Rucete ✏ Campbell Biology In a Nutshell
Unit 4 MECHANISMS OF EVOLUTION — Concept 25.2 The Fossil Record Documents the History of Life
The fossil record provides critical evidence for life's evolutionary history. Fossils allow scientists to reconstruct the sequence of evolutionary changes and date major events in the history of life.
The Nature and Formation of Fossils
Fossils form mainly in sedimentary rocks, layered deposits called strata.
Fossils can also be preserved through:
Mineralization (organic matter replaced by minerals).
Amber (organisms trapped in hardened tree resin).
Frozen soil or ice (preservation of entire organisms).
Although informative, the fossil record is incomplete:
Many organisms never fossilize.
Fossils can be destroyed by geological processes.
Fossilization is biased toward species with hard parts, widespread distribution, or long duration.
Dating Rocks and Fossils
Relative dating determines fossils' ages by their position within rock layers (strata).
Radiometric dating measures radioactive isotopes' decay to determine fossils' absolute ages:
Carbon-14 dating useful up to 75,000 years old.
Isotopes like uranium-238 (half-life 4.5 billion years) date older fossils indirectly by dating volcanic rocks surrounding them.
The Origin of New Organism Groups
Fossils document the gradual evolution of major organism groups.
Example: The evolution of mammals from synapsid ancestors:
Synapsids had jaws with multiple bones, evolving gradually into single-boned jaws of mammals.
Unique mammalian features (three middle-ear bones, specialized teeth) arose gradually, as shown by transitional fossils.
Importance and Limitations of the Fossil Record
Provides evidence of evolutionary changes, extinction, and transitional forms.
While it is incomplete, the fossil record continues to improve through ongoing discoveries.
In a Nutshell
The fossil record is crucial for understanding life's evolutionary history. Through relative and radiometric dating, it reveals patterns of speciation, extinction, and major evolutionary transitions, despite its limitations and biases.