Geology Reference
In-Depth Information
Trace fossils:
Remains of an organism's activity, such as movement (a footprint) or
lifestyle (a burrow) but without any indication of the organism's actual body.
Earth did not always support life. In Chapter 18, I describe the very early earth as a life-
less, hot, atmosphere-free planet in the early years of the solar system's formation. It
took billions of years before simple, single-celled organisms appeared, and their origins
are still a scientific mystery.
Simple, single-celled life ruled earth for many millions of years before the
Cambrian ex-
plosion.
Chapter 19 describes this period of relatively sudden increasing complexity and
abundance of life on earth, as well as the millions of years that followed when life was
lived almost entirely in the oceans until amphibians emerged on the land.
Chapter 20 delves into the Age of Reptiles, when dinosaurs ruled the earth and reptiles
filled the skies and seas. At the end of this period, all the earth's continents were coming
together like puzzle pieces to create a supercontinent called
Pangaea.
Evidence for
Pangaea is still visible in the coastal outlines of South America and Africa — indicating
where they used to be attached as part of the supercontinent.
In relatively recent time, geologically speaking, mammals took over from reptiles to rule
the earth. The Cenozoic era (beginning 65.5 million years ago), which we are still experi-
encing, is the most recent and therefore most detailed portion of earth's history that can
be studied in the geologic record (the rocks). All the geologic features of the modern
earth, such as the Grand Canyon and the Himalayan Mountains, were formed in this
most recent era. In Chapter 21, I describe the evolution of mammal species (including
humans) and the geologic changes that occurred to bring us to today.
At various times in the history of earth, many different species have disappeared in
what scientists call
mass extinction events.
In Chapter 22, I describe the five most dra-
matic extinction events in earth's history. I also explain a few of the common hypo-
theses for mass extinctions, including climate change and asteroid impacts. Finally, I ex-
plain how the earth may be experiencing a modern-day mass extinction due to human
activity.
