural selection accumulate and, over long periods of time, produce organisms that differ markedly from their ancestors. This is the basic mechanism by which new species come into existence, although, as we shall see, the exact way that this process occurs is a subject of debate among scientists today. We should point out that normal rates of evolution are easily sufficient to produce all the complexity of life we see around us. Scientists have estimated, for example, that if a population of mice began to change at a rate observed in many organisms today, those mice could be as big as elephants in a few tens of thousands of years!
EVIDENCE FOR EVOLUTION
Research from the earth and life sciences provides copious evidence for the common ancestry and continuous change of life on Earth, beginning billions of years ago.
Molecules of Life
The molecular makeup of life provides compelling evidence for evolution from one single cell. All life is built from the same small subset of organic molecules. All of Earth’s living things, from slime mold to tea roses to humpback whales, have the exact same DNA-based genetic code, with the molecules following right-handed (never left-handed) spirals. Of all the hundreds of different possible amino acids, only twenty different types form all proteins in every organism. It is reasonable to argue that some or all of these chemical oddities arose in the first cell and have been locked in ever since. If more than one cell had arisen independently, then life would surely possess more than one chemical vocabulary.
Cells
The cellular architecture of all life also points to a common ancestry. Every living thing is made of cells, all of which share many of the same chemical and physical structures. There is also an intimate connection between single-celled and multicelled organisms. Even large and complex animals and plants are collections of cells that are often capable of separate existence. Individual human cells are highly specialized to serve as skin or muscle or nerve or organ. But isolate those cells and they revert to single-celled behavior. Single human cells can adopt amoeba-like form, and they feed and duplicate just like bacteria. In one sense humans and other mammals are colonial organisms, formed from trillions of cooperating cells.
Fossils and Evolution
The most dramatic evidence for evolution comes from fossils—the rockbound remains of past life. Fossils form when organisms die and are buried. Mineral-rich waters flowing underground gradually replace the atoms in the organism’s hard parts until at last we have a fossil—a replica in stone of the original. Ocean floors are littered with shells, scales, teeth, and other durable remains. River valleys and lake bottoms accumulate animal bones and tree trunks, leaves and insects. Fossil relicts appear in sediments from every geological age, but the nature of past life revealed by those fascinating petrified remains changed in striking fashion through the ages. Fossils prove that for almost four billion years of Earth history life has evolved, increasing in both complexity and diversity.
There are, of course, limitations to the fossil record. Most living things do not possess hard parts, so the majority of species are rarely, if ever, preserved in rock. Even preservation of shells and bone is a chancy thing. Most organisms die, decay, and weather away without a trace. The fossils that remain, therefore, are at best a spotty historical record of Earth’s life.
THE STORY OF EVOLUTION
Geologists and paleontologists divide Earth history into several major eons and eras, based on the type of life that dominated the lands and oceans at a particular time. The Hadean and Archean eons (4.5 to 2.5 billion years ago) saw the formation of the solid Earth, the filling of ocean basins, and the chemical evolution of single-celled life. The Archean atmosphere had no ozone to block the sun’s harsh ultraviolet radiation (see Chapter 19), so life was not possible on land, but there is abundant evidence for simple life in the early oceans.
The most ancient ocean sediments contain remains of only single-celled life, commonly microscopic bacteria-like spheres or rods. Such tiny creatures are observed by preparing paper-thin slices of rock, which are studied with a microscope. Most of the fossil bacteria are rather nondescript, isolated blobs, but occasionally death caught them in the act of dividing. Other primitive life formed mats of algae, with clearly delineated layers and filaments similar to modern-day Australian algal mats found in ponds and coastal pools. The rock record is clear: one-celled life crowded Earth’s seas for three billion years.
More complex life and an oxygen-rich atmosphere evolved during the Proterozoic eon (2.5 billion to 542 million years ago). The first multicellular plants and animals are found in rocks about one billion years old. Jellyfish, soft-bodied worms, and multicellular algae came to rest in sediments that now form parts of Australia, Europe, and North America. Our knowledge of these ancient flora and fauna is limited because none of these organisms had hard parts. Their preservation required an unusual combination of circumstances: rapid sedimentation, calm waters, and lack of scavenging bacteria.
Life on Earth changed dramatically about 542 million years ago, at the start of the Paleozoic era (542 to 251 million years ago), when animals evolved the ability to make hard shells. The fossil record shows a remarkable increase in the diversity of sea life in the space of a few million years. Corals and other colonial animals built vast reef systems near continents. Segmented lobster-like creatures, precursors of modern snails, starfish, sea urchins, and a wide variety of bivalve shells also abound in ocean sediments from half a billion years ago.
Few of the life-forms in that ancient world would be familiar to today’s skin divers, but as the eons passed more and more modern types joined the fossil record. The first jawed fish, land plants, and insects arose perhaps 400 million years ago, while vertebrates crawled from sea to land about 360 million years ago. Great forests of cycads and ferns developed along with winged insects at the 300-million-year mark, and shortly thereafter large reptiles roamed the surface.
Dinosaurs and other reptiles ruled the land, sea, and air for most of a quarter of a billion years during the Mesozoic era (251 to 66 million years ago). Tyrannosaurus, Stegosaurus, Triceratops, and other giant dinosaurs are only the most famous of hundreds of curious beasts that evolved, along with the trees, flowering plants, modern-looking shellfish, and the first small mammals, to inhabit almost every corner of the planet. The heyday of the spectacular reptiles lasted for almost 200 million years, but ended suddenly 66 mil
lion years ago. With the death of the dinosaurs, who up to that time had dominated the battle for resources, mammals were gradually able to evolve, adapt to, and exploit many different environments.
The most recent, Cenozoic era (66 million years ago to the present) saw the rise of mammals, which have evolved many diverse forms, including Homo sapiens. By 10 million years ago life on Earth had a distinctly modern cast. Bats, cats, dogs, and rodents were common. There were a few oddities: elephants were hairy with strangely directed tusks, giant sloths stood as tall as a house, and horses had toes. But birds, fish, insects, and other everyday animals were much like those of today’s forests and streams, while the oceans contained a recognizable cast of whales, sharks, and reef life. Still, one prominent modern life-form—the hominids—was missing.
Homo sapiens, the human species, is a remarkably recent product of evolution. Scientists who study fossil hominids reckon that the evolutionary event that separated the human ancestors from the ancestors of the chimpanzee happened about eight million years ago in Africa. The fossil record leading to the first creatures we could call human is rather sparse, both because there were relatively few animals to be fossilized (think of modern-day baboons) and because sedimentary rocks of that age are rare on Earth’s surface. Nevertheless, we do have a few fossils that exhibit intermediate features between the first humans and more primitive primates.
Science Matters Page 28