The basic timeline of Earth is a 4.6 billion year old Earth, with (very approximately):
about 3.5 - 3.8 billion years of simple cells (prokaryotes)
3 billion years of photosynthesis
2 billion years of complex cells (eukaryotes)
1 billion years of multicellular life
600 million years of simple animals
570 million years of arthropods (ancestors of insects, arachnids and crustaceans)
550 million years of complex animals
500 million years of fish and proto-amphibians
475 million years of land plants
400 million years of insects and seeds
360 million years of amphibians
300 million years of reptiles
200 million years of mammals
150 million years of birds
130 million years of flowers
65 million years since the non-avian dinosaurs died out
2.5 million years since the appearance of Homo
200,000 years since humans started looking like they do today
25,000 years since Neanderthals died out
Mass Extinctions
Extinctions are part of natural selection. Species often go extinct when their environment changes and they do not have the traits they need to survive. Only those individuals with the traits needed to live in a changed environment survive (Figure below).
Figure 7.35
Humans have caused many extinctions by introducing species to new places. For example, many of New Zealands birds have adapted to nesting on the ground. This was possible because there were no land mammals in New Zealand until Europeans arrived and brought cats, fox and other predators with them. Several of New Zealands ground nesting birds, such as this flightless kiwi, are now extinct or threatened because of these predators.
Mass extinctions, such as the extinction of dinosaurs and many marine mammals, happened after major catastrophes such as volcanic eruptions and major earthquakes changed the environment. Scientists have been looking for evidence of why dinosaurs went extinct over fairly short periods. Many scientists are examining the theory that a major cataclysmic events, such as an asteroid colliding with Earth, may have caused the extinction of dinosaurs 65 million years ago (Figure below).
Figure 7.36
The fossil of Tarbosaurus, one of the land dinosaurs that went extinct during one of the mass extinctions.
Since life began on Earth, there have been several major mass extinctions. If you look closely at the geological time scale, you will find that at least five major massive extinctions have occurred in the past 540 million years. In each mass extinction, over 50% of animal species died. The total number of extinctions could be as high as 20 mass extinctions during this period.
The fossil record tells the story of these mass extinctions: millions of species of fish, amphibians, reptiles, birds, mammals, mosses, ferns, conifers, flowering plants, and fungi populated the seas and covered the Earth - as continents crashed together and broke apart, glaciers advanced and retreated, and meteors struck, causing massive extinctions. Two specific extinctions occurred at the end of the Permian period and when the dinosaurs went extinct.
At the end of the Permian, an estimated 99.5% of individual organisms perished. Several factors may have contributed, and one factor relates again to the supercontinent Pangaea. Marine biodiversity is greatest in shallow coastal areas. A single continent has a much smaller shoreline than multiple continents of the same size. Perhaps this smaller shoreline contributed to the dramatic loss of species, for up to 95% of marine species perished, compared to “only” 70% of land species. Although the exact cause remains unknown, fossils clearly document the fact of Earth’s most devastating extinction.
Figure 7.37
The supercontinent Pangaea encompassed all of todays continents in a single land mass. This configuration limited shallow coastal areas which harbor marine species, and may have contributed to the dramatic event which ended the Permian - the most massive extinction ever recorded.
The dramatic extinction of all dinosaurs (except those which led to birds) marked the end of the Cretaceous period. A worldwide iridium-rich layer, dated at 65.5 million years ago, provides evidence for a dramatic cause for their ultimate extinction. Iridium is rare in the Earth’s crust, but common in comets and asteroids. Scientists associate this layer with a huge crater in the Yucatan and Gulf of Mexico. A collision/explosion between the Earth and a comet or asteroid could have spread debris which set off tsunamis, altered the climate (including acid rain), and reduced sunlight 10-20%. A consequent reduction in photosynthesis would have caused a drastic decrease in food chains, leading to the extinction of the dinosaurs. The fossil record obviously depicts the presence of dinosaurs on Earth, and the absence of dinosaur fossils after this extinction event demonstrates the relationship between the fossil record and evolution.
Figure 7.38
The fossil record demonstrates the presence of dinosaurs, which went extinct over 65 million years ago.
After each mass extinction, open ecological niches are quickly filled by other species. This is well documented in the fossil record. This episodic speciation following an event such as a mass extinction also shows the relationship between evolution and the fossil record.
Figure 7.39
Mammals and birds quickly invaded ecological niches formerly occupied by the dinosaurs. Mammals included monotremes (A), marsupials, and hoofed placentals (B). Modern sharks (C) patrolled the seas. Birds included the giant flightless (D).
Lesson Summary
During the 1800s, geologists, paleontologists and naturalists found several forms of physical evidence that confirmed that the earth is very old.
Fossils of ancient sea life on dry land far from oceans supported the idea that the earth changed over time and that some dry land today was once covered by oceans.
The many layers of rock represent the order in which rocks and fossils appeared.
Indications that volcanic eruptions, earthquakes and erosion that happened long ago shaped much of the earth’s surface.
Radiometric dating allows scientists to measure the age of rocks by measuring the radioactivity of certain minerals in rocks.
The oldest rock minerals found on Earth so far are zircon crystals that are at least 4.404 billion years old.
Some of the oldest fossils of life forms on Earth are at least 3.5 billion year old fossils of blue green algae found in Australia.
Scientists believe the early earth contained no oxygen gas, but did contain other gases, including nitrogen, carbon dioxide, carbon monoxide, water vapor, hydrogen sulfide and probably a few others.
Geologists and other earth scientists use geologic time scales to describe when events occurred throughout the history of Earth.
The geological time scale of Earth's past is organized according to events which took place during different periods on the time scale.
Life on Earth began about 3.5 to 4 billion years ago.
The first life forms were single cell organisms, prokaryotic organisms, similar to bacteria.
The first multicellular organisms did not appear until about 610 million years ago in the oceans. Some of the first multicellular forms included sponges, brown algae, and slime molds.
Plants and fungi appeared roughly 500 million years ago. They were soon followed by arthropods (insects and spiders).
Amphibians evolved about 300 million years ago, followed by mammals around 200 million years ago and birds around 100 million years ago.
Extinction of species is common; in fact, it is estimated that 99% of the species that have lived on the earth no longer exist.
Mass extinctions, such as the extinction of dinosaurs and many marine mammals, happened after major catastrophes such as volcanic eruptions and major earthquakes changed the environment.
There have been at least five major massive extinctions have occurred in the past 540 million years.
In
each mass extinction, over 50% of animal species died.
Review Questions
How do scientists determine the age of a rock or fossil today?
How do we know the maximum possible age of the Earth?
How do we know the minimum possible age of the Earth?
How old is the Earth, based on current evidence?
Why is it difficult to determine how life started on Earth?
How long ago did life start on Earth?
When did mammals first appear on Earth?
What kinds of events are recorded on a geological time scale?
Further Reading / Supplemental Links
Stein, Sara, The Evolution Book, Workman, N.Y., 1986.
Yeh, Jennifer, Modern Synthesis, (From Animal Sciences).
Darwin, Charles, Origin of the Species, Broadview Press (Sixth Edition), 1859.
Ridley, Matt, The Red Queen: Sex and the Evolution of Human Nature, Perennial Books, 2003.
Ridley, Matt, Genome, Harper Collins, 2000.
Sagan, Carl, Cosmos, Edicions Universitat Barcelona, 2006.
Carroll, Sean B., The Making of the Fittest: DNA and the Ultimate Forensic Record of Evolution, Norton, 2006.
Dawkins, Richard, The Blind Watchmaker, W.W. Norton & Company, 1996.
Dawkins, Richard, The Selfish Ge Oxford University Press, 1989.
Diamond, Jared, The Third Chimpanzee: The Evolution and Future of the Human Animal, HarperCollins, 2006.
Mayr, Ernst, What Evolution Is, Basic Books, 2001.
Zimmer, Carl, Smithsonian Intimate Guide to Human Origins, Smithsonian Press, 2008.
http://en.wikipedia.org/
Vocabulary
Cambrian explosion
A sudden burst of evolution that may have been triggered by some environmental changes that made the environment more suitable for a wider variety of life forms.
extinct
Something that does not exist anymore; a group of organisms that has died out without leaving any living representatives.
mass extinction
An extinction when many species go extinct during a relatively short period of time.
radiometric dating
A method to determine the age of rocks and fossils in each layer of rock; measures the decay rate of radioactive materials in each rock layer.
stromolites
Fossils made of algae and a kind of bacteria; some of the oldest fossils on Earth.
Points to Consider
The next chapter focuses on prokaryotic organisms. Remember, prokaryotes lived on this planet for two billion years before eukaryotic cells even existed.
Discuss with your class what you think are some of the characteristics, and some of the differences, of prokaryotic organisms.
Chapter 8: Prokaryotes
Lesson 8.1: Bacteria
Lesson Objectives
Describe the cellular features of bacteria.
Explain the ways in which bacteria can obtain energy.
Explain how bacteria reproduce themselves.
Identify some ways in which bacteria can be helpful.
Identify some ways in which bacteria can be harmful.
Check Your Understanding
How do prokaryotic and eukaryotic cells differ?
Answer: Eukaryotic cells have a membrane-bound nucleus while prokaryotes do not.
What are some components of all cells, including bacteria?
Answer: cell membrane, cytoplasm, etc.
Introduction
About 3.5 billion years ago, long before the first plants, people, or other animals appeared, prokaryotes were the first life forms on Earth. Recall that prokaryotes are single-celled organisms that lack a nucleus, and that the prokaryotes include bacteria and archaea. For at least a billion years, Bacteria and Archaea ruled the Earth as the only existing organisms. Even though life is much more diverse on Earth today, bacteria (singular: bacterium) are still the most abundant organisms on Earth. You probably know bacteria as “germs” that cause disease, but as you will see, they can also do many helpful things for the environment and humankind.
Characteristics of Bacteria
Bacteria are so small that they can only be visualized with a microscope. When viewed under the microscope, they have three distinct shapes. These three shapes allow bacteria to be classified by their shape. The bacilli are rod-shaped, the cocci are sphere-shaped, and the spirilli are spiral-shaped (Figures below, below and below).
Figure 8.1
Bacteria come in many different shapes. Some of the most common shapes are bacilli (rods), cocci (spheres), and spirilli (spirals). Bacteria can be identified and classified by their shape.
Figure 8.2
is an example of bacteria that are rod-shaped, or bacilli.
Figure 8.3
is an example of bacteria that are sphere-shaped, or cocci.
Bacteria are surrounded by a cell wall consisting of peptidoglycan, a complex molecule consisting of sugars and amino acids. The cell wall is important for protecting the bacteria. In fact the cell wall is so important that some antibiotics, such as penicillin, work to kill bacteria by preventing the proper synthesis of the cell wall. In parasitic bacteria, which depend on a host organism for energy and nutrients, capsules or slime layers surround the cell wall help defend against the host’s defenses.
Recall that all prokaryotes, including the bacteria, lack the membrane-bound organelles and nucleus of eukaryotic cells (Figure below). Like eukaryotic cells, however, prokaryotic cells do have cytoplasm, the fluid inside the cell; a plasma membrane, which acts as another barrier; and ribosomes, where proteins are assembled. The DNA of bacteria is mostly contained in a large circular strand, forming a single chromosome, that is compacted into a structure called the nucleoid. Many bacteria also have additional small rings of DNA known as plasmids.
Figure 8.4
The structure of a bacterial cell is distinctive from the eukaryotic cell because of features such as an outer cell wall and the circular DNA of the nucleoid, and the lack of membrane-bound organelles.
Some bacteria also have tail-like structures called flagella (Figure below). The flagella assist the bacteria with movement. As the flagella rotate, they spin the bacteria and propel them forward.
Figure 8.5
The flagella facilitate movement in bacteria. Bacteria may have one, two, or many flagella - or none at all.
Obtaining Food and Energy
Bacteria obtain energy and nutrients from a variety of different methods. Bacteria known as decomposers break down wastes and dead organisms into smaller molecules to obtain nutrients and energy. Photosynthetic bacteria use the energy of the sun, together with carbon dioxide, to make their own food (discussed in the Cell Functions chapter). Briefly, in the presence of sunlight, carbon dioxide and water is converted into glucose and oxygen. The glucose is then converted into usable energy. Glucose is, in essence, the "food" of the bacteria. An example of photosynthetic bacteria is cyanobacteria, as seen in Figure below.
Figure 8.6
Cyanobacteria are photosynthetic bacteria. These bacteria carry out all the reactions of photosynthesis within the cell membrane and in the cytoplasm; they do not need chloroplasts.
Bacteria can also be chemotrophs. Chemotrophs obtain energy by breaking down chemical compounds in their environment, such as nitrogen-containing ammonia. They do not use the energy from the sun. This process is important, for example, for the cycling of nitrogen through the environment. As nitrogen can not be made by living organisms, it must be continually recycled. Organisms need nitrogen to make organic compounds, such as DNA.
Some bacteria depend on other organisms for survival. For example, mutualistic bacteria live in the root nodules of legumes, such as pea plants, and make nitrogen available to the plants; in this relationship both the bacteria and the plant benefit. Other bacteria are parasitic and can cause illness. In a bacterial parasitic relationship, the bacteria benefit and the other organism is harmed. Harmful
bacteria will be discussed later in the lesson.
Reproduction in Bacteria
Bacteria reproduce asexually through binary fission. During binary fission the chromosome copies itself (replicates), forming two genetically identical copies, then the cell enlarges and divides into two new daughter cells. The two daughter cells are identical to the parent cell (Figure below).
Binary fission can happen very rapidly. Some species of bacteria have been shown to double their populations in less than ten minutes! (Figure below)
Figure 8.7
Bacteria cells reproduce by binary fission, resulting in two daughter cells identical to the parent cell.
Figure 8.8
Bacteria can divide rapidly. This image is of a growing colony of bacteria. In the right environment the growth and division of two can form a colony of hundreds of bacteria in just a few hours.
Sexual reproduction does not occur in bacteria, but genetic recombination, the combining and exchange of DNA, does happen in bacteria through three different methods: conjugation, transformation, and transduction. In conjugation, DNA passes through the sex pilus, a hairlike extension on the surface of many bacteria, that temporarily joins two bacteria. In transformation, bacteria pick up pieces of DNA from their environment. In transduction, bacteriophages, viruses that infect bacteria, carry DNA from one bacteria to another.
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