|
by factmonster.com
It is now
generally assumed that planets are formed by the gathering and binding of
gas and dust in a cosmic cloud, but there is no way of estimating how long
this process takes. Our Earth acquired its present size, more or less,
between 4,000 million and 5,000 million years ago. Life on Earth
originated about 2,000 million years ago, but there are no good fossil
remains from periods earlier than the Cambrian, which began about 550
million years ago. The largely unknown past before the Cambrian Period is
referred to as the Pre-Cambrian and is subdivided into the Lower (or
older) and Upper (or younger) Pre-Cambrian—also called the Archaeozoic and
Proterozoic Eras.
The known
geological history of Earth since the beginning of the Cambrian Period is
subdivided into three giant chunks of time, or eras, each of which
includes a number of shorter periods. They, in turn, are subdivided into
even shorter subperiods. In a subperiod, a certain section may be
especially well known because of abundant fossil finds. Such a section is
called a formation, and it is usually identified by a place name.
Paleozoic Era
This era began
550 million years ago and lasted for 355 million years.
|
Period
|
Years
Long1 |
Subperiods |
Creatures That Appeared |
|
Cambrian
|
70 |
Early
Cambrian
Middle Cambrian
Late Cambrian |
Invertebrate sea life of many types appear and multiply during this
and the following period. |
|
Ordovician |
85 |
Early
Ordovician
Late Ordovician |
First
known fishes. |
|
Silurian |
40 |
Early
Silurian
Late Silurian |
Gigantic
sea scorpions. |
|
Devonian
|
50 |
Early
Devonian
Late Devonian |
Number of
fishes and other forms of sea life increases; land still largely
lifeless. |
|
Carboniferous |
85 |
Mississippian (Early)
Pennsylvanian (Late) |
Early
insects and first known amphibians. |
|
Permian
|
25 |
Early
Permian
Late Permian |
Early
reptiles and mammals; earliest form of turtles. |
1. In millions of years.
Mesozoic Era
This era began
195 million years ago and lasted for 135 million years. Popular name: Age
of Reptiles.
|
Period
|
Years
Long1 |
Subperiods |
Creatures that appeared |
|
Triassic
|
35 |
Buntsandstein (Early)
Muschelkalk (Middle)
Keuper (Late) |
Early
reptiles which resembled lizards. |
|
Jurassic
|
35 |
Black
Jurassic, or Lias (Early);
Brown Jurassic, or Dogger (Middle)
White Jurassic, or Malm (Late) |
Many
reptiles fit for going to sea; early large dinosaurs; somewhat
later, flying reptiles; earliest known birds. |
|
Cretaceous |
65 |
Early
Cretaceous
Late Cretaceous |
More
dinosaurs; more birds; creatures with pouches (marsupials). |
1. In millions of years.
Cenozoic Era
This era began 60
million years ago and includes the geological present. Popular name: Age
of Mammals.
|
Period
|
Years
Long1 |
Subperiods |
Creatures That Appeared |
|
Tertiary
|
c. 60 |
Paleocene
Eocene
Oligocene
Miocene
Pliocene |
First
mammals other than marsupials; formation of amber (a substance left
by a fossil), abundant insect population; early bats; steady
increase of large mammals; mammals closely resembling today's
animals. |
|
Pleistocene (popular name: Ice Age) |
1 |
Four
major glacial formations named Günz, Mindel, Riss, and Würm,
originally the names of rivers. Last glaciation ended 10,000 to
15,000 years ago |
Various
forms of early humans. |
|
Holocene
|
|
The
present |
The last
3,000 years are called “history”. |
1. In millions
of years.
Return
to Table of Contents
|