BIRTH
OF A PLANET :
EARTH
FORMS
Earth grew from a cloud of dust and rocks surrounding the
young Sun. Earth formed when some of these rocks collided.
Eventually they were massive enough to attract other rocks
with the force of gravity, and vacuumed up all the nearby
junk, becoming the Earth, around 4.56 billion years
ago. The Moon probably formed soon after, when a
planet-sized chunk of rock smashed into the Earth and
threw up a huge cloud of debris. This condensed into the
Moon.
PLANETARY
CONDITIONS STABILIZE AND WE
GET AN EARLY ATMOSPHERE
It took 300 million years for the planet to be cool
enough for water to start condensing in liquid form.
Cataclysmic meteorite bombardment is ongoing, bringing
many molecules and chemicals to the surface, in violent
fashion. Earth's atmosphere becomes mostly carbon
dioxide, water vapor, methane, and ammonia. Formation of
carbonate minerals on the surface starts reducing
atmospheric carbon dioxide.
ORIGIN
OF LIFE : FIRST ORGANISMS
Nobody knows exactly when life began. The oldest confirmed
fossils, of single-celled microorganisms, are 3.5 billion years
old. Life may have begun a bit earlier than that, but probably not
while huge rocks were still raining down on Earth. Life may have
begun in warm alkaline vents on the seabed, harnessing energy
from chemosynthesis- inorganic molecules (such as hydrogen
sulfide) or methane are combined with an oxygen source (in this
case seawater) to create simple sugars. Miller-Urey
experiments have shown it’s possible to get self-replicating
organic molecules to form from inorganic ones, with lightning as
the catalyst in an atmosphere like Early earth had, showing that
this step likely came first.
LIFE HARNESSES THE POWER OF THE SUN:
EARLY PHOTOSYNTHESIS BEGINS
All life needs energy to survive, and the biggest source of
energy for life on Earth today is the Sun. Some of the
earliest microorganisms evolved the capture of energy from
sunlight. But unlike green plants today, the first
photosynthesizing organisms did not release oxygen as a
waste product, so there was still no oxygen in the air.
Cyanobacteria were the first microorganisms to release
oxygen during photosynthesis. They were able to form small
colonies of individual cells that could share resources,
becoming abundant.
Cyanobacteria colonies.
THE BEGINNING OF PLATE TECTONICS :
CONTINENTS FORM
Today, Earth's surface is divided into a few dozen plates of
rock, one of which sometimes ploughs under another to be
destroyed in the planet's molten heart. This process, called
plate tectonics, is thought to have begun around 3 billion
years ago. Only when plate tectonics had come into
operation could the first continent, nicknamed 'Ur', come
into being.
THE GREAT OXIDATION EVENT:
FREE OXYGEN ACCUMULATES IN
THE
ATMOSPHERE
For the first half of Earth's history, there was hardly any oxygen in
the air. But then some bacteria began harnessing sunlight to make
sugar from carbon dioxide and water, just like green plants today.
These microbes pumped out oxygen as a waste product, creating
the oxygen-rich atmosphere we have today. Anaerobic organisms
are poisoned by oxygen, likely causing a mass extinction. The first
oxygen is theorized to have been the cause of the entire planet
freezing over into a 'Snowball Earth', by stripping the greenhouse
gas methane from the air- this first ice age lasted ~300 million
years.
ENDOSYMBIOSIS : RISE OF
COMPLEX CELLS
The first organisms were simple cells like modern bacteria, but
some of them became much more complex inside after the first
long ice age was over. These 'eukaryotes' developed lots of
specialized equipment within their cells. They also had a new
source of energy: sausage-shaped objects called mitochondria, or
some had mitochondria and chloroplasts, which were once freeliving bacteria, but which were absorbed in a process called
endosymbiosis. Every animal and plant you've ever seen is a
eukaryote. The complex cell with the first nucleus, common
ancestor to all plants, animals, and fungi alive today, is estimated to
have evolved 1.6 billion years ago.
ORIGIN
OF MATING : THE FIRST SEX
The fossil record looks fairly dull at first – so much so that
the period is called the 'Boring Billion'. But behind the
scenes plenty was happening. For one thing sex may have
evolved for the first time. It's not clear why, or when, some
organisms stopped simply dividing in two and started the
messy business of sex. But it was definitely going on 1.2
billion years ago: there are fossils of red algae from that
time that were clearly forming specialized sex cells such as
spores.
BEGINNING OF BIG ORGANISMS:
FIRST FOSSIL OF A MULTICELLULAR
(A WORM )
ORGANISM
For the first time, life was not just made up of single cells.
Now cells were teaming up to form larger organisms with
things like mouths, limbs and sense organs. The oldest
multicellular fossils found so far were in Gabon, of large
organisms that date back 2.1 billion years. Different
categories of organisms probably evolved multicellularity
independently, with plants managing it before animals.
FIRST BIG ICE AGE :
A FROZEN SNOWBALL
WORLD
Earth froze over, and it may have lasted as long as 300 million
years. The ice may well have stretched all the way from the
poles to the equator. There are many possible causes; eruption
of a supervolcano, reduction in atmospheric greenhouse gases,
changes in solar energy output, or changes in the Earth's orbit.
Regardless of the trigger, the climate changed, increased ice and
snow on the one massive continent at the equator, which reflected
more solar energy back to space, further cooling the Earth and
further increasing ice. Volcanoes may have had a role in
replenishing CO2, possibly ending this global ice age.
CAMBRIAN EXPLOSION: LEAPS IN BIODIVERSITY
BEGIN WITH A WARMING PLANET
The first complex organisms, weird tube- and frond-shaped
things called the Ediacarans, appeared soon after the first
major ice age. Soon after that, evolution went through a
major growth spurt. In the Cambrian Explosion, it seems
almost every group of modern animals appeared within tens
of millions of years, all still in the ocean. This apparent
'explosion' may be partly due to better fossilization, as many
animals now had hard shells, and tons of them began
appearing in the fossil record 540 million years ago.
1ST MASS EXTINCTION :
ORDOVICIAN ORDEAL CAUSES 85% OF LIFE TO
GO EXTINCT
The Ordovician period was a time when life flourished, mostly in
the sea, except some plants. But towards its end, the world cooled
dramatically and ice sheets spread from the poles. Plants colonized
land ~ 10 million years prior and CO2 in the atmosphere was slowly
depleted. The ensuing ice age is called the Andean-Saharan,
because the evidence of it comes from the Andes mountains and
the Sahara Desert. The deep freeze led to the third-largest mass
extinction on record. Most life was still confined to the sea, and
85% of marine species were wiped out. In the aftermath, fish
survived and then became much more common.
OUT OF THE SEA:
PLANTS COLONIZE
THE LAND
Some animals ventured onto land as far back as 500 million
years ago, but they only visited briefly – perhaps to lay eggs
in a place without predators. Plants were the first to take up
permanent residence on land 440 million years ago.
The first land plants were relatives of green algae, but they
rapidly diversified into mosses, ferns, and liverworts. As a
result, CO2 in the atmosphere starting going down.
2ND MASS EXTINCTION: LATE D EVONIAN
SHALLOW SEA POISONING
75% of all species on Earth died out in the Late Devonian mass
extinction, though it may have been a series of extinctions over
several million years, rather than a single event. Life in the shallow
seas were the worst affected, and reefs took a hammering, not
returning to their former glory until new types of coral evolved
over 100 million years later. In fact, much of the sea bed became
devoid of oxygen, rendering it effectively out of bounds for
anything except bacteria. Changes in sea level, asteroid impacts,
climate change and new kinds of plants messing with the soil have
all been blamed for these extinctions.
FROM
FINS TO LEGS :
FISH
WALK ON LAND
With plants well-established on land, the next step was for
animals to move out of the water. Insects were among the
first, around 400 million years ago. But they were followed
soon after by big, backboned animals such as Tiktaalik, a fish
that looked a bit like a salamander. Fish like Tiktaalik would
eventually evolve four limbs, and give rise to amphibians,
reptiles and mammals. It may be a good thing it left the
water when it did, as soon afterwards the Late Devonian
Extinction wiped out many marine animals, including some
terrifying-looking armored fish.
SUPERCONTINENT : PANGEA FORMS
For the last time, plate tectonics caused all Earth's
continents came together to form one giant supercontinent.
Known as Pangaea, it was surrounded by a world-spanning
ocean called Panthalassa. It lasted until ~175 million years
ago, when tectonics again tore it apart over tens of millions
of years. Its migrated remnants became the familiar modern
continents present today (for now!).
3RD MASS EXTINCTION
End Permian, the great dying
Just as the reptiles were flourishing, life on Earth faced
perhaps its greatest challenge. The Permian extinction was
the worst mass extinction in the planet's history, obliterating
up to 96% of marine species and similar numbers of land
animals. We don't know for sure what caused it, but massive
volcanic eruptions – creating what is now the Siberian Traps
– may have been to blame. In the aftermath, the first
dinosaurs evolved.
HAIRY
BEASTS : FIRST MAMMALS
At the same time that the dinosaurs were spreading and
diversifying, the first mammals evolved. Their ancestors
were reptiles called cynodonts, whose faces looked a little
like those of dogs and may have had fur or whiskers. Early
mammals such as Morganucodon were small and shrew-like,
and probably only active at night. This may have spurred
them to evolve warm-bloodedness: the ability to keep their
body temperature constant.
4TH MASS EXTINCTION
End of Triassic period, dinosaurs unleashed
The dinosaurs were flourishing on land, and in the sea giant
reptiles called ichthyosaurs had become the top predators.
Then another disaster struck. We’re not sure what caused
the Triassic extinction, but it killed off around 80% of species
that were alive then. In the aftermath, the dinosaurs became
the dominant land animals and eventually reached titanic
sizes. The biggest species whose mass is accurately known,
Dreadnoughtus schrani, weighed about 59 tons!
FEATHERED FLIGHT
Birds evolved from feathered dinosaurs – modern birds
are essentially Velociraptors with beaks instead of
snouts and wings instead of arms. The most famous
early bird, Archaeopteryx, lived 150 million years
ago. But in recent years slightly older fossils, such as
Xiaotingia and Aurornis, have been found in China.
PLANT REVOLUTION
This may sound strange, but flowers are a recent
invention. There have been land plants for 465 million
years, yet there were no flowers for over two-thirds of
that time. Flowering plants only appeared in the
middle of the dinosaur era. The equallyfamiliar grasses appeared even more recently. The
oldest fossil grasses are just 70 million years old,
although grass may have evolved a bit earlier than that.
THE FIFTH MASS EXTINCTION
Boom, you're extinct. 65 million years ago, a huge
chunk of rock from outer space smashed into what is now
Mexico. The explosion was devastating, but the longer-term
effects were worse. Dust was thrown into the upper
atmosphere and blocked out sunlight, and in the ensuing
cold and darkness Earth suffered its fifth and last mass
extinction. The dinosaurs were the most famous casualties,
but pterosaurs and giant marine reptiles were also wiped
out.
LIVING
IN THE TREES
Almost immediately after the dinosaurs were wiped out,
mammals evolved the ability to nourish their young inside
their wombs using a placenta, just like modern humans.
Soon, some of these early placental mammals evolved into
the first primates. They would ultimately give rise to
monkeys, apes and humans. But the first ones were small
creatures. The oldest known primate skeleton is of a species
called Archicebus achilles, which weighed no more than 30
grams. They lived in the hot and humid rainforests of Asia.
THE ROAD TO HUMANITY
The first apes appeared in Africa around 25 million years
ago. Then at some point, the group split into the ancestors
of modern humans and the ancestors of modern apes. It's
hard to say exactly when, but thanks to modern genetics
and a host of fossil discoveries, we have a rough idea. The
oldest known hominid was Sahelanthropus tchadensis, which
lived about 7 million years ago.
THE THINKING APE
Our species, Homo sapiens, is ridiculously young. We have
only existed for 1/5 of a million years. In that time we have
expanded from our African birthplace to reach every
continent, and even outer space. Our activities have
precipitated the sixth mass extinction and unleashed the
fastest episode of climate change in Earth's history. Yet we
are also the only species that has ever managed to piece
together the history of Earth and seem to have an
unprecedented ability to influence the evolution of life on
Earth. How humans will collectively use this ability is yet to
be seen.
END OF LAST I CE AGE, L ARGE MEGAFAUNA
BECOME EXTINCT
Who or what killed off North America's mammoths and
other megafauna 13,000 years ago? There are several
theories. First, that the continent's first human hunters,
fresh from Siberia over the land bridge, killed the
megafauna off as they colonized the newly discovered land.
Another theory is that climate changes at the end of the
Pleistocene epoch triggered the collapse.
#
Early Earth & Origin of Life
years ago
1
End of last Ice Age; large megafauna become extinct
10,000
2
First homo sapiens, the thinking ape
200,000
3
Road to humanity, first primate in genus Homo
7,000,000
4
Into the trees: first primates
50,000,000
5
5th mass extinction, asteroid caused climate cooling, killed dinosaurs
66,000,000
6
First flowering plants
145,000,000
7
Feathered flight
150,000,000
8
4th mass extinction Triassic-Jurassic, 80% life killed, dinosaurs unleashed
200,000,000
9
First Mammals
210,000,000
10
3rd mass extinction: global warming, the great dying; 96% of species go extinct
250,000,000
11
Supercontinent Pangea forms
300,000,000
12
Fish first ‘walk’ onto land
375,000,000
13
2nd mass extinction: Devonian, 70% of marine go extinct, shallow sea poisoning
395,000,000
14
Plants first colonize land
440,000,000
15
1st mass extinction, 85% life extinct, plants depleted CO2 in atm, global cooling
444,000,000
16
Cambrian explosion of biodiversity
540,000,000
17
Climate change, ice age, snow ball earth
850,000,000
18
First fossil of a multicellular organism (a worm)
1,100,000,000
19
First sexual reproduction
1,200,000,000
20
Endosymbiosis, rise of eukaryotes
1,600,000,000
21
First fossil of multicellular organisms
2,100,000,000
22
First Evidence of Free O2 in atmosphere
2,500,000,000
23
Beginning of plate tectonics
3,000,000,000
24
Life harnesses sun’s energy
3,400,000,000
25
First evidence of life
3,500,000,000
26
Early atmosphere forms CH4, CO2, NH4, liquid water now present
4,280,000,000
27
Earth forms
4,560,000,000
note: there are 1,000 millions in a billion