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Antiquity of Life

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Terms

You should have a working knowledge of the following terms:

  • carbon-14 dating
  • Cenozoic era
  • continental drift
  • dendrochronology
  • fossil
  • geological time scale
  • half-life
  • isomer
  • isotope
  • Mesozoic era
  • Paleozoic era
  • Pangaea
  • plate tectonics
  • Precambrian era
  • racemization
  • radiocarbon dating
  • radiometric dating
  • sedimentary rock


Introduction and Goals

Macroevolution and major events that lead to speciation was discussed here. This tutorial continues to address evolution on a broad scale by looking at biological diversity in the context of a changing planet. As you will learn, Earth's environment has had a varied past. By the end of this tutorial you should have a basic understanding of:

  • The age of the universe, the solar system, and life
  • The dynamic structure of Earth
  • The geologic time scale
  • The distinction between relative and absolute dating methods
  • The basic principles behind radiometric dating, dendrochronology, and racemization.

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. Matter has been hurling through space, undergoing a number of changes as it traverses the cosmos.

Since the beginning, this vast amount of matter has been undergoing constant change. 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 understand the origin of the universe and solar systems.

Early Earth

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 birth. In fact, Earth's surface is dynamic.

Earth's crust consists of large plates that move with respect to each other (as depicted here). This movement, termed plate tectonics, is due to 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. (Climatic changes also influence water distributions.) In the last 4.5 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. 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 animals on separate landmasses.
Go to the following Web site, and return prepared to answer a question concerning the glaciation events that provided evidence for Pangaea's existence. Evidence for Pangaea


Figure. Continental Drift and Plate Tectonics. (Click image to enlarge)


Figure. Continental Drift and Pangaea's History. (Click image to enlarge)

How Do We Know What We Know?

The fossil record

This figure 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 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.


Figure. Sedimentary Rock and Fossil Formation. (Click image to enlarge)

Relative dating

Probably the best source of fossils is sedimentary rock, which is formed by layers of minerals that settle in water. Over time these minerals build up and pack the layers lying beneath them, along with any organisms that have settled with the sediments. The location of fossils in sedimentary rock is indicative of their relative age. This means that one fossil can 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. Fossils are divided into age groups according to a geological time scale, whereby explosions of life or mass extinctions mark the general age of Earth. 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. Review the Cambrian explosion in tutorial 13. Layers of rock bearing fossils suggest a rapid diversification of reptiles that belong to 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, Mesozoic, and Cenozoic; and you should know that we are currently in the Quaternary period of the Cenozoic era.

Early Geological Eras

Recent Geological Eras

Absolute dating

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, dendrochronology and racemization. These methods will be explored below.

Radiometric Dating

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.

This illustration describes 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 (Carbon 14 dating) and read the section on "The 14C Method" and return prepared to answer the following question. What does carbon-14 decay into?


Figure. Radiometric Dating. (Click image to enlarge)

How Do We Know That C-14 Dating is Accurate?

From your reading at the carbon-14 Web site, you should now know how C14 is formed and how it enters the food chain. When a living organism dies, it stops absorbing C14 and the C14 that is already in the organism begins to decay. C14 decays at a slow, steady rate and reverts to N14. The rate at which carbon decays (its half-life) is typically between 5600 and 5700 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 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 are the same. In addition, the argon/potassium-40 dating method has been used to date Mount Vesuvius's 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

Dendrochronology is an absolute dating technique. This science relies on the fact that tree growth occurs in spurts, depending on seasonal variations in water and sunlight. Trees have annual growth rings that reflect their ages, and that often reveal something about past climate conditions. Remarkably, global fluctuations of weather patterns appear to be the best indicators of annual growth ring changes; wood obtained from Egyptian tombs has been matched to tree growth ring patterns found in ancient trees located in California. This figure depicts the basic strategy used to date a piece of wood.


Figure. Dendrochronology. (Click image to enlarge)

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 4,767 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.

Amino Acid Racemization

Amino acids (subunits of proteins; to be discussed in tutorial 16) exist in two isomer forms: a left-handed (L-form) isomer and a right-handed (D-form) isomer. During protein synthesis, organisms only use the L-form amino acids; however, when an individual dies, its L-form amino acids are slowly converted to D-form amino acids. This chemical conversion is termed racemization, and knowing the rate of racemization for a given amino acid allows scientists to date some fossils. While this absolute dating technique has its uses, it is constrained by the fact that racemization is temperature-sensitive. Therefore, this method can only be used if the climate has remained relatively stable. It also cannot be used on fossils that have mineralized.

Go to the following Web site and read about the use of this method in dating a silk specimen from an ancient Egyptian mummy. Be prepared to answer a question about how researchers knew the silk specimen was not modern. The Use of Silk in Ancient Egypt

Summary

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 about 1 billion years later. 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 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 diversity of organisms that we see today.

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), 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 5600 years for 10 molecules of C14 to decay into 5 molecules of N14. In other words, C14 has a half-life of about 5600 years. 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. 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).

To continue with our discussion of life's origin, we will address what we know about the basic requirements for life, along with what early life on Earth might have looked like.