The History of Life

Deep Time. The story of life unfurls against a backdrop of time, of deep time: the length of time the universe has existed, the length of time that Earth has been a planet, the length of time that life has been on Earth. We are better at understanding things that we can have some experience of, but it is impossible to experience deep time. Most of us can relate to a period of one hundred years; a person in his fifties might reflect that one hundred years ago, his grandmother was a young woman. A person in her twenties might be able to imagine what life was like for a great-grandparent one hundred years ago. Thinking back to the time of Jesus, two thousand years ago, is more difficult; although we have written descriptions of people's houses, clothes, and how they made their living in those times, there is much we do not know of official as well as everyday life. The ancient Egyptians were building pyramids five thousand years ago, and their way of life is known in only the sketchiest outlines.

And yet the biological world of five thousand years ago was virtually identical to ours today. The geological world five thousand years ago would be quite recognizable: the continents would be in the same places, the Appalachian and Rocky mountains would look pretty much as they do today, and major features of coastlines would be identifiable. Except for some minor remodeling of Earth's surface due to volcanoes and earthquakes, the filling in of some deltas due to the deposition of sediments by rivers, and some other small changes, little has changed geologically. But our planet and life on it are far, far older than five thousand years. We need to measure the age of Earth and the time spans important to the history of life in billions of years, a number that we can grasp only in the abstract.

one second is a short period of time. Sixty seconds make up a minute, and sixty minutes make up an hour. There are therefore 3,600 seconds in an hour, 86,400 in a day, 604,800 in a week, and 31,536,000 in a year. But to count to 1 billion seconds at the rate of one per second, you would have to count night and day for approximately thirty-one years and eight months. The age of Earth is 4.5 billion years, not seconds. That is an enormous amount of time. As Stephen Jay Gould remarked, "An abstract, intellectual understanding of deep time comes easily enough—I know how many zeros to place after the 10 when I mean billions. Getting it into the gut is quite another matter. Deep time is so alien that we can really only comprehend it as metaphor" (Gould 1987: 3).

Figure 2.1 presents divisions of geological time used to understand geological and biological evolution. The solar system formed approximately 4.6 billion years ago; Earth formed about 4.5 billion years ago. The emergence of life was probably impeded by the bombardment of the Earth and moon by comets and meteorites until about 3.8 billion years ago, because only after the bombardment stopped do we find the first evidence of life. As discussed in the section "Astronomical and Chemical Evolution," there was a period of hundreds of millions of years of chemical evolution before the first structures that we might consider alive appeared on Earth: primitive one-celled organisms, less complex than any known bacterium.

After these first living things appeared between 3.5 billion and 4 billion years ago, life continued to remain outwardly simple for more than 2 billion years. Single-celled living things bumped around in water, absorbed energy, and divided—if some other organism didn't absorb them first. Reproduction was asexual: when a cell divided, the result was almost always two identical cells. very slow changes occur with asexual reproduction, and this is probably an important reason that the evolution of life moved so slowly during life's first few billion years. Yet some very important evolutionary changes were taking place on the inside of these simple cells: the earliest living things gave rise to organisms that developed a variety of basic metabolic systems and various forms of photosynthesis.

Nucleated Cells. The first cells on Earth got along fine without a nucleus or a membrane around their DNA; in fact, bacteria today generate energy, carry out other cell functions, and reproduce new daughter bacteria without having nucleated DNA. Nucleated (eukaryotic) cells didn't evolve until about 1.5 billion years ago. Around 2 billion years ago, great changes in Earth's surface were taking place: continents were moving, and the amount of oxygen had increased in the atmosphere. Where did this oxygen come from? oxygen is a by-product of photosynthesis, and indeed, oxygen produced by photosynthesizing bacteria built up in the atmosphere over hundreds of millions of years. This would explain the appearance of large red-colored geological deposits dating from this time: dissolved iron oxidized in the presence of free oxygen. In the words of researcher William Schopf, "The Earth's oceans had been swept free of dissolved iron; lowly cyanobacteria—pond scum—had rusted the world!" (Schopf 1992: 48). The increase of oxygen in the atmosphere resulted in a severe change in the environment: many organisms could not live in the new "poisonous" oxygenated environment. others managed to survive and adapt.

The surface of Earth had been inhospitable for life: deadly radiation would have prevented life as we know it from existing at the planet's surface. The increase in oxygen as a result of photosynthesis resulted in the establishment of an ozone layer in the stratosphere. oxygen is o2; when ultraviolet radiation in the stratosphere strikes oxygen, ozone, which is O3, is formed. The ozone shield protects living things from ultraviolet radiation, which permitted the evolution of life at the surface of the planet and eventually of the evolution of organisms composed of more than one cell (multicellular organisms, or metazoans).

Figure 2.1

Timescale of Earth's history. Courtesy of Alan Gishlick.














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Eukaryotic cells may have evolved from unnucleated cells that were able to enclose their DNA in an interior membrane (forming the nucleus), and that incorporated other cells within their cell membranes. Nucleated cells have structures called organelles within their cytoplasm that perform a variety of functions having to do with energy capture and use, cell division, predation, and other activities. Some of these structures, such as mitochondria and chloroplasts, have their own DNA. Similarities between the DNA of such organelles and that of some simple bacteria have supported the theory that, early in evolution, the ancestors of eukaryotes absorbed certain bacteria and formed a cooperative, or symbiotic, relationship with them, whereby the newcomers functioned to enhance performance of metabolism, cell division, or some other task (Margulis 1993). The nucleus itself may have been acquired in a similar fashion, from "recycled" parts obtained after the absorption of other bacteria. Evidence for these theories comes, of course, not from the fossil record but from inferences based on biochemical comparisons of living forms.

Once nucleated cells developed, sexual reproduction was not far behind. Sexual reproduction has the advantage of combining genetic information from more than one individual, thus providing more variation to the population. Having more variation allows both the individual organism and the population of organisms to adjust to environmental change or challenge. Some researchers theorize that geological and atmospheric changes, together with the evolution of sexual reproduction, stimulated a burst of evolutionary activity during the late Precambrian period, about 900 million years ago, when the first metazoans (organisms composed of many cells) appear in the fossil record.

The Precambrian and the Cambrian Explosion. The first evidence we have of mul-ticelled organisms comes from the Precambrian period, about 900 million years ago, and consists of fossils of sponges and jellyfish. Sponges are hardly more than agglomerations of individual cells; jellyfish are composed of two layers of cells that form tissues. Jellyfish, then, have a more consistent shape from organism to organism than do sponges, yet they lack a head and digestive, respiratory, circulatory, or other organs. Early echinoderms, represented today by starfish, sea urchins, and sea cucumbers, also occur in the Precambrian. Like the other Precambrian groups, early echinoderms have a rather simple body plan, but they do have a mouth and an anus, three tissue layers, and organs for digestion.

In the Cambrian, about 500 million years ago, there was rapid divergent evolution of invertebrate groups. New body plans appeared: "inventions" like body segmentation and segmented appendages characterized new forms of animal life, some of which died out but many of which continue to the present day. These new body plans appear over a geologically sudden—if not biologically sudden—period of about 10 million to 20 million years. Crustaceans, brachiopods, mollusks, and annelid worms, as well as representatives of other groups, appear during the Cambrian.

Evolutionary biologists are studying how these groups are related to one another and investigating whether they indeed have roots in the Precambrian period. In evolutionary biology, as in the other sciences, theory building depends on crosschecking ideas against different types of data. There are three basic types of data used to investigate the evolutionary relationships among the invertebrate groups: size and shape (morphological) comparisons among modern representatives of these groups, biochemical comparisons among the groups' modern descendants, and the fossil record. Largely because of problems in the preservation of key fossils at key times—and the fact that the evolution of these basic body plans might have taken "only" tens of millions of years, an eye blink from the perspective of deep time—the fossil evidence currently does not illuminate links among most of the basic invertebrate groups. Nonetheless, much nonfossil research is being conducted to understand similarities and differences of living members of these groups, from which we may infer evolutionary relationships.

one particularly active area of research has to do with understanding the evolution and developmental biology (embryology) of organisms, a new field referred to as "evo-devo."

Evo-Devo. Advances in molecular biology have permitted developmental biologists to study the genetics behind the early stages of embryological development in many groups of animals. What they are discovering is astounding. It is apparent that very small changes in genes affecting early, basic structural development can cause major changes in body plans. For example, there is a group of genes operating very early in animal development that is responsible for determining the basic front-to-back, top-to-bottom, and side-to-side orientations of the body. other early-acting genes control such bodily components as segments and their number, and the production of structures such as legs, antennae, and wings. Major changes in body plan can come about through rather small changes in these early acting genes. What is perhaps the most intriguing result of this research is the discovery of identical or virtually identical early genes in groups as different as insects, worms, and vertebrates. Could some of the body plan differences of invertebrate groups be the result of changes in genes that act early in embryological development?

Probable evolutionary relationships among the invertebrate groups are being established through anatomy, molecular biology, and genetics, even if they have not been established through the fossil record. one tantalizing connection is between chordates, the group to which vertebrates belong (see the subsequent section), and echinoderms, the group to which starfish and sea cucumbers belong. on the basis of embryology, RNA, and morphology, it appears that the group to which humans and other vertebrates belong shared a common ancestor with these primitive invertebrates hundreds of millions of years ago. Although adult echinoderms don't look anything like chordates, their larval forms are intriguingly similar to primitive chordates. There are also biochemical similarities in the way they use phosphates—but read on to find out more about chordates!

Vertebrate Evolution. our species belongs to the vertebrates, creatures with a bony structure encircling the nerve cord that runs along the back. Vertebrates are included in a larger set of organisms called chordates. Although all vertebrates are chordates, not all chordates are vertebrates. The most primitive chordates look like stiff worms. Characteristically, chordates have a notochord, or rod, running along the back of the organism with a nerve cord running above it. At some time in a chordate's life, it also has slits in the neck region (which become gills in many forms) and a tail. An example of a living chordate is a marine filter-feeding creature an inch or so long called

Figure 2.2

Amphioxus shows the basic body plan of chordates in having a mouth, an anus, a tail, a notochord, and a dorsal nerve chord. Courtesy of Janet Dreyer.

dorsal nerve chord

Figure 2.2

Amphioxus shows the basic body plan of chordates in having a mouth, an anus, a tail, a notochord, and a dorsal nerve chord. Courtesy of Janet Dreyer.

dorsal nerve chord

amphioxus. To look at it, you wouldn't think it was very closely related to vertebrates, but it is. Amphioxus lacks vertebrae, but like vertebrates, it has a notochord, a dorsal nerve cord, a mouth, an anus, and a tail. Like vertebrates, it is the same on the right side of the body as it is on the left (i.e., it is bilaterally symmetrical), and it has some other similarities in the circulatory system and muscle system that are structurally similar to vertebrates. It is probably fairly similar to an early chordate, but because it has been around the planet for a long time, it has evolved as well. Still, it preserves the diagnostic features of chordates in a relatively simple form (Figure 2.2).

Amphioxus is iconic in biological circles. There aren't very many evolution songs (there are far more antievolution songs!), but one that many biologists learn is sung to the tune of "It's a Long Way to Tipperary":

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