FACT
Approximately
850–630 million
years ago, a ’global
glaciation’ event is
thought to have
occurred, also
known as an
’Snowball Earth’. At
this time
temperatures
dropped
dramatically and
large parts of the
Earth’s surface were
covered in ice.
During this period it
is thought that any
life forms
dependent on light
would have gone
extinct.Proterozoic: over the next 800 million years, the earliest forms of sexual reproduction de-
veloped, thus greatly increasing the diversity of organisms. The first multicellular organisms
consisting of cells specialised to perform specific functions began to evolve. The fossils
known asSwartpuntiafirst appeared approximately 600 million years ago and went extinct
by the start of the Cambrian, 543 million years ago. Swartpunia are some of the first soft-
bodied organisms in the fossil record.Swartpuniafossils were discovered in Namibia and
are some of the oldest fossils known to exist from before the Cambrian period.
Figure 11.7: Earliest soft-bodied animals found in Namibia. A simplified reconstruction ofSwartpun-
tia.
Cambrian explosion DUMMY
From approximately 580–500 million years ago, a large diversity of creatures appeared.
This event is called theCambrian Explosion, and almost all of the animals alive today can
trace their beginnings to this rapid expansion of diversity. The Cambrian Explosion was the
relatively rapid appearance over millions of years of most of the main animal groups found
in the fossil record.
In Cambrian times there was no life on land and little or none in freshwater — the sea was still
very much the centre of living activity. From 580–540 million years ago, oxygen began to
accumulate in the atmosphere, allowing for the formation of the ozone layer which blocked
the damaging rays of the ultraviolet light from the sun, permitting the colonisation of land.
The Cambrian explosion was just the first period of thePaleozoic era. It is famous because
of the sudden explosion in diverse morphologies, and the fact that is ”sowed the seeds” for
the continued evolution that occurred throughout the Paleozoic.
Chapter 11. History of Life on Earth 325