LABORATORY # 6
EVOLUTION, TIME AND THE FOSSIL RECORD
LABORATORY OBJECTIVES
Upon completion of this laboratory the student will:
REFERENCE
Textbook: chapter 3, p 45
chapter 13, pp 272
chapter 22, p462
chapter 23, pp 490, 491, 496 and 497
INTRODUCTION
Evolution of life means change of life through time. It explains how the various forms of life on earth came to be. The study of evolution involves study of the process whereby species change both physically and physiologically through time and thus become adapted to their environment. The implication is that as the environment changes, species must also change or risk extinction. The mechanism of evolution, proposed by Charles Darwin, is natural selection. Specifically, there are many different selective agents in the environment, which function to either remove organisms directly from the environment by death and/or illness or to produce a situation that is not favorable for maximum reproductive success. Variation of traits among individuals of a species causes some to be more likely to survive natural selective agents than others. They are said to be the most fit. Survival of the fittest means that those organisms which have the most advantageous combination of variations are able to live to reproduce and pass on their advantageous traits to their offspring. Over time the species has adapted to its environment. Modern biology is in large part built around the maxim of organic evolution. It explains the seeming contradiction of the enormous diversity of life forms, and similarities or unity in life. It has been said that nothing much makes sense in biology unless it is studied in the light of evolution by natural selection. While the exact mechanisms of evolution continue to be debated by scientists, evidence for evolution continues to be amassed. There are few scientists who doubt the validity of the theory of evolution by natural selection.
Evidence in support of evolution must show that organisms have changed over the ages but that the most closely related still have many features in common. This laboratory will study two types of data in support of evolution: fossil evidence and biochemical comparison.
To appreciate evolution, especially speciation, one must have some idea of the vastness of time over which the earth's surface and life have had to change. In this lab you will participate in activities meant to give you a good "feel" for the vastness of geologic time. Another integral part of natural selection is mutation. Mutation is the only original source of new variation.
The key to survival in a changing environment is not resistance to change, but meeting change with change. Even though a single species must inevitably be modified or become extinct, continued life is assured through evolution.
THE FOSSIL RECORD
Some of the best evidence in support of evolution is supplied by the fossil remains of organisms found in sedimentary rocks of the earth's crust. These rocks, deposited as layers of sediment in the sea bed or along rivers and later uplifted to form dry land, can be found in many places (including Middle Tennessee). One of the most striking examples, however, is found in the southwestern U.S., where the Colorado River has carved the Grand Canyon to expose a veritable history of life on the earth.
The word “fossil” is from the Latin word “fossilis” which means to dig up. For our purpose a fossil is the “evidence of past life.” We restrict past evidence to items that are pre-historic or older than around 10,000 years before present.(older than the Holocene/Recent epoch).
Evidence can be an original part of the plant or animal, such as a leg bone of a dinosaur. However, evidence can also be the footprint made by the leg of that dinosaur. Tracks, trails, burrows, and coprolites are some of the more common trace fossils. Often trace fossils are more informative as to the activities, modes of life, and food sources than are body fossils. In some cases, the skeletal remains or hard parts are dissolved away but the impression of the hard part is left behind. This impression is called a mold. If this mold is then filled with material then a replica of the original organism is formed and is termed a cast.
Preservation of Fossils
When an organism dies it begins to break down chemically and physically. Especially important in this breakdown is scavenging of the remains by other organisms and the exposure to the elements of nature. The factors that resist this breakdown are (1) rapid burial, (2) hard parts that are more durable and (3) a mineral composition that is resistant to decay.
The following list is of ways in which organisms can resist this breakdown and be preserved.
1. Complete Preservation – This is the rarest type of preservation and is achieved by separating the organism from the means in which it could bread down. Some examples are the frozen mammoths in the glacial ice or insects in amber. Mummies are completely preserved but are not fossils. Read the intro again if you don’t know why.
2. Carbonization – This is the most common preservation mode for soft-parts (non-mineral parts). Heat and pressure drive off hydrogen, oxygen, and nitrogen. This leaves only a dark film of carbon showing the outline of the fossil.
3. Replacement – This is the removal of the original mineral, usually by dissolution and the subsequent filling of the space with a different mineral precipitated from solution.
4. Permineralization – This is the later filling of pore spaces of a hard part by minerals. A petrified tree has minerals that have crystallized in its woody cells.
Activities:
I. Examples of Fossils
You will be given several fossils to examine. Consider each fossil and determine what kind of fossil it is: imprint, cast, completely preserved, carbonized, permineralized, trace, or other.
What kinds of organisms tend to fossilize most readily?
II. Evolutionary Trends in the Horse
As an example of how the evolutionary history of an animal taxon may be preserved in its fossil remains, we will consider the horse. Horses give us one of the best evolutionary sequences known, because their fossil record is so complete.
The horse was a native of North America from near the end of the Paleocene Epoch (55 million years ago) to Late Pleistocene time (about 2 million years ago). Horses underwent most of their evolution in North America, and fossils from rocks of different ages reveal gradual changes in teeth, limbs, feet and body size. The earliest horse, Hyracotherium (also known as "Eohippus"), appeared in Late Paleocene time and was about the size of a medium-sized dog. There were many different species of this early horse, but all were small, slender and characterized by three-toed hind feet and four-toed front feet. The teeth of Hyracotherium were relatively primitive with bluntly-cusped, low-crowned molars suitable for browsing on soft forest vegetation.
Mesohippus, a three-toed Early Oligocene horse was about the size of a sheep, but looked more like the horse as we know it today. The teeth were still low-crowned, but somewhat higher than those of Hyracotherium.
In Late Oligocene and Early Miocene time, grassy plains expanded and horses adapted to the changing environment. There was a reduction in toes that permitted faster locomotion, and the teeth increased in height. Dentin was inter-layered with enamel in their higher and more complex crowns. These teeth were better adapted for grinding coarse vegetation such as grass. Pliohippus (the first one-toed horse) appeared in Early Pliocene time. Its head and body were quite horse-like and its high-crowned teeth and long jaws were well adapted for grazing.
By Middle Pliocene time most of the horse's basic evolution had occurred and horses looked much as we see them today. Near the end of the Pliocene Epoch, Pliohippus gave rise to Equus, the modern horse. Equus was widespread in North America and remained there until near the end of the Pleistocene "Ice Age." Then, for some unexplained reason, the horse became extinct in North America and was not present again until reintroduced by Spanish explorers in the sixteenth century. In Europe and Asia, the fossil record of the horse dates from Eocene, but the species were different from American horses and there was no ancestral line that survived the Tertiary. In the Pleistocene Epoch, horses migrated to Asia, probably via the Bearing Strait land bridge.
Procedure: Have one member of your group procure a set of fossil horse teeth from the front desk. Using the information provided in the preceding paragraphs, determine the phylogenetic sequence of the specimens. The first fossil in the sequence should represent the earliest evolutionary stage, as revealed by these particular horse teeth replicas. The last fossil should represent the highest stage of evolution illustrated by this group of specimens.
Which of the replicas illustrates a type of tooth that had an extensive, complex crown surface that would have been best adapted to grinding up rough vegetation?
What feature did you use to establish this evolutionary sequence? What is the proper specimen number sequence, from simplest to most complex? Ask your instructor to verify this sequence.
On what continent did most of the horse's evolution occur?
TIME
It is extremely difficult for many people to comprehend how life could have begun as primitive bacteria and changed over time to give us all the diversity that we see on earth. For this evolution of life to seem plausible, one must have some feel for “deep time.” Best current estimates for the age of the earth range from four and one half to five billion years. Most human beings, to whom a century is a very long time, have difficulty comprehending the vast span of time involved here.
Activities:
I. Absolute Ages of Rocks
The ability to determine the absolute ages of rocks is a fairly recent discovery and is based on the radioactive decay of certain chemical elements. Radioactive decay occurs when isotopes of these chemical elements undergo changes by the emission of particles to form a more stable form of the element or even a new element. This is a random process in nature, and the geologists who work with these processes seek to determine the decay constant, or the probability that a certain proportion of the atoms of a given element will decay in a unit of time. Since the number of atoms available to decay decreases through time, it is possible to estimate the length of time it takes for one half of the parent atoms to decay. This period of time is called the half-life and it is a time unit unique to each isotope. Thus, we speak of the half-life of Uranium-238 (the parent) to form a stable product Lead-206 (the daughter) as being 4.5 billion years. If we can measure the ratio of the parent to the daughter and we know the decay constant, we can determine the age of the rock. The basic formula in a simplified version is as follows:
T = K(D/P)
Where T = age of the rock in years before the present
K = the decay constant
D = amount of daughter produce present today
P = amount of parent product present today
Common parent-daughter combinations used for absolute age determination are uranium-lead, thorium-lead and potassium-argon.
What is the decay constant of a radioactive isotope?
What is the half-life of an element?
If one began with 48 grams of a radioactive isotope, how many grams would remain after the passage of 4 half-lives?
If the daughter-parent ratio of an isotope pair in rock sample X is 1/4 and the decay constant (K), expressed in years is 1.3 billion, what is the absolute age of rock sample X?
II. Geologic Time Scale
This exercise is designed to help you understand the geologic time frame in which organic evolution has played out. This graphic representation of the earth's history described below will assist you in grasping this concept.
Materials Required:
Adding machine tape
Masking tape
Sharp pencil
Magic Markers of various colors
Meter stick
Ruler
Geologic Time Scale
Procedure:
1. Divide into groups of three to five students, as directed by your instructor.
2. Each group will procure a materials set and proceed to the hall where it can work unimpeded. (Remember, other classes are going on, so keep the noise down.)
3. Tape the end of the adding machine tape to the floor and mark a "Present" time-line. This is the line from which all other times will be measured.
4. Using a scale of 1 cm = 2,000,000 years, calculate, measure, mark and label a section of the tape representing the elapsed time from "Time Present" to "Time Zero," the beginning of the earth, as indicated on your Geologic Time Scale. Use the estimate of 4.6 billion years as the time span. Tear off the tape and stick it down.
5. Continue in similar fashion, calculating, measuring, marking and labeling each of the periods of time indicated on the Geological Time Scale. Use different colors for Pre-Cambrian (began 4.6 billion years ago; ended 590 million years ago), Paleozoic (began 590 mya; ended 248 mya), Mesozoic (began 248 mya; ended 65 mya), and Cenozoic (began 65 mya; up to the present time) time periods.
6. Now locate and mark and label the following events:
a. Life began in the late Archean, about 3.2 billion years ago.
b. Soft-bodied invertebrate animals were present about 700 million years ago.
c. Hard-bodied invertebrates were abundant about 600 million years ago, at the beginning of the Cambrian Period.
d. The first vertebrates (fishes) appeared about 450 million years ago in the late Ordovician Period.
e. Plants invaded the land about 415 million years ago in the Silurian Period.
f. Vertebrate land animals, in the form of Amphibians, appeared in the Devonian Period around 350 million years ago.
g. Reptiles evolved in the Pennsylvanian Period about 300 million years ago.
h. About 210 million years ago, in the Triassic Period, the first mammals appeared on earth.
i. The first birds evolved about 150 million years ago in the Jurassic Period.
j. In the late Cretaceous Period, about 70 million years ago, the first primates appeared.
k. The first hominids, represented by "Lucy," roamed East Africa about 4 million years ago.
l. Homo sapiens, in the form of Neanderthal, lived in Europe as long as 500,000 years ago.
m. The first humans made it to North America about 40,000 years ago.
n. Christ was born approximately 2,000 years ago.
The dinosaurs became extinct at the close of the Cretaceous Period. Compare their stay on the earth with that of our own species. Is the common view of dinosaurs as "big dumb beasts who couldn't cope with change" justified? Why?
What is the most surprising thing that you learned from this activity?
If one were to put this history of the earth in terms of a 24-hour day, when (how long ago) would life have begun? When would reptiles have evolved? When would Homo sapiens have arrived on the scene?
How is radiometric dating technique used to establish the geologic time scale? How can one determine the age of a particular fossil? How can one determine when an extinct species appeared, how long it existed, and when it disappeared?