History of Life – Lab Exercises

Exercise 1: Timeline of Life of Earth

For the exercises in this lab, please work in groups of 4-5.


  • meterstick
  • calculator tape
  • pen or pencil
  1. Measure and cut a length of calculator tape that is 4.54 meters long. You will use this to represent Earth’s history. The timeline MUST be to scale. In other words, each centimeter of the timeline will represent the same number of years. 
  • Determine your scale.
  • How many years does a meter represent?__________________
  • How many years does centimeter represent?________________
  • How many years does a millimeter represent?________________
  • How many years does a micrometer represent?________________
  • Place the major geologic Eras and Periods on the timeline. Please note that these start and end times are approximate and have been rounded to the nearest 0.5 million years.
EraPeriodStart (mya)End (mya)
Paleozoic 541252
Mesozoic 25266
Cenozoic 66current

*These dates follow the current time scale as published by the International Commission on Stratigraphy.

  • Place each of the events in the table below on the timeline.
Oldest prokaryote fossils13500 mya
Great oxidation event22400-2000 mya
Oldest eukaryote fossils32100 mya
Oldest multicellular fossils41200 mya
Oldest animal fossils5600 mya
Cambrian explosion 541-509 mya
Oldest fossils of fungi6440 mya
Oldest land plant fossils7425 mya
Oldest amphibian fossils8370 mya
Oldest seed plant fossils9365 mya
Oldest reptile fossils10315 mya
Oldest beetle fossils11280 mya
Oldest dinosaur fossils12230 mya
Oldest mammal fossils13160 mya
Oldest flowering plant fossils14130 mya
Oldest diatom fossils15115 mya
Oldest primate fossils1656 mya
Oldest Homo fossils172.3 mya
Oldest human (Homo sapiens) fossils180.3 mya

*Dates are approximate and some are disputed. However, it should be noted that the general sequence and approximate ages are agreed upon, but each specific date has a +/- range and is subject to revision pending future discoveries. 

Exercise 2: Coal Ball Peels


  • coal ball slab
  • thick glass grinding plate (1’ square)
  • 400 grit carborundum powder
  • 5% HCl acid bath
  • water bath
  • acetone
  • 3 mil cellulose acetate sheet
  • tray containing gravel


  1. Obtain one of the coal ball slabs.
    • Select the side that you wish to peel.
  2. Polish the cut surface with 400 grit carborundum (silicon carbide) powder:
    • Add water and carborundum powder to make a “paste” on the surface of a glass grinding plate.
    • Grind the surface using a “figure 8” motion until smooth (this should not take long – 30 seconds to a minute).
    • Rinse the surface in the sink to remove grit.
  3. Etch the surface using dilute (5%) HCl:
    • Holding the coal ball slab by its edges upside down (polished side down), immerse the polished surface in a bath of 5% HCl for 15 (± 3) seconds.
      • Caution: Use eye protection (goggles), gloves and a lab coat for this step.  5% HCl will not harm your hands, but it will sting if you have a cut and damage jewelry and clothing.  Keep the acid container covered when not using.
  4. Immediately rinse the etched surface with a gentle stream of water or place in a water bath.  
    • Caution: Do not touch the etched surface of the coal ball, or allow it to be touched by another surface/object.  Etching leaves the plant cell walls standing in relief about 30-40 micrometers above the un-etched surface of the coal ball.  These cell features are very easily crushed.
  5. After a brief rinse, place the etched coal ball in a gravel tray, etched side up, so that the etched side is leveled (just “eyeball” this).
  6. Allow the etched surface to air dry (about 30 minutes) or rinse several times with a gentle stream of acetone, and permit the surface to dry for about 5 minutes after the final rinse.  The surface must be completely dry prior to peeling.
    • Caution: Acetone should be used only in the fume hood (for this and the next step).
  7. Make a peel:
    • Cut a square of 3 mil (= 0.003 inch) thick cellulose acetate film that will be sufficient to cover the etched surface and extend beyond its edges for ½ to1”.
    • Flood the leveled and etched surface with acetone.
    • Carefully “roll” the cellulose acetate sheet on to the etched surface, in order to exclude air bubbles.
      1. Caution: Do not place flat on to the specimen (this usually results in air bubbles).
      1. Caution: Do not touch the acetate sheet (i.e. don’t tamp it down).
      1. Caution: Do not attempt to shift the sheet once it has been rolled on to the specimen.
  8. Finish:
    • Allow the new peel to dry for at least fifteen minutes.  Letting it dry for an hour or more will help keep the peel from curling.
    • Starting at one side of the peel (preferably a narrow side), carefully pull the peel up from the specimen.  If the peel sticks, use a razor blade to help lift the peel.
  9. Take a picture of your peel using the microscope. Make to sure to center and crop appropriately, and add a scale bar. Submit your picture via Canvas.

Exercise 3: Interpreting Coal Ball Peels

  1. Examine several of the coal ball peels using a dissecting microscope.  Can you identify any of the plant structures that are present?
  2. Why have most of the stems, leaves, and roots seen in these peels been assigned to “organ genera?”

Exercise 4: Examine prepared fossils

Examine the fossils that have been set out for you to observe. Each includes an approximate age. Place each fossil on the timeline you have constructed. 

Prepared fossil set #1: Ironstone Concretions

Coal balls contain random assemblages of plant matter (i.e. whatever “fell into” the coal swamp) that has been permineralized in fine detail.  These fossils show fine internal structure, but because they are masses of compacted plant material from different individuals, they are not as informative in regards to plant morphology.  Plant morphology is far more easily observed by compression/impression fossils.

The Carterville Formation in Southern Illinois contains ironstone concretions.  These are nodules that when cracked open often show compressions and impressions of Pennsylvanian plants (and less commonly animals).  These fossils are approximately the same age as the coal balls examined in exercise 2.

  1. Observe several ironstone concretions that have been cracked open.  What types of plant structures have been preserved?
  2. How does the information provided by these specimens differ from that provided by the coal balls?
  3. Add the Pennsylvanian coal swamps to your timeline

Prepared fossil #2: Rhynia from the Rhynie Chert

  • The Rhynie Chert contains a famous permineralized fossil assemblage from near Aberdeenshire, Scotland.  It is early Devonian (Pragian: about 410 million years old) in age and contains members of a silicified “bog” community.  The plants, animals, and fungi that were preserved show extremely fine preservation.  This was an extremely interesting time in the evolution of terrestrial organisms (soon after the evolution of the earliest members of the plant kingdom and colonization of the land surface), and this formation contains the best preserved specimens of early plants that is currently known. 
    • Examine the stem cross section of Rhynia.
    • Examine the stem cross section of Asteroxylon.

Prepared fossil set #3: Ostracoderms from the Beartooth Butte Formation

  • The Beartooth Butte Formation is a series of “channel fill deposits” from northern Wyoming and southern Montana.  They have been dated to the lower Devonian Period (Emsian: about 400 million years old), and contain a mixture of primitive plant and animal compressions and impressions.
    • Examine the remains of primitive armored fish

Prepared fossil set #4: Green River Formation Fish and Plants

  • The Green River Formation from Wyoming is famous for its finely preserved compression fossils of fish.  This formation was formed during the Eocene (epoch) of the Paleogene Period (about 50 million years old).
    • Observe the fish and plant fossils.  Do these look like they might belong to recognizable groups?

Exercise 5: Split shale to find trilobites

  • Looking at your timeline, to which geologic Era and Period does the Wheeler Shale belong?

Wheeler Shale contains fossils of many different organisms, including brachiopods, sponges, and crustaceans. However, it is most famous for its trilobites. We anticipate that several trilobites will be found. There are at least six trilobite species that have been discovered, but two species are far more common than the others. Elrathia kingii and Asaphiscus wheeleriare your most likely finds.

Text Box:   
The most common trilobites of the Wheeler Shale. Left is Elrathia kingii and right is Asaphiscus wheeleri. Image credits: John Alan Elson and Wikipedia user Dwergenpaartje. Licensed under GFDL and CC BY-SA3.0.
The most common trilobites of the Wheeler Shale. Left is Elrathia kingii and right is Asaphiscus wheeleri. Image credits: John Alan Elson and Wikipedia user Dwergenpaartje. Licensed under GFDL and CC BY-SA 3.0.

Important: Please wear eye protection while splitting shale, or while watching your classmates split shale!

Important: Please use care when using the rock hammers. We want to split the rock, NOT your fingers or the countertop.

References for events table

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4.               Bengtson, S., Sallstedt, T., Belivanova, V. & Whitehouse, M. Three-dimensional preservation of cellular and subcellular structures suggests 1.6 billion-year-old crown-group red algae. PLOS Biol. 15, e2000735 (2017).

5.               Chen, J.-Y. et al. Small Bilaterian Fossils from 40 to 55 Million Years Before the Cambrian. Science 305, 218–222 (2004).

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10.             Müller, J. & Reisz, R. R. The phylogeny of early eureptiles: comparing parsimony and Bayesian approaches in the investigation of a basal fossil clade. Syst. Biol.55, 503–511 (2006).

11.             Béthoux, O. The Earliest Beetle Identified. J. Paleontol. 83, 931–937 (2009).

12.             Martinez, R. N. et al. A Basal Dinosaur from the Dawn of the Dinosaur Era in Southwestern Pangaea. Science 331, 206–210 (2011).

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15.             Bridoux, M. C. & Ingalls, A. E. Diatom microfossils from cretaceous and eocene sediments contain native silica precipitating long-chain polyamines. Geobiology11, 215–223 (2013).

16.             Dunn, R. H. et al. New euprimate postcrania from the early Eocene of Gujarat, India, and the strepsirrhine–haplorhine divergence. J. Hum. Evol. 99, 25–51 (2016).

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18.             Richter, D. et al. The age of the hominin fossils from Jebel Irhoud, Morocco, and the origins of the Middle Stone Age. Nature 546, 293–296 (2017).