Geological and paleontological evidence
has allowed us to piece together a picture of how life on earth evolved from the
earliest single-celled organisms to the diversity of single- and multi-celled
organisms that have lived through history.
Figure2.1: History of Life on Earth
Mass Extinctions
The history of life on earth is punctuated by a series of mass extinctions,
during which the number of species on earth plummeted. Each of these
extinctions changed the landscape of global diversity and allowed new types of
organisms to thrive in the aftermath.
The first mass extinction occurred in the Ordovician period, about 500 million
years ago (500 mya). A second extinction occurred in the late Devonian period,
about 360 mya. This extinction paved the way for a diversification of land
plants, insects, and amphibians in the Carboniferous period. The next mass
extinction took place at the end of the Paleozoic era in the Permian period,
about 250 mya. After this extinction, marine and reptile diversity increased
greatly, eventually leading to the "Age of the Dinosaurs." The dinosaur age
crashed to a halt in the late Cretaceous period (70 mya), when the most recent
mass extinction eliminated the last dinosaurs and allowed mammals to become a
dominant form of life.
Milestones in Evolution
The first fossil evidence of life is found in the Archaen era, around 3500 mya.
During this era, prokaryote
life forms diversified, the first photosynthesis appeared, producing oxygen and
radically changing the atmosphere of the earth, thereby allowing for the
development of aerobic respiration. The first
eukaryotes do not appear
until well into the Proterzoic era, approximately 1900 mya. Around 640 mya the
first multicellular organisms appear.
The Paleozoic era shows many major changes. After the Ordovician extinction,
plants and insects begin colonizing the land. Vertebrates such as the bony fish
diversify, and amphibians begin coming out of the water in the Devonian period.
In the Permian period, about 290 mya, the earth's continents form one large land
mass called Pangaea. At this time, reptiles begin to dominate over amphibians
on land.
The Mesozoic era, commonly known as the "Age of the Dinosaurs", sees two major
changes. The first is the diversification and domination of reptiles on land.
The second is the separation of Pangaea into what will eventually become the
modern continents.
The Cenozoic era, which followed the mass extinction of the late Cretaceous
period, takes geological history up to modern times. In this era, beginning
only 65 mya, mammals and flowering plants diversify and dominate over the
previously prevalent reptiles and gymnosperms. Only in the Quaternary period,
the most recent period, do we see the evolution of humans.
