CONCEPTS IN BIOLOGY
PART V. THE ORIGIN AND CLASSIFICATION OF LIFE
19. The Origin of Life and the Evolution of Cells
19.6. The Geologic Timeline and the Evolution of Life
A geologic timeline shows a chronological history of living organisms based on the fossil record. The largest geologic time units are called eons. From earliest to most recent, the geologic eons are the Hadean, Archaean, Proterozoic, and Phanerozoic. The Phanerozoic eon is divided into the eras Paleozoic, Mesozoic, and Cenozoic. Each of these eras is subdivided into smaller time units called periods. For example, Jurassic is the period of the Mesozoic era that began 208 million years ago (figure 19.12).
FIGURE 19.12. A Geologic Time Chart
This chart shows the various geologic time designations, their time periods, and the major events and characteristics of each time period.
Note on the geological time chart in figure 19.12 that its divisions are associated with the major events in the evolutionary history of living things. For example, the Ordovician period is characterized by the appearance of the first primitive land plants and a major extinction of marine animals. The Carboniferous period is characterized by large swamps of primitive plants and cone-bearing plants, and reptiles arose by the end of the period. The Tertiary period is the time when most forms of modern terrestrial organisms arose. In each instance, the dominance of a particular plant or animal group resulted from adaptive radiation. It is also important to note that the ends of many geologic time designations are associated with major extinctions. These extinctions appear to be related to major changes in climate or sea level or asteroid impacts. Following each major extinction, a new group of organisms became dominant.
An Aquatic Beginning
Recent evidence suggests that the first living thing most likely came into existence in the ocean approximately 3.8 billion years ago during the Archaean eon. Prokaryotic cell types (domain Bacteria) appear about 3.5 billion years ago in the fossil record. One of the common fossils is of stromatolites, photosynthetic Bacteria that grew in layers and formed columns in shallow oceans. Modern-day stromatolites still exist in western Australia (figure 19.13). Also about this time, the Archaea diverged from the Bacteria. For approximately 2 billion years, the only organisms on Earth were Bacteria and Archaea that lived in the ocean. For most of its existence Earth was dominated by prokaryotic organisms. The photosynthetic cyanobacteria are thought to have been responsible for the production of the molecular oxygen (O2) that began to accumulate in the atmosphere about 2.4 billion years ago. The presence of oxygen made conditions favorable for the evolution of other types of cells. The first members of the Domain Eucarya, the eukaryotic organisms, appeared approximately 1.8 billion years ago.
FIGURE 19.13. Stromatolites in Australia
This photo of stromatolites was taken at Hamelin Pool, Western Australia, a marine nature preserve. The dome-shaped structures are composed of cyanobacteria and materials they secrete, they grow to 60 centimeters tall. Similar structures are known from the fossil record. Taking samples from fossil stromatolites and cutting them into thin slices produces microscopic images that show some of the world’s oldest cells.
There is fossil evidence of multicellular algae at about 1 billion years ago and multicellular animals at about 0.6 billion years ago. During the Cambrian period of the Paleozoic era, an evolutionary explosion of multicellular animals occurred. Examples of most of the present-day kinds of marine invertebrate animals (e.g., echinoderms, arthropods, mollusks) are found in the fossil record at this time.
Several other “evolutionary explosions,” or adaptive radiations, followed.
Adaptation to a Terrestrial Existence
For about 90% of Earth’s history, life was confined to the sea. Primitive land plants probably arose about 430 million years ago and the first land animals (ancestors to present-day centipedes and millipedes) at about 420 million years ago. In order to live on land, organisms needed several characteristics:
1. an ability to exchange gases (particularly oxygen and carbon dioxide) with the air,
2. a way to prevent dehydration,
3. some sort of skeleton for support, and
4. an ability to reproduce out of water.
Modern plants exchange gases through openings in their leaves. Plants with vascular tissue were able to obtain water from the soil and regulate its loss with a waxy coating on their exterior. The cellulose cell walls allowed for support. The development of pollen grains allowed sperm to be transferred to the egg through the air. More primitive plants like mosses and ferns have swimming sperm and need water for sexual reproduction.
The first major group of land animals to become abundant was the insects, followed by vertebrates. Both groups are very successful but solved the problems associated with life on land in different ways. The marine ancestors of insects already had two characteristics that were valuable. They had an external skeleton and they had legs that they could use to walk on the ocean bottom or on land. In this sense they were somewhat preadapted for life on land. Marine ancestors of insects would have had gills, and some terrestrial arthropods (spiders and land crabs) have modified gills that work on land. However, modern insects have a system of tubes that permeate their bodies and carry oxygen to each cell. A waxy coating on the exterior reduces water loss and internal fertilization and an egg that resists drying allow for reproduction on land.
The conquest of a terrestrial environment by vertebrates appears to have involved several steps. Among the vertebrates, the first land animals most likely evolved from a lobefinned fish of the Devonian period. They possessed two important adaptations: lungs and paired, lobed fins which had a skeletal structure. The lobe fins allowed the organisms to pull themselves onto land and travel to new water holes during times of drought. They were probably the ancestors of the first amphibians. Early amphibians would have found a variety of unexploited terrestrial niches, resulting in the rapid evolution of new amphibian species and their dominance during the Carboniferous period. Although early amphibians had a skeleton with legs and could breathe air, they probably lost water through their skin and returned to water to reproduce as modern amphibians do.
Reptiles are the first truly terrestrial vertebrate organisms. In addition to having lungs and a supportive skeleton like their amphibian ancestors, they also had a relatively impermeable skin that reduced water loss and two reproductive adaptations that allowed for reproduction on land. One change allowed males to deposit sperm directly within females. Because the sperm could directly enter females and remain in a moist interior, it was no longer necessary for the animals to return to the water to mate, as the amphibians still had to do. However, the developing young still required a moist environment for early growth. A second modification, the amniotic egg, solved this problem. An amniotic egg, such as a chicken egg, protected the developing young from injury and dehydration while allowing for the exchange of gases with the external environment. See chapter 23 for a discussion of the nature of an amniotic egg. With these adaptations, the reptiles were able to outcompete the amphibians in most terrestrial environments. The amphibians that did survive were the ancestors of present-day frogs, toads, and salamanders. With extensive adaptive radiation, the reptiles took to the land, sea, and air. A particularly successful group of reptiles was the dinosaurs, which were the dominant terrestrial vertebrates for more than 100 million years.
Both birds and mammals are descendants of reptiles. They have a relatively impermeable skin and internal fertilization but have diverged somewhat in the way they reproduce. All birds lay eggs and most mammals have a uterus in which the young develop prior to birth. A more complete discussion of terrestrial adaptations can be found in chapter 22 for plants and chapter 23 for animals.
19.6. CONCEPT REVIEW
19. Describe four problems organisms had to overcome to be successful on land.
20. For each of the following pairs of terms, select the one that is the earliest in geologic time.
d. flowering plant—cone-bearing plant
e. aerobic respiration—photosynthesis
Current theories on the origin of life speculate that either the primitive Earth’s environment led to the spontaneous organization of organic chemicals into primitive cells or primitive forms of life arrived on Earth from space.
The spontaneous origin of living things on Earth would require:
• the formation of organic molecules,
• a genetic system,
• a membrane that separated the organic molecules from their surroundings, and
• a method of obtaining energy.
There are two different theories about the way in which the first living things would have obtained energy. They were either anaerobic heterotrophs or chemosynthetic autotrophs. Regardless of how the first living things came to be on Earth, these basic units of life were probably similar to present-day prokaryotes. The primitive cells could have changed through time as a result of mutation. A changing environment would have selected for new combinations of characteristics.
The recognition that prokaryotic organisms can be divided into two distinct types has led to the development of the concept that there are three major domains of life: the Bacteria, the Archaea, and the Eucarya. The Bacteria and Archaea are similar in structure, but the Archaea have metabolic processes that are distinctly different from those of the Bacteria.
The origin of the Eucarya is more clear-cut. Similarities between cyanobacteria and chloroplasts and between aerobic bacteria and mitochondria suggest that they have a common origin. The endosymbiotic theory proposes that eukaryotic cells are the result of combining two or more ancient cell ancestors into one cellular unit and that both of the original separate cells benefit from the new combination.
The accumulation of geologic information has allowed several key events in the history of life to be placed in sequence (figure 19.14).
FIGURE 19.14. An Evolutionary TimeLine
This chart displays how science sees the order of major, probable events in the origin and evolution of life from the “Big Bang” to the present day.
1. The reproduction of an apple tree by seeds is an example of
a. spontaneous generation.
d. None of the above is correct.
2. The first organisms on Earth would have carried on aerobic respiration. (T/F)
3. Endosymbiosis involves one cell invading and living inside another cell. (T/F)
4. Which of the following suggests that organic molecules may have formed on Earth from inorganic molecules?
a. Experiments that simulated the early Earth’s atmosphere have produced organic molecules.
b. Organic molecules have been detected in interstellar gases in space.
c. Meteorites contain organic molecules.
d. All of the above are correct.
5. Oxygen in today’s Earth atmosphere is the result of the process of _____.
6. The oldest fossils of living things are about _____ years old.
a. 3.5 million
b. 3.5 billion
c. 4.5 billion
d. 4.5 million
7. The first genetic material was probably
8. An organism that requires organic molecules for food is known as a(n) _____.
9. A prokaryotic organism that is a chemoautotroph and lives in a very hot environment is probably in the domain
d. Any of these are correct.
10. Which one of the following evolved before all of the others?
a. terrestrial plants and animals
b. prokaryotic cells
c. eukaryotic cells
d. multicellular organisms
11. The first terrestrial organisms were
c. simple plants.
12. Chloroplasts are probably derived from
d. None of the above is correct.
13. Louis Pasteur performed an experiment that showed that _____ did not occur.
14. Stanley Miller performed an experiment that showed that
a. life originated on Earth.
b. life originated in outer space.
c. membranes form around cells.
d. organic molecules can be formed from inorganic molecules in a rather than a reducing atmosphere.
15. In order to be successful as a terrestrial organism, any organism must be able to retain water. (T/F)
1. b 2. F 3. T 4. d 5. photosynthesis 6. b 7. c 8. heterotroph 9. a 10. b 11. c 12. b 13. spontaneous generation 14. d 15. T
Thinking Like an Astrobiologist
Imagine that there is life on another planet in our galaxy (Planet X). Using the following data concerning the nature of this life, determine what additional information is necessary and how you would verify the additional data needed to develop a theory of the origin of life on Planet X.
1. The age of the planet is 5 billion years.
2. Water is present in the atmosphere.
3. The planet is farther from its Sun than our Earth is from our Sun.
4. The molecules of various gases in the atmosphere are constantly being removed.
5. Chemical reactions on this planet occur at approximately half the rate at which they occur on Earth.