The Record in the Rocks
Life first appeared on Earth about three and a half billion years ago. This date is not from a time machine but from the “record in the rocks”—from fossils found in the rocks and from radiometric dating. Radiometric dating tells how old something is by measuring its radioactive decay, but fossils are what we want to look at here. Fossils are the remains or signs of ancient life found preserved in rocks.
Most animals and plants decay and disappear when they die, but sometimes under the right conditions, their bodies are buried and become preserved in sand or mud. Over millions of years, the surrounding material becomes rock, and the plant and animal remains become fossils.
Fossils can be teeth, seeds, bones, or shells. Leaves and feathers may be saved as delicate impressions in the rock. Even footprints and burrows can leave evidence of past life.
Paleontologists (scientists who study fossils) look at rocks and fossils to understand the history of certain plants and animals, and to figure out how the characteristics of land and sea have changed over time. Comparing today’s creatures and conditions with those in the fossil record can be very helpful.
The entire time that life has been on Earth can be shown on the imaginary calendar below, where today is December 31 and each day equals 10 million years ago. Life first appeared 3,650 million years ago, or 365 imaginary days ago, on January 1.
See if you can figure out the dates for the appearances of the other plants and animals listed below. For each organism, decide how many days ago it would have appeared, count that number backwards on the calendar below from December 31, and then print its letter in the correct square. “A” has been done for you. (Hint: Figure out whether we added, subtracted, multiplied, or divided to get 365 days ago—and why we did.)
| Millions of Years Ago | |
|---|---|
| A. first life | 3,650 |
| B. fish | 450 |
| C. land plants | 400 |
| D. birds | 140 |
| E. reptiles | 300 |
| F. multicelled creatures | 700 |
| G. humans | 2 |
| H. mammals | 200 |
| I. Tyrannosaurus rex | 70 |
| J. whale | 60 |
Extra Challenge: Find A (first life) on the calendars. Then find when simple, one-celled animals evolved into multicelled creatures (F). What a long time! Find a real calendar and count the days between A and F. Then do the math. Number of days ___ x 10,000,000 years = years.
