Chapter 17: The Cambrian Explosion and BeyondOverview |
Once the fundamental processes of DNA replication, protein synthesis, respiration, and cell division had evolved, a spectacular diversification of life ensued. Innovations like photosynthesis and the nuclear membrane evolved. These events, along with others we reviewed in Chapter 14, spanned some 3.2 billion years and created the deep branches on the tree of life. During this interval all organisms, with the exception of some small red, brown, and green algae, were unicellular. Chapter 17 of your textbook explores this interval in Earth history, called the Phanerozoic (“visible life”) eon.
17.1 The Nature of the Fossil Record In Chapter 2, we introduced the geological time scale that was established by paleontologists in the early 19th century. We also looked at how 20th-century geochronologists are using radioactive isotopes to estimate the absolute age of each eon, era, period, and epoch.
Refer to section 17.1 of your textbook to explore how life has changed through time by analyzing the fossil record.
17.2 The Cambrian Explosion Almost all of the animal phyla currently recognized by biologists make their first appearance in the fossil record during the Cambrian period—a span of just 40 million years. This amount of time is little more than the blink of an eye, geologically speaking.
Turn to section 17.2 to learn about just how much evolutionary change occurred during the interval.
17.3 Macroevolutionary Patterns Understanding the great innovations in evolution is fundamental. That is why we spent most of Chapter 16 and much of this one on subjects like the origin of life, endosymbiosis, and multicellularity. But documenting evolution’s “greatest hits” is only part of historical biology’s portfolio.
Turn to section 17.3 to learn more about the broad patterns in the fossil record and how the microevolutionary processes of mutation, natural selection, drift, and gene flow have produced an almost overwhelming diversity of life forms.
17.4 Mass Extinctions Extinction is the ultimate fate of all species. Over the course of Earth’s history, what patterns occur in the rate of extinction?
Turn to section 17.4 to explore the answer to this question.