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Exploring how fossils provide a chronological record of life and transitional forms.
What if you found a 'photo' of a creature that lived 300 million years ago buried deep beneath your feet? Fossils are nature's time capsules, but they only form under 'one-in-a-million' conditions.
Most living things simply disappear after death, recycled by scavengers and bacteria. To become a fossil, an organism must be buried quickly by sediment (like mud or sand) to protect it from oxygen and decay. Over millions of years, minerals replace the organic material in a process called permineralization, turning bone into stone. Because this requires very specific environments—usually near water—the fossil record is incomplete. It is like a book where 90% of the pages have been ripped out; we see the highlights of history, but not every single detail.
Quick Check
Why are we more likely to find a fossil of a hard-shelled clam than a soft-bodied jellyfish?
Answer
Hard parts like shells and bones decay much slower than soft tissues, providing more time for burial and mineralization to occur.
Scientists use two methods to read the clock of the Earth. Relative dating uses the Law of Superposition, which states that in undisturbed rock layers, the oldest layers are at the bottom. This tells us the sequence of events but not the exact year. To find the actual age, we use absolute dating (radiometric dating). This relies on the steady decay of radioactive isotopes. By measuring the ratio of 'parent' isotopes to 'daughter' isotopes, we can calculate exactly how many millions of years have passed.
Imagine a volcanic rock contains a radioactive isotope with a half-life () of 700 million years. 1. A scientist finds that only (25%) of the original parent isotope remains in the sample. 2. Since , we know exactly 2 half-lives have passed. 3. Age = million years = billion years old.
Quick Check
If Fossil A is found in a rock layer 50 feet below Fossil B, which one is likely older according to relative dating?
Answer
Fossil A is older because it is in a deeper layer (Law of Superposition).
If evolution is a gradual process, we should see 'links' between major groups. These are transitional fossils. A famous example is Archaeopteryx, which has the feathers and wishbone of a bird but the teeth and long bony tail of a reptile. These fossils act as evidence for the 'intermediate steps' in evolution, showing how a lineage of dinosaurs slowly adapted for flight. They prove that life doesn't just jump from one form to another; it transitions through functional, intermediate stages.
Whales evolved from four-legged land mammals. The fossil record shows a clear progression: 1. Pakicetus: A land-dweller with an ear bone unique to whales. 2. Ambulocetus: The 'walking whale' with webbed feet for swimming and walking. 3. Basilosaurus: A marine giant with tiny, useless hind legs (vestigial structures). 4. Modern Whales: No external hind limbs, fully adapted for deep-sea life.
Which process describes minerals replacing organic matter to create a fossil?
If an isotope has a half-life of 10,000 years, how much of the original isotope remains after 20,000 years?
The fossil record is considered 'complete' because we have found fossils of every species that ever lived.
Review Tomorrow
In 24 hours, try to explain the difference between relative and absolute dating to a friend without looking at your notes.
Practice Activity
Research the 'Tiktaalik' fossil and identify which two major groups of animals it appears to bridge.