Introduction and Learning Objectives
This tutorial will address evolution on a broad scale by looking at biological diversity in the context of a changing planet, and discusses how we determine the dates of events in Earth’s history. As you will learn, Earth's environment has changed dramatically in the 4.5 billion years since it was formed.
By the end of this tutorial you should have a basic understanding of:
- The age of the universe, the solar system, and Earth
- The differences between the environment of the early Earth and Earth today
- The chemical evolution of life
- The dynamic structure of Earth
- The geologic time scale
- The distinction between relative and absolute dating methods
- The basic principles behind radiometric dating and dendrochronology
- The three main branches of the tree of life
- Know the age of Earth and explain how this has been determined
- Describe the conditions on early Earth and their effect of on the evolution of life
- Understand the difference between relative and absolute dating methods
- Be able to determine the half life of a fossil based upon radiometric dating info
- Name the three domains of life and describe the primary difference between prokaryotes and eukaryotes
How Old is the Earth?
The available astronomical data indicate that the universe is about 13.7 billion years old. This estimate is based, in part, on the observation that the universe is constantly expanding. Astronomers have plotted the trajectories (directional movement) of various stars and galaxies and determined that all matter in the known universe arose from a common point. In an event often referred to as the "Big Bang," the universe arose in a relatively brief moment in time and matter was flung outward from this central origin. Since this beginning, this vast amount of matter has been hurling through space, undergoing a number of many changes as it traverses the cosmos.
By using radiometric dating techniques (discussed herein) of pristine meteorites, astronomers have determined that our own solar system formed 4.55 - 4.56 billion years ago.
The Hubble telescope was dispatched in 1990 to explore the far reaches of our galaxy and to photograph events (e.g., star formation). Astronomers use images captured by the Hubble telescope to further our understanding of the origin of the universe and solar systems.
The Conditions on Early Earth
The atmosphere on early Earth was strikingly different from that of today. Primitive Earth's atmosphere consisted of methane, ammonia, hydrogen, water vapor, and a negligible amount of free oxygen. This is an important point because if there was a lot of free oxygen available, life probably could not have arisen from inorganic compounds. This is because oxygen tends to oxidize substances, which means that electrons are removed. Importantly, the early atmosphere was highly reducing (capable of gaining electrons and forming more complex molecules). Oxidation and reduction will be examined in more detail in Tutorial 23.
Support for the Chemical Evolution of Life
Charles Darwin wrote of "some warm little pond" in which life could have begun. While this idea was initially scorned, in reality, he might not have been too far from the truth. About forty years later the Scotsman, John Haldane, and the Russian, Alexander Oparin, independently developed a model that has been called the primordial soup model for the origin of life. Basically this model suggests that complex, metabolically active molecules could arise from a slurry of simple chemicals occurring in the right proportions. This idea was tested in 1953 by Stanley Miller and Harold Urey in their classic but quite simple experiment.
In the Miller-Urey experiment, a mixture of gases (methane, ammonia, hydrogen, and water vapor) representing the primitive atmosphere was subjected to an electrical discharge. Figure 1 shows their experimental set up. The compounds formed were then condensed and sampled. The solution was found to contain a variety of organic compounds, including aldehydes, carboxylic acid, and amino acids. Thereby, Miller and Urey affirmed that organic compounds could arise in abiotic conditions similar to those that existed on early Earth.
Figure 1. Abiotic synthesis of organic molecules in a model system. (Click image to enlarge)
The Miller-Urey experiment has often been repeated, and similar set-ups have generated various organic compounds (including ATP when phosphate is added to the initial slurry). However, there are still various aspects about abiotic synthesis of organic compounds that stimulate much debate. For instance, many scientists feel that the essential ingredients for these early chemical reactions could have come from deep-sea vents in Earth's oceans or from submerged volcanoes, rather than from the early atmosphere. Also, the first cells might have been autotrophs, using inorganic sulfur and iron compounds to gain energy, rather than consuming organic molecules (heterotrophs). Ultimately, these laboratory simulations give us insight into how the chemicals necessary for life could have formed. We will discuss the chemistry of life further in Tutorial 3.
Our own limited lifespan could lead us to believe that Earth is a stable structure, however, geological data indicate that Earth has changed radically since its origin 4.5 billion years ago. In fact, Earth's surface is dynamic.
Earth's crust consists of large plates that move with respect to each other (Figure 2). This movement, termed plate tectonics, is due to activity at spreading centers and subduction zones. As a result of this crust movement, the continents are constantly rearranging themselves in a phenomenon known as continental drift. When two plates intersect, any number of geological events (e.g., mountain building, volcanic eruptions, and earthquakes) can result. Continental drift can be measured by a variety of techniques, and geologists have discovered that different land masses drift at different rates. The slowest rates of movement are about 2.5 cm/year, whereas the fastest are around 15 cm/year. Given the vast amount of water on the planet, this redistribution of land also influences ocean partitioning and water levels. (changes in climate also influence water distributions.) In the last 4 billion years, many changes in landmasses have occurred on the planet.
About 250 million years ago, all of the landmasses were joined as a supercontinent that geologists named Pangaea (Figure 3). About 180 million years ago, Pangaea began to break apart into the Northern Laurasia and Southern Gondwana landmasses, which eventually separated to create the landmasses that exist today. The history of a supercontinent explains many of the similarities observed in modern organisms on separate landmasses.
Go to the following Web site to learn about some of the evidence for Pangaea's existence. Be sure to click on the “rejoined continents” figure. USGS Historical Perspective on Pangaea
Figure 2. Continental Drift and Plate Tectonics. (Click image to enlarge)
Figure 3. Continental Drift and Pangaea's History. (Click image to enlarge)
The Fossil Record
The best evidence about the origin of life and evolution of living organisms comes from the fossil record. Fossils can be preserved remnants of organisms (e.g., bones and teeth) or whole organisms embedded in amber, acid bogs and tar pits, or anywhere bacteria can't decompose a dead body. Fossils can also be found in other forms (e.g., an animal's footprint). A scientist who studies fossils is a paleontologist.
Fossils are divided into age groups according to a geological time scale, a classification of different periods in Earth’s history. For example, layers of rock bearing evidence of the origin of most modern animal phyla would be classified as belonging to the Cambrian period in the Paleozoic era. We will discuss the Cambrian explosion in Tutorial 18. Layers of rock bearing fossils suggest a rapid diversification of reptiles took place during the Permian period in the Paleozoic era.
While you will not need to know the different periods and epochs for this course, you should be familiar with the four great eras of the geological time scale: Precambrian (oldest), Paleozoic (Figure 5), Mesozoic, and Cenozoic (Figure 6); and you should know that we are currently in the Quaternary period of the Cenozoic era.
Early Geological Eras
Recent Geological Eras
Probably the best source of fossils is sedimentary rock, which is formed by layers of minerals that settle out of water. Over time these minerals build up and pack the layers lying beneath them, along with any organisms that have settled with the sediments. Figure 6 summarizes the major events associated with the formation of sedimentary rock. Note that each stratum, or layer, represents a particular period in Earth's history and is characterized by a collection of fossils of organisms that lived at that time. The location of fossils in sedimentary rock is indicative of their relative age. This means that one fossil can generally be classified as older than another based on their relative position in the sedimentary rock. Deeper layers were formed earlier and their fossils are older than those found in more shallow layers of rock. This method of estimating the relative age of a fossil is known as relative dating.
Figure 6. Sedimentary Rock and Fossil Deposition. (Click to enlarge)
A variety of methods are used to estimate a fossil's age in years, rather than simply in relative terms. These absolute dating methods include radiometric dating and dendrochronology. These methods will be explored below.
The identity of a particular atom is determined by the number of protons it has in its nucleus. However, a collection of atoms of a particular element can have varying numbers of neutrons. For example, hydrogen atoms can have no neutrons (the most common form, sometimes called protium), one neutron (deuterium) or two neutrons (tritium). These are hydrogen's three isotopes. Moreover, different isotopes have different (but predictable) stabilities. Many elements have radioactive isotopes; their nuclei decay at predictable rates (unique for each isotope) and give off energy and atomic particles. When the decay leads to a change in the number of protons, it transforms the atom to an atom of a different element. For example, potassium-40 decays from a radioactive parent atom to a stable daughter atom of argon-40. Scientists have identified several elements with radioactive isotopes that decay at known rates; they can be used to date various rocks, including those containing fossils. This procedure is known as radiometric dating, and as mentioned earlier, this absolute dating method has been used to date material as old as meteorites. Let's take a closer look at how this method works.
Figure 7 illustrates how, over time, radioactive parent atoms decay and the number of stable daughter atoms, relative to the number of parent atoms, increases exponentially. Note how the two lines intersect at the first half-life point. In other words, a radioisotope's half-life is the amount of time it takes for one-half of the parental atom population to decay into daughter atoms.
It takes 1.25 billion years for half of the potassium-40 in a sample to decay to argon-40; in other words, the half-life of potassium-40 is 1.25 billion years.
Because living things contain so much carbon, radiometric dating using carbon-14 (C14), also called radiocarbon dating or carbon-14 dating, is quite common. Go to this Web site (It's Elemental!) and return knowing the answer to the following question: What does carbon-14 decay into?
Figure 7. Radiometric Dating. (Click image to enlarge)
How Do We Know That C-14 Dating is Accurate?
So how does subatomic physics help establish an absolute dating method for fossils? Easy, well in theory at least; the technical details of how this is done can be a bit complex. As long as an organism is alive, it incorporates C14 and C12 into its body. At death, however, assimilation ceases. The carbon that is immobilized into the hard tissues is quite insoluble and resists decomposition. Slowly, but at a predictable rate, the C14 disappears, decaying to its daughter isotope. The rate at which carbon decays (its half-life) is approximately 5730 years. After about 50,000 years, the amount of C14 remaining is so small that material can't be dated reliably.
How do we know this radiometric “clock” is accurate? In order to have confidence in a scientific fact, corroboration is required. In other words, without an independent means of verifying a fact, its scientific value is questionable. The more a fact is checked using different methods, the more reliable the scientific community judges that fact. Many experiments have verified the usefulness and accuracy of carbon-14 dating. For example, archaeologists know from the historical record the age of various Egyptian tombs. Wood taken from these tombs has been dated using the carbon-14 dating method, and the two dates agree. In addition, the argon/potassium-40 dating method has been used to date the time of the Mount Vesuvius eruption in Italy, and this date coincides with the date reported by Roman historians. Many other corroborative dates have validated the use of radiometric methods.
Dendrochronology is also an absolute dating technique. This science relies on the fact that trees have annual growth rings that reflect the age of the tree – one ring is produced each year. The width of the ring may vary, depending on variations in water and sunlight; a tree ring from a dry year will be more narrow than a tree ring from a year with abundant rainfall. Therefore, the pattern of the rings can also often reveal something about past climate conditions. Remarkably, global fluctuations of weather patterns appear to be the best indicators of annual growth ring changes; growth rings from wood obtained from Egyptian tombs has been matched to tree growth ring patterns found in ancient trees located in California. Figure 8 depicts the basic strategy used to date a piece of wood.
Dendrochronology and radiometric dating have been used to date many bristlecone pine trees in the White Mountains of California. The oldest tree has been named Methuselah, and dendrochronology and carbon-14 dating methods have determined the tree to be 4,844 years old. Moreover, by matching tree growth ring patterns with dead wood laying in the vicinity, a tree growth ring catalog has been created that spans back 9,000 years.
Figure 8. Dendrochronology. (Click image to enlarge)
In addition to providing information on climate, tree rings can also provide insight into other environmental conditions. Read this story and make a note of the information that Dr. Thomas Swetnam from the University of Arizona has discovered through his examination of tree ring sections and what this evidence shows about how human actions can impact our environment.
Think like a scientist - which technique should you choose?
You are reading a blog written by a student who is working at an archeological dig in Cyprus. She mentions that they just uncovered an ancient burial site in which the skeletal remains of a human are found next to those of a cat. The arrangement of the bones suggests that the cat was buried with its owner and may have been a pet. Remarkably, the location of the grave relative to other known artifacts suggests it is 10,000 years old. If this is correct, then this is the earliest evidence for cat domestication. The blog author suggests that dendrochronological analysis of some of the wood fragments found nearby, together with C14 analysis should confirm the dates. You are not so sure these two methods will work equally well.
- Which method is most likely to shed accurate insight into the age of these two skeletons? Why?
- Which one of these techniques is most reliable?
The Diversity of Life
"It is difficult for us to appreciate exactly how far back life has existed on Earth. One way to look at it is to imagine that all 4.5 billion years of geologic time is compressed into one 24-hour day. During the first second past midnight, Eearth forms. The origin of life would have occurred around 4:00 am. The oldest fossils known would have been deposited at 5:30 in the morning, just when the sun is coming up. The age of microscopic life starts then and continues all the way past noon, past 3:00, past 6:00, and in fact, it isn't until 9:00 at night that larger organisms appear. All of that evolution that you normally learn about in textbooks, from trilobites to fish to amphibians to reptiles to birds and mammals, all takes place in the last 3 hours of the day. Humans don't appear until just a scant few seconds before midnight." - J. William Schopf, UCLA paleobiologist
The Earth has been a living planet for about 4 billion years. During that time, life has evolved and diversified into the approximately 1.8 million species that have been described so far, with the possibility of between 10 and 100 million species living on the planet today. All life on Earth appears to have arisen from a single common ancestor based on the evidence that all organisms studied so far use virtually the same genetic code to transform the information in their DNA into proteins (we will cover this in Tutorial 35). Life has diversified into three major domains, the Bacteria, Archaea, and Eukarya. The Bacteria and Archaea have more simple cells (prokaryotes), while member of the Eukarya have complex cells with a nucleus and an array of organelles that perform different functions within the cell (eukaryotes). Figure 9 shows the relationships among these groups.
Figure 9. The Three-Domain System of Classification. (Click to enlarge)
This tutorial explored the age of Earth and some techniques for investigating Earth's past. What do we know about events that occurred in Earth's distant past? To answer this question, biology has turned to the sciences of geology, physics, and astronomy to learn about the conditions that existed in Earth's distant past.
The available astronomical data indicate that the universe is about 15 billion years old. This estimate is based, in part, on the observation that the universe is constantly expanding. Astronomers have plotted the trajectories of various stars and galaxies and determined that all matter in the known universe arose from a common point. In an event, often referred to as the "Big Bang," the universe arose in a relatively brief moment in time and matter was flung outward from the central origins. Matter has been hurling through space and undergoing a number of changes as it traverses the cosmos.
Our solar system formed about 4.5 billion years ago. It appears that Earth began forming shortly thereafter, and evidence suggests that life first appeared by the end of the first billion years. The available evidence indicates that evolutionary events have been ongoing over the last 3.5 billion years and that these processes have led to the diversity of life we see around us today. In thinking about evolution, remember that all extant (living) species represent "modern" forms of life. The challenge that faces evolutionary biologists is to reconstruct events that have occurred in Earth's distant past, and to infer the nature of ancient events that have contributed to the current diversity of organisms.
Most researchers who tackle questions on the origin of life think that organic compounds first formed abiotically. The experiments of Urey and Miller demonstrated that these compounds could have formed in an atmosphere similar to that which existed on early Earth. It is also possible that organic compounds might have formed in deep-sea vents. Another hypothesis, panspermia, is that organic compounds (or perhaps life itself) arrived on the planet from extraterrestrial sources.
One of the first challenges is to know the temporal sequences of historical events. Fortunately, geologic events provide valuable information. The Earth's crust is not a static structure. It changes as a result of plate tectonics (which leads to continental drift, which is measured at about 1 inch/year, on average), as well as changes in the water levels of the oceans (due to redistributions during one of several ice ages), wind, the successive layering of sediments (material in the oceans sink), and erosion (wind and water pound on the exposed terrestrial ground). The bottoms of these ancient oceans exist in the form of sedimentary rock in many places on the planet. Many extinct species have left their fossilized impressions in these sedimentary layers.
Geologists have studied these layers around the world and have noted common species' patterns of fossils in these various layers. Generally speaking, the deeper the layers, the older the sediment and the more ancient the fossilized species. This relationship is so pervasive that geologists use fossils to describe the layer they are examining. (This has certain practical uses, as geologists looking for oil use the presence of certain fossilized species to know when their drills are approaching oil-bearing strata.) The geological time scale is based on the relative distribution of fossils in sedimentary layers. A close examination of these layers reveals various patterns in the distribution of various fossilized species at different depths (i.e., relative ages). Various changes in the patterns of fossilized species are noted and these are used to delineate four major eras: the Precambrian, the Paleozoic, the Mesozoic, and the Cenozoic. The geological time scale provides a relative time scale, but it does not directly establish an absolute time scale. Atomic physicists can determine absolute time scales.
Matter is composed of subatomic particles. Each atom of a particular element is characterized by the number of protons that exist within its central core. Also, in each core are a variable number of neutrons. Carbon, a prominent atom found in living organisms, has 6 protons. The most prevalent form of carbon has 6 protons and 6 neutrons and is referred to as carbon-12, or C12. However, a much less common form (and one that is unstable) also exists and is known as carbon-14, or C14 . This unstable form decays at regular intervals. The key here is "regular intervals." It takes about 5730 years for 10 molecules of C14 to decay into 5 molecules of N14. In other words, C14 has a half-life of about 5730 years. These two isotopes of carbon can be analytically separated and the ratio of the two can provide an absolute dating method for "carbon-dating" a sample. This method's validity has been independently verified using dendrochronology. The carbon-14 dating method is fairly reliable for samples that are less than about 50,000 years old. For samples that are older, or that do not contain carbon, isotopes of other elements can be used. A new method that measures the decay of K20 into Ar20 shows great promise in dating samples as young as 2,000 years and as old as several billions years (the half-life of K20 is 1.25 billion years).
Dendochronology, the counting of tree rings, cannot go back into deep time; it is limited to determining dates within the past 9000 years. However, it can provide information on climate year by year through examination of the width of these growth rings.
Life has been evolving on Earth for approximately 3.5 billion years. The three major domains of life (Bacteria, Archaea, and Eukarya) are distinguished by fundamental differences in their cell structure.
After reading this tutorial, you should have a working knowledge of the following terms:
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