Science Matters (37 page)

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 descendants is the full measure of biological success.

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!

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.

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.

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.

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.

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
million 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.

The oldest fossils that we could call human are from the genus
Australopithecus
(“southern ape”), found in sediments in what is now Ethiopia. About three and a half million years old, these fossils are of a creature who walked upright, was about three feet tall, and had a brain the size of a modern newborn baby. There are well over a dozen different types of
Australopithecus
and early
Homo
that lived in Africa between three and one million years ago, and deciding which of these are in the actual line of descent of modern humans is still a matter that is intensely debated. In any case, individuals of what are called anatomically modern humans,
Homo sapiens
, appeared on the scene in Africa about 200,000 years ago. A close cousin,
Homo neanderthalensis
, or Neanderthal man, lived in Europe and the Middle East from about this time to roughly 35,000 years ago, when it went extinct. Thus, although there have been many close relatives of modern humans in the past, there are none alive today.

Most of today’s evolutionary scientists focus not on whether evolution occurred, but how it occurred. When Darwin first proposed his theory, he argued that evolution proceeds at a slow, steady rate, and that small changes gradually accumulate to produce large ones. This view is known today as gradualism. In the early 1970s, two American paleontologists—Stephen Jay Gould and Niles Eldredge—proposed an alternative theory that goes
under the name of punctuated equilibrium. In their view, evolution is characterized by long periods of little change, interspersed (punctuated) by short periods of rapid change.