Page 58 - Ultimate Visual Dictionary (DK)

PREHISTORIC EAR TH

The changing Earth

THE EARTH FORMED FROM A CLOUD OF DUST and gas drifting Small mammals
through space about 4.6 billion years ago. Dense minerals sank to appeared (e.g.,
Crusafontia)
the center while lighter ones formed a thin rocky crust. However,
Dinosaurs
the first known life-forms—bacteria and blue-green algae—did became
not appear until about 3.4 billion years ago, and it was only about extinct
700 million years ago that more complex plants and animals
began to develop. Since then, thousands of animal and
Global
plant species have evolved; some, such as the dinosaurs, mountain
survived for many millions of years, while others died building
occurred CRETACEOUS
out quickly. The Earth itself is continually changing.
Although continents neared their present locations
about 50 million years ago, they are still drifting Multicellular soft-
bodied animals
slowly over the planet’s surface, and mountain
appeared (e.g.,
ranges such as the Himalayas—which began to worms and
form 40 million years ago—are continually jellyfish)
being built up and worn away. Climate is also
subject to change: the Earth has undergone Shelled
a series of ice ages interspersed with invertebrates
warmer periods (the most recent appeared (e.g.,
trilobites)
glacial period was at its height
about 20,000 years ago).
Marine plants
flourished

Land plants ORDOVICIAN CAMBRIAN PRECAMBRIAN TIME
appeared
(e.g., Cooksonia)

Unicellular
organisms appeared SILURIAN DEVONIAN
(e.g., blue-green
algae)

Amphibians
More complex appeared (e.g.,
types of algae Ichthyostega)
Earth Vertebrates appeared
formed
Coral reefs appeared (e.g.,
appeared Hemicyclaspis)
GEOLOGICAL TIMESCALE
MILLIONS OF
YEARS AGO (MYA)
4,600 570 510 439 409 363 323 290
MISSISSIPPIAN PENNSYLVANIAN
(NORTH AMERICA) (NORTH AMERICA)
CAMBRIAN ORDOVICIAN SILURIAN DEVONIAN CARBONIFEROUS
PRECAMBRIAN TIME PALEOZOIC

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