Earth is the third planet from the Sun and the only body in the Solar System
known to harbour life. It is the largest terrestrial object in the Solar
Plate tectonics was first proposed by Alfred Wegener, a German meteorologist,
in 1904. However, the idea did not catch on until the 1960's when fresh
interpretation of this fundamental mechanism began.
The Earth is a very dynamic body. Plate tectonic processes are driven by
the large amounts of internally-produced heat, changing the face of the
planet more rapidly than any other body in the Solar System. The shape of
oceans and land-masses can change completely over periods of only a few
tens of millions of years.
Water plays a vital role not only in the preservation of life, but in plate
tectonics as well. As oceanic crust is created along oceanic ridges (such
as the Mid-Atlantic Rift or the East Pacific Rise), water is chemically
combined with the molten rock materials and locked into it. The rock is
consequently much denser than rocks formed on dry land. This causes oceanic
plates to subduct beneath continental plates when they collide, which induces
melting of the rock, rising of the magma, and eruption of the water-laden
rocks onto the surface, releasing large amounts of water back into the atmosphere.
Geologists have been able to study the Earth in great detail - it being
our home planet helps! Consequently, a detailed history of the Earth has
been built up, especially since the 1960's, to which the geological timescale
is central. In the table below, time is measured in millions of years before
the present (Ma). (Source: Harland et al., 1989)
- As in the late Jurassic the global environment
was warm and balmy in the Early Cretaceous, with the general drying
of the climate slowly progressing.
- Conditions remained good for plants, with tropical
and temperate conifer forests remaining in abundance, along with
some dryer open plains.
- The continents continued to spread apart, with
new seas and oceans appearing. These included the Atlantic, which
opened some time after 130 Ma.
- The separation of the continents caused the
isolation and subsequent independent evolution of many groups of
animals and plants that were iniitially common between them.
- Two giant dinosaurs evolved in South America;
the largest [titanosaur] sauropod, Argentinosaurus, and
the largest [allosaur] theropod, Giganotosaurus.
- At ~100 Ma, in the mid-Cretaceous, the global
climate began to cool, perhaps due to the spread of the Atlantic.
The cooling process accelerated, particularly as the Cretaceous
drew to an end.
- Flowering plants (or angiosperms)
made their first appearance in the late Cretaceous as small shrubs
and weeds. They proved to be a milestone in plant evolution as reproduction
via flowers enabled plants to evolve more quickly; by the end of
the period the broad-leaved angiosperms dominated forest canopies,
as they do today.
- The tyrannosaurs enjoyed most success
as a group in the late Cretaceous, with the well-known Tyrannosaurus
rex evolving only ten million years or so before all dinosaurs
- The ichthyosaurs began to decline, and became
extinct ~90 Ma. The plesiosaurs survived until the K-T extinction
event, as did a new group of marine reptiles that evolved ~95 Ma,
the mosasaurs. Some sea turtles became huge; 4.5m long
in some cases.
- Some pterosaurs also became huge, with wingspans
of 9+ metres. Such animals may have been able to migrate large distances.
However, they declined in number towards the end of the period,
with only a few species living beyond ~70 Ma. The remainder perished
in the extinction at the end of the Cretaceous. BIRDS......
- Mammals began to increase in number (although
they remained morphologically small), particularly as the Cretaceous
drew to a close - maybe because they were well adapted to cooler
- At the end of the Cretaceous period an extinction
event occurred which wiped out approximately 40% of all animal families,
including the dinosaurs, pterosaurs, most marine and aquatic reptiles
(except the crocodilians and turtles), some arthropods such as the
ammonites, and some fish. Most groups that survived were
badly affected and took some time to recover in the Tertiary period.
The extinction seems to have taken place over a period of 2-3 million
years, beginning at ~67 Ma, and was most likely initiated by the
global cooling of the climate. However, the impact of an asteroid
at 65 Ma - the crater of which sits on the edge of the Yucatan peninsula
- finished off the process. The fallout from the impact left a thin
tell-tale layer of rock containing iridium around the world, which
marks the K-T (Cretaceous-Tertiary) boundary.
- Pangaea continued to break up......
- As the continents separated the climate became
warm and more humid, and monsoons would have periodically swept
across lowland areas, creating large river basins and seasonal floods.
- Conifer forests dominated polar regions, and
the tropics were ruled by conifers, ferns and cycads. Vegetation
was clearly lush, prospering in the more tropical climate, as extensive
coal beds were laid down in the Jurassic.
- Dinosaurs became the dominant land vertebrates.
Being highly adaptable and diversifying hugely they spread to all
continents, becoming a highly successful group.
- The pterosaurs (flying reptiles) also diversified
and enjoyed great success in the Jurassic.
- The largest fish ever known evolved in the
Jurassic. Leedsichthys, which was a 25m long filter-feeding
teleost fish, evolved alongside rather less friendly giant
carnivorous pliosaurs such as 25m long Liopleurodon
- the largest marine predator ever known. Marine crocodilians also
- The first true mammals appeared at ~205 Ma
in the early Jurassic, probably evolving from the cynodonts.
- The climate in the tropics became somewhat
drier in the mid-Jurassic, and arid conditions spread in many areas.
- The drying of the climate resulted in extensive,
almost savannah-like open areas appearing, but it was not dry enough
to cause desertification.
- At around this time, ~190 Ma, a herbivorous
dinosaur group called the sauropods appeared. They became
a large group, particularly in late Jurassic and early Cretaceous
times, and produced the largest land animals the world has ever
known - including the longest (among the diplodocids),
and the tallest and heaviest (among the brachiosaurids).
- All of the large [land-dwelling] carnivores
from the early Jurassic until the end of the Cretaceous were members
of the bipedal theropod group. The first tyrannosaurs
appeared in the mid- to late-Jurassic.
- The Earth remained hot and dry, although rainfall
sustained much plant life. Even at the poles, temperatures remained
- Plant life was globally represented predominantly
by ferns, gingkos and primitive conifers.
- There were few remaining large predators or
many competitors for various niches, which meant that a few well
adapted species came to represent rather high percentages of all
vertebrate life on land. Due to the lack of variety and to take
advantage of the available niches, animal evolution accelerated.
This resulted a wide range of new species and genres.
- Among these were aquatic/marine reptiles (the
sauropterygia), which came to dominate Mesozoic seas. These
included the nothosaurs and more fish-like ichthyosaurs.
- In the Late Triassic, Pangaea began to break
up into smaller continents; initially Laurasia and Gondwana,
separated by the Tethys ocean. The intense volcanism associated
with this event may have played some role in the Triassic-Jurassic
- The previously dominant therapsid ('mammal-like')
reptiles declined - apart from the cynodonts, which went
on to become true mammals.
- A new group of reptiles called the archosaurs
rose to dominance at ~240 Ma, whose success would last throughout
the Mesozoic era. The group included the dinosaurs, pterosaurs
(or 'flying lizards') and crocodilians.
- The dinosauria consisted of two major groups;
the saurischia (lizard-hipped) and ornithischia
(bird-hipped). Among the dinosaurs, the prosauropods (mostly
herbivores) and theropods (carnivores) emerged in the Late
- An extinction event at the end of the Triassic
affected life in the oceans particularly badly: many invertebrate
groups suffered great losses, the conodonts became extinct
and all marine reptiles apart from plesiosaurs and ichthyosaurs
disappeared. Life on land fared better, although several groups
of small reptiles, primitive dinosaurs and some amphibians died
- Cool, dry conditions pervaded at the beginning
of the Permian.
- Ferns and primitive conifers quickly moved into
open areas left by retreating tropical forests.
- Amphibians survived the change in climate but
were not well suited to the new conditions. The reptiles fared rather
better and, forced to innovate in the cool climate, rose to dominate
life on land.
- Late in the Permian period the continents all
met up to form a supercontinent called Pangaea, parts of
which extended to the poles.
- The climate became hot and dry, leading to
extensive desert environments.
- A group of 'mammal-like' reptiles, the therapsids,
adapted well to the desert conditions and survived into the Triassic
period (and beyond).
- By the close of the Permian the climate
had become extreme. This, combined with the problem of a single
landmass (and therefore a limited set of ecosystems), caused conditions
for life to worsen. Intense competition amongst organisms of all
kinds in the limited habitats available began to result in many
extinctions. Eventually, 96% of all marine life and over 70% of
land vertebrates became extinct; the most severe extinction event
in the history of life on Earth.
- The Earth had a globally hot, humid climate
that extended into the Arctic and Antarctic circles.
- The atmospheric concentration of oxygen was
very high, allowing the evolution of giant arthropods.
- Densely forested swamps dominated lowland areas;
lycopsid trees (up to 50m tall), tree-sized ferns and horsetails
were common. Huge coal deposits were laid down during the Carboniferous
due to prolific plant life.
- Amphibians were ideally suited to such humid,
swampy environments and thrived, dominating vertebrate life on land.
- Reptiles first appeared ~340 Ma. They
are adapted to be less reliant on wet environments than amphibians,
and have a great advantage in the ability to lay their eggs away
from water. However, they were not particularly successful until
the Permian period.
- Sharks and bony fish dominated life in the
oceans, and coral reefs began a slow recovery.
- An ice age began at the end of the Carboniferous
at ~290 Ma, with ice caps extending from both poles.
- The ice age dramatically altered climatic
conditions around the world, forcing a retreat of tropical forests
and their associated fauna, leading to the extinction of many species.
- Pioneer vegetation soon developed into primitive
forests that followed rivers and estuaries. They included the first
- Millipedes and the precursors to spiders, the
trigonotarbids, dominated land-dwelling animals.
- In the sea, the evolution of the jaw in fish
lead to a rapid increase in predatory varieties; giant placoderms
(armoured fish, including the 8m-long Dunkleosteus) evolve
alongside the first sharks and teleost (bony) fish. Sharks
would prove to be more successful than the placoderms, and may have
forced their extinction (by 355 Ma, in the early Carboniferous).
- Some [jawed] fish make the move onto land,
and the first amphibians evolve.
- Towards the end of the Devonian period a
series of extinction events (or one prolonged event) occurred, beginning
at ~364 Ma. This lasted for approximately 3 million years and primarily
affected life inhabiting warm, shallow seas. Trilobites fared badly
but a few survived until the Permian extinction. Reef-building organisms
were particularly badly affected and did not recover fully until
the Mesozoic era. -> LINK
- Arthropods continued to dominate sea life.
Some became capable of briefly emerging onto land. Complex reef
systems developed in the tropics.
- Towards the end of the Silurian, small plants
and fungi pioneered the colonisation of land - but remained near
to streams and rivers.
- Permanently land-dwelling arthropods soon followed
(millipedes were relatively common). Most were herbivores, a few
- At ~420 Ma, fish evolved armour plating and
a movable lower jaw.
- Animal life remained in the sea, dominated
by the arthropods. Trilobites were numerous. The first
- Land remained largely barren. Some lichens and
slime moulds inhabited river and stream banks.
- Extinction event, perhaps due to global
- Gondwanaland glaciation ??? - --> LINK
- Animals evolved hard parts, aiding fossilisation
and therefore also our knowledge of such forms of life.
- The complex eye also evolved. This is often
credited with increasing the rate of evolution of animals as complex
predator-prey relationships resulted.
- Life remained confined to the sea, but several
major animal groups emerged, including arthropods, molluscs,
echinoderms and vertebrates (in the form of primitive
- Complex multi-cellular (but soft-bodied)
life emerged in warm, shallow coastal waters following the thaw
at ~600 Ma. Evidence of this stage of life exists as Ediacaran fossils.
- There is much evidence that from ~750 Ma until
~600 Ma a severe ice age occurred in which the average global temperature
dropped to -40°C and all the oceans froze over - the controversial
'Snowball Earth' theory.
- Eukaryotic (complex-celled) life evolved
- Not long after, simple multi-cellular organisms
appeared such as jellyfish-like and filter feeding sponge-like animals.
- By 2,000 Ma the atmosphere contained a persistent
level of oxygen (i.e. it was being produced in sufficient quantities
that it could not all react with the environment), some of which
formed an ozone layer.
- Cyanobacteria evolved sometime before 3,500
Ma. This form of life could extract energy from sunlight (photosynthesis).
The first fossil evidence of life dates from this era; colonies
of cyanobacteria form stromatolites.
- Life first appeared ~3,850 Ma in the primitive
oceans, existing as simple prokaryotic bacteria.
4560 Ma to
|Basin Groups 1-9
Atmosphere, climate and ecosystems
Precambrian organism evolution
Sea-dwelling animal evolution
Land-dwelling animal evolution
Life probably evolved spontaneously on Earth, but many astronomers are
convinced that it is extra-terrestrial in origin. Many outer Solar System
natural satellites contain large amounts of ice consisting of not only water
but also hydrocarbons. Spectral analysis of material in cometary tails and
comas has also revealed the presence of hydrogen, carbon, oxygen and nitrogen,
which could have reached Earth very easily.