Page 53 - The Restless Earth Fossils

52 Fossils

boundaries between Paleozoic, Mesozoic, and Cenozoic Eras.
Fossils show that 90% of Paleozoic species disappeared 251 mil-
lion years ago between the Permian and Triassic Periods and 65%
died 65 million years ago between the Cretaceous and Tertiary
Periods. (Remember all that Cold oysters Congeal Quickly
stuff at the end of Chapter 3? The Tertiary is the first period at
the beginning of the Cenozoic Era.) Paleontologists study these
mass extinction events carefully, hoping to understand them
so that Homo sapiens can avoid turning up on a list of extinction
fatalities.

precaMBrian eternities

So what was going on during that long stretch from when the
Earth was congealing from hot star stuff just beyond your left
fingertip to your right wrist? Life existed during much of that
Precambrian time, but it was microscopic. Sometimes, it left
little more than chemical traces in ancient rocks until about
3,500 million years ago. Then, some chains of simple plant
cells—informally referred to as pond scum—coiled together into
some amazing pillars of cells and sand called stromatolites.
Fossil stromatolites look a bit like giant concrete stalks of cauli-
flower, but they changed the course of history forever by evolving
the process of photosynthesis—a technique for using light energy
to turn water and carbon dioxide into sugars with the aid of a
“helper” molecule called chlorophyll. Stromatolites, as a result,
had to “pass a little gas.” Unlike the methane that escapes from
animals from time to time, plants release oxygen—the waste prod-
uct of photosynthesis.
The earliest living cells made energy by breaking the chemi-
cal bonds in methane and various sulfur compounds. Oxygen,
because it reacts with vital chemicals very quickly, was a deadly
poison. Stromatolites were so successful that they drove the first
living cells to “hellish” places like hot sulfur springs (like those
in Yellowstone National Park) and deep-sea hydrothermal vents

RE_Fossils2print.indd 52 3/17/09 8:59:43 AM