LIFE'S BIG QUESTIONS: How Did Earth Get Animals?

Frontiers travels to China, where biologist Andrew Knoll and his Chinese colleague investigate one of the great mysteries of biology -- how microscopic life evolved into big animals in a relatively short time. The answers may lie in sedimentary rocks in southwestern China. The rocks provide the evidence Knoll is looking for to confirm his theory that geologic activity about 580 million years ago brought about a change in the oxygen cycle that made an explosion of life possible.

Activity: Get a Half-Life
Consider This!

 BIOLOGY Heterotroph Hypothesis,taxonomy CHEMISTRY nuclear EARTH SCIENCE atmosphere,natural radioactivity,tectonics

 GEOLOGY sedimentary rock LIFE SCIENCE ecology PHYSICAL SCIENCE radioactive dating, sedimentation

ACTIVITY: GET A HALF-LIFE

On this segment of Frontiers, you watched scientists look for fossils in a Chinese quarry. These fossil finds helped confirm Andrew Knoll's theory about the explosion of life that occurred about 570 to 580 million years ago. How do scientists calculate the age of fossils such as those found in the quarry? One technique that helps determine the approximate age of a fossil sample is radioactive dating.

Everything that is alive or ever was alive on the earth contains a set amount of radioactive materials. For example, the ratio of radioactive carbon to nonradioactive carbon is thought to have remained the same for millions of years. So you probably have about the same ratio of radioactive to nonradioactive carbon in you as did a velociraptor or saber-toothed tiger.

The radioactivity of an element decays at a predictable rate. Different radioactive elements decay at different rates. Uranium 238 decays over billions of years, while polonium decays in mere seconds.

Suppose you have 1kg of pure uranium, and you lock it away in a safe place for 4.5 billion years. When you return to get your uranium, you find about 1/2kg of uranium left. What happened to the other 1/2 kg? It decayed and turned into lead.

Geologists use the term half-life to describe the breakdown of radioactive materials. Half-life is the time it takes for one-half a sample of radioactive material to change into what is known as a stable daughter element. At what point would your sample above contain no uranium? Does exactly half the sample break down during a half-life? Let's find out.

In this activity, you will perform a statistical analysis of recurring decay events.

MATERIALS
• 100 pennies
• 100 nickels
• shoe box with top
• graph paper
• pencil
• notebook paper
(you may also use a spreadsheet program to record and interpret your data)

PROCEDURE
1. Read all instructions first. Before performing the experiment, predict the shake numbers at which you think each half-life should occur. Write your predictions on a piece of paper.

2. Work with one partner or alone. Place all the nickels head-side up in the box. Put the top on the box. Shake vigorously for several seconds.
Note: You will repeat this step many times, so you must remember how you shook the box and duplicate the action each time.

3. Remove all tail-side-up nickels and record the number in a data chart (see example below) or on a spreadsheet program on a computer. Replace the "spent" nickels with an equal number of pennies. Circle the shake number each time you reach or pass a half-life.

4. Continue until all the nickels are gone. Repeat the procedure enough times so that within your class 50 or more trials have been completed. If you are using a spreadsheet, all data should be entered into one file. Copy the file for as many teams as you have. Create a double line graph that plots a shake number against half-life and shake number against total change.

DATA CHART

 RECORDINGS: SHAKE #1 SHAKE #2 Started with # Nickels: Started with # Pennies: Ended with # Nickels: Ended with # Pennies: Total Change # Nickels: Total Change # Pennies:

...etc.

QUESTIONS

1. What do the nickels and pennies represent?

2. Determine the total number of half-lives possible for this activity (round where necessary).
.
3. Use all class data to calculate mean shake numbers at which each half-life occurs.

5. How did the actual results compare to your predictions?

6. Assuming your experiment follows the same general trend as nature, does half-life follow a predictable and even pattern? How would you describe it?

1. Nickels represent radioactive material (isotopes). Pennies represent one of the daughter elements.

2. 7, using these rounded numbers: 50, 25, 13, 7, 4, 2, 1.

4. Probably not. Many students will opt to use the number 7 as their basis for prediction. Their predicted results may be quite orderly. Since the process is completely random, there is no order to it. Results should be different.

6. No. It could be described as an average pattern with close to predictable half-life graduations.

CONSIDER THIS!
• In The Beak of the Finch (Knopf, 1994), author Jonathan Weiner details the work of Peter and Rosemary Grant, evolutionary biologists who have spent years studying Darwin's finches on a remote island in the Galapagos. The birds endured a drought that killed many of them. Only those whose beaks were best suited to find seeds hidden in hard-to-reach places survived. How does this example illustrate natural selection?

• What are some other examples of mutating species? (Hint: Think small. One example is bacteria, which have been mutating since their earliest days on this planet and continue to mutate, thus foiling the antibiotics created to treat harmful strains and presenting a hazard to public health. The tuberculosis bacterium is known to be evolving into resistant strains. See Frontiers Show 302, "The Return of Tuberculosis.")

 Scientific American Frontiers Fall 1990 to Spring 2000 Sponsored by GTE Corporation, now a part of Verizon Communications Inc.