This timeline is broken into the three following parts:
Part A) The formation of the earth and the changes in the earth’s physical characteristics that made it
hospitable to life.
Part B) The development of cellular life.
Part C) The development of multicellular life. Here we consider the development of our ancestors: worms,
fish, amphibians, reptiles, mammals and apes. We will also consider the development of some of our non-
ancestral cousins including: plants, insects, dinosaurs and birds.
Part A) This section recounts the noteworthy occurrences that took place during the formation of the earth.
More specifically it outlines the changes in the earth’s physical characteristics that made it hospitable to life.
4,600,000,000 years ago: The Solar System Forms Out of the Remnants of a Supernova
Our earth, our sun and the other 9 planets in our solar system are material remnants of a tremendous
interstellar explosion – a type II supernova. Over 4.6 billion years ago this huge explosion created a shock
wave that helped to compress some of its own left over matter into tiny clumps. Eventually these clumps of
matter and gas coalesced to form the sun, the planets, the moons, the asteroids and the comets of today’s solar
The earth formed into a homogeneous (uniformly mixed) solid by the process of planetessimal accretion (it
used its gravity to attract and retain loose debris floating around in the solar system). For a long time the earth
was a red hot ball of matter. The surface eventually cooled, but as we know, even today the center is still
extremely hot and the molten, partially liquid core affects the earth’s surface in many ways (the effects are
evident in volcanoes, hydrothermal vents, and plate tectonics). The heat from the center of the earth (which is
fueled by radioactive decay as well as residual heat from the formation of the earth) is one of the two sources
of energy (the other being the light from the sun) that allowed life on earth to form and florish.
Life necessitates energy and these two forms of energy, chemical energy and light energy, were always
available on earth. Despite this other physical characteristics of the earth had to change in order to allow for
the formation of life.
4,300,000,000 years ago: As the Earth Cools Outgassing Allows for an Atmosphere and Oceans
Melting of the earth’s core due to radioactive and gravitational heating leads to its differentiated interior
composition. The heating also leads to the process of outgassing. This refers to the process where solids
under the earth’s crust would heat up and boil into gasses. These gasses then escaped out of the earth’s crust to
form the early atmosphere. This atmosphere was composed of certain atoms and simple molecules such as
water vapor, methane, ammonia, hydrogen, nitrogen, and carbon dioxide. These molecules are relatively heavy
(heavier than hydrogen and helium which, by the way float away into space because the earth’s gravity is too
weak to retain them) and so they are kept close to the surface of the earth (again in the form of an atmosphere),
because of the earth’s gravity.
Before the earth went through its outgassing period it had no atmosphere, and thus could not contain life. Life
most probably first formed underwater, but without an atmosphere it is impossible for water to condense on the
surface of the earth. This is because a liquid at room temperature will boil away at a vaccum. The pressure
created by a true atmosphere is the only way for bodies of water to remain on the surface of the earth for an
extended time. If our atmosphere today was suddenly removed, our oceans would boil off very quickly.
The creation of the ozone layer was important for living organisms as well. Water vapor molecules that made
up part of the early atmosphere underwent photodissociation after being bombarded with ultraviolet light from
the sun. The process of photodissociation causes the H20 molecules to separate into oxygen atoms and
hydrogen molecules. The oxygen atoms, because of their inherent lightness, formed the ozone layer which
later on protected early life forms from the high energy ultra violet light from the sun. This is the same ozone
layer that protects us from ultraviolet rays today.
4,000,000,000 years ago: The Earth is a Less Dangerous Place
The bombardment of the Earth by planetessimals (small planets careening around our early solar system that
added their mass to the earth’s mass during the earth’s early stages) ceases. A more peaceful earth soon
becomes home to some of the simplest precursors to life, complex organic molecules. For an understanding
of how such molecules got a start deep within the earth’s oceans please see the The Stanley Miller
Experiment. At this time the atmosphere does not contain any free oxygen. Oxygen is a reactive, destructive
molecule and if it had been present it might have impeded the formation of life.
3,800,000,000 years ago
The Earth’s crust becomes solid. The oldest rocks found and carbon dated by man appear around this time.
3,800,000,000 years ago
Water vapor found in the atmosphere, that originated from the outgassing of the earth condenses into water to
form the earth’s early oceans. Frozen water from the icy comets that bombarded the earth, melt into liquid to
supplement these oceans. The precursors to cells- self-replicating RNA molecules appear around this time.
To read more about self-replicators click here.
Part B) This section describes how cellular life developed from interactions between carbon, hydrogen and
3,500,000,000 - 2,800,000,000 years ago: We Have Cells, Thus We Have Life
The first cells appear around this time and a major distinction is drawn at this point. According to convention
nothing is referred to as living matter unless it is a cell or composed of cells. Many combinations of complex
molecules that led up to the formation of cells might have seemed life like but were not alive until they formed
the first cell. The complex macromolecules, the amino acid chains and the protein strands, even the self
replicating RNA molecules that led up to the formation of these first cells are not considered living matter.
Therefore this time period saw the formation and evolution of the first life forms on the planet.
Our oldest fossils that are evidence for the existence of life date back to this time period. The processes that
led up to the formation of cells must have started well before this period. A great deal of scientific research is
still being conducted to understand the events that led up to the first cells.
The early cells resembled prokaryotic cells (as opposed to the more advanced eukaryotic cells which we are
composed of) and were very similar in appearance and functionality to what we know as bacteria. These
prokaryotic cells are distinguished from eukaryotic cells (which you and I are made from), because they lack
membrane bound nuclei and organelles (the functioning structures that float freely inside of the cell bodies).
Also prokaryotic DNA is not organized into chromosomes and it is generally more simplistic in structure and
function. These simple prokaryotic cells actually merged to form the first eukaryotic cells by a process
The First Cellular Organisms:
These first organisms were chemoautotrophs, this means that they supply themselves with energy from
chemicals in their environment. Unlike heterotrophs (like you and I, all animals and fungi and some types of
bacteria) they did not need to feed off of other forms of life to stay alive. Also unlike photoautotrophs (like
green plants and some bacteria) they did not need sunlight for energy either.
These organisms probably developed on the beds of the ocean floor near underwater hotspots, also known as
hydrothermal vents. Here the chemoautotrophs took energy from the chemicals in the hot waters and used
them to create and maintain their bodies. They used carbon dioxide as a source of carbon (the most important
element in living organisms) and they metabolized inorganic materials to supply themselves with energy.
These early unicellular organisms and the multicellular organisms that evolved to be able to consume them
were probably our very first cellular ancestors. Many forms of chemoautotrophic bacteria and the organisms
that feed off of them (shrimp, worms and fish) still exist today in hydrothermal vents.
The Atmosphere Gains Oxygen
Very simplistic aquatic plants evolved from the earliest unicellular organisms. Blue-green algae began to
acquire energy in a new and different way. It began to gather energy from sunlight by using a process called
photosynthesis. This algae began to release oxygen molecules into the oceans and these molecules quickly
found their way to the atmosphere above.
The accumulation of oxygen in the atmosphere increased the ozone layer and changed the atmosphere from a
chemically reducing atmosphere to a chemically oxidizing one. This new atmosphere featured a positive
feedback mechanism which quickly increased the supply of oxygen in the atmosphere. The large amount of
oxygen was toxic to some early organisms (anoxic organisms), but it also allowed other organisms that burned
energy more efficiently (by oxidation) to come about. These new organisms used oxygen in their form of
cellular respiration and it is obvious that they are our direct progenitors because our bodies are made up of
cells that undergo the same type of cellular respiration.
1,500,000,000 years ago
The first Eukaryotic organisms develop. These are single cell organisms that have membrane enclosed nuclei
and organelles. Eukaryotic organism had more evolutionary potential and because of some of their
fundamental properties allowed for a great variety of different creatures.
Part C) As multicellular organisms evolved from their unicellular ancestors they moved out of the oceans,
onto the land and even into the air
1,500,000,000 – 600,000,000 years ago
The rise of multicellular organisms. Unicellular or single celled life is no longer the only type of life on
earth. Most animals though could not survive without unicellular organisms and so even today there is still a
type of symbiosis between multicellualr and unicellular organisms.
545,000,000 years ago
The cambrian period featured an explosion of hard bodied organisms. These organisms were mostly marine
invertebrates, such as trilobites.
500,000,000 - 450,000,000 years ago
The rise of fish was seen by this time period. Fish are actually the first vertebrates and at this time they were
the most complex, most intelligent animal type on the earth. At this time fish were much simpler than many
modern day fish. For example, none of the fish at this time had yet evolved jaws. It took several million years
for fish to slowly evolve the jaw that we see them with now. Interestingly, some fish, and other fish like animals
(including lancelets, hagfish and lampreys) on the earth today do not have jaws. We ultimately inherited our
own jaws, our eye structure and our vertebrae, not to mention countless other homological structures, from
these fish. They were truly the evolutionary pioneers of our planet.
430,000,000 years ago
Waxy coated algae begin to live on the dry land. This form of algae was definitely one of the first terrestrial
forms of life on the earth. Plant life on the surface evolved from some of these first dry algae plants.
420,000,000 years ago
Millipedes are the first land animals. Millipedes along with all other insects share common marine ancestors
with humans (like sponges) but not terrestrial ancestors. Humans evolved from amphibians that moved on to
the land. Insects on the other hand evolved from arthropods that moved onto the land. Arthropods are marine
animals like crab, lobster and shrimp. The similarities between arthropods and insects are obvious, the
antennae, the segmented body, the paired and jointed limbs and the exoskeletons are unmistakably similar.
375,000,000 years ago
The Appalachians are formed due to a plate collision involving modern day North America, Africa and Europe
375,000,000 years ago
Primitive sharks appear. Sharks along other marine animals with cartilage instead of bone (rays and skates)
actually came before most fish.
350,000,000 – 300,000,000 years ago
Fish begin to form legs from their fins and lungs from their gills. One fish in particular, a lobe finned fish in
the Devonian period called the Eusthenopteron evolved the limb bones in its fins that were later necessary for
the transition to land. Early amphibians such as Ichthyostega and Acanthostega were probably closely related to
this lobe finned fish. The first of these amphibians moved onto the land very slowly and they represent the
transition from aquatic life to terrestrial life. Later, reptiles which could no longer breathe under the water
(and which are born terrestrially) evolved from these early amphibians.
350,000,000 years ago
A large variety of primitive insects have evolved on the earth’s surface.
350,000,000 years ago
Primitive ferns evolve and become the first plant to use roots.
300,000,000 – 200,000,000 years ago
Rise of reptiles. Reptiles evolve from amphibians and develop into many shapes and sizes.
300,000,000 years ago
Winged insects evolve. At this point winged insects represent the only form of life that can fly. Birds will
develop wings much later in the evolutionary history. Bats will develop wings much later still, after the rise of
280,000,000 years ago
Beetles and weevils evolve
250,000,000 years ago
The Permian period mass extinction is the largest mass extinction ever recorded. It kills many types of animals
including the majority of marine invertebrates.
250,000,000 – 210,000,000 years ago
The dinosaurs appear in the late Triassic and dominate from the Triassic, through the Jurassic and die out in the
Cretaceous. Archosaurs were a group of large reptiles that came to prominence in the Triassic. They included a
wide range of carnivores and herbivores and were the ancestors to dinosaurs, birds, crocodiles, and pterosaurs.
230,000,000 years ago
Roaches and termites evolve
225,000,000 years ago
Modern ferns and modern bees have evolved
200,000,000 years ago
Pangaea, the huge super-continent, begins to break up into the continents that we know today. First this super-
continent breaks into Gondwanaland in the south and Laurasia in the north. Laurasia went on to break up into
North America, Europe and Asia. Gondwanaland, on the other hand would break up into India, Australia, Africa,
Antarctica and South America.
200,000,000 years ago
Primitive crocodiles have evolved
200,000,000 years ago
Very small very simplistic mammals evolve from reptiles. These mammals have traits that are vastly different
from reptiles. Please keep in mind though that it took many thousand of years for these traits to evolve. It is
thought that one of the first true intermediaries between reptiles and mammals was the cynodont, a small
carnivorous reptile. Mammals developed live birth (viviparous birth), teat milk (lactation) and warm blood
(endothermy). The dinosaurs were also warm blooded. It seems that being warm blooded allowed animals to
be more active and energetic.
145,000,000 years ago
A dinosaur- the Archaeopteryx- which may have been the first flying vertebrate appears. This animal was
considered a reptile, more specifically a dinosaur, but its descendants are birds. It may or may not be obvious
to you by their movements, habits and vocal calls that birds evolved directly from dinosaurs.
136,000,000 years ago
Primitive kangaroos have evolved
120,000,000 years ago
The earliest known placental mammal, the mouse-sized Eomaia appears. Before placental mammals came
around though, two other groups of mammals were created as we diverged from reptiles- the marsupials and the
100,000,000 years ago
Primitive cranes have evolved
90,000,000 years ago
Modern sharks have evolved
65,000,000 years ago
The K-T boundary- the extinction of the Dinosaurs has taken place and mammals begin to proliferate.
Mammals were kept very small before this time as the dinosaurs truly dominated the earth before the mass
extinction that killed them out. There have actually been many mass extinctions (as observed in the fossil
record) this one was not the biggest but it killed off almost all of the dinosaurs (except for birds). Finally,
mammals had their chance to evolve from very small shrew like animals to some of the bigger mammals that
we see today. One interesting thing to note is that many of the early mammals began to look slightly different
very early on. Very early camels, horses, rhinos, sheep, pigs, dogs, and cats all looked somewhat similar at one
point in time. They were able to grow larger and they became vastly different looking as they began to fill
different ecological niches. They began to take up different spots on the food chain and began to move to
vastly different climates, this is why we see so much variety in modern mammals.
60,000,000 years ago
Rats, mice and squirrels have evolved
55,000,000 years ago
Rabbits, hares and the first primates evolve
50,000,000 years ago
Primitive monkeys evolve. The most primitive of monkeys (prosimians), which are still extant (the opposite of
extinct) today, were very similar in appearance to the other early mammals.
28,000,000 years ago
Koalas have evolved
20,000,000 years ago
Parrots and pigeons have evolved
20,000,000 -12,000,000 years ago
Chimpanzees, and hominids (our bipedal ancestors) evolve.
4,000,000 years ago
Hominids (the family of primates in which homo sapiens is the only extant species) evolve from apes after
bipedalism (the ability to walk on two feet) develops. This ability to walk on two feet was thought to be a
response to some of the factors that jeopardized the lives of early hominids. Most apes, as opposed to humans,
knuckle walk. They use all four “legs,” (their hands and feet) to move around in their environment. Most
monkeys would not benefit from bipedalism because they find most of their safety and food in trees, and they
are well adapted to tree living.
Early hominids were forced to hunt for food on the plains, away from trees. The ability to walk on two feet
allowed them to see from a higher vantage point, over tall grasses and gave them an advantage in hunting and
avoiding predators. Bipedalism also frees the hands allowing hominids to carry heavy objects, tools or
weapons long distances. Finally bipedalism is a much more energy efficient means of locomotion. Without
bipedalism, early hominids could never have gathered enough food on the plains to support their energy
consuming knuckle walking.
4,000,000 – 1,000,000 years ago
3,500,000 years ago
The Australopithecus Lucy walks the Earth.
2,000,000 years ago
Widespread use of stone tools
2,000,000 – 10,000 years ago
The most recent ice age occurs
1,600,000 – 200,000 years ago
Homo erectus exists
1,000,000 - 500,000 years ago
Homo erectus uses fire as a tool
300,000 years ago
Geminga supernova occurs and creates an explosion nearly as bright as the moon from 60 parsecs away.
200,000 – 30,000 years ago
Homo sapiens neanderthalensis exist.
200,000-50,000 years ago
Modern humans, Homo sapiens, have developed a brain that is extremely massive in comparison to their body
weight. Their brain size and cognitive capacity reflects their reliance on social interaction and teamwork and
also on complex visuo-spatial, conceptual and verbal problem solving. At this point nature had developed an
animal that was able to modify and develop its own behavior. It was probably bipedalism, society and an
opposable thumb that allowed human brains to enlarge to such a relatively huge size.
100,000 years ago
It is estimated that modern (or very close to modern) humans migrated out of Africa at this time. Before this
mass migration all Homo sapiens were located in Africa and the Middle East; in Asia there was Homo erectus
and in Europe there was Homo Neanderthalensis. It is not known weather Homo sapiens killed the Homo
erectus and Homo neanderthalensis after moving out of Africa, or if they interbred. Either way, homo sapiens
were the only ones that made it to modern times.
50,000 – 0 years ago
Homo sapiens sapiens appear.
40,000 – 12,000 years ago
Homo sapiens sapiens enter Australia from southeastern Asia. They also enter North America from
northeastern Asia by crossing the Bering Strait land bridge.
25,000 – 10,000 years ago
The most recent glaciation creates an ice sheet that covers most of the northern United States.
20,000 years ago
Homo sapiens sapiens paint the Altamira cave
12,000 years ago
Homo sapiens sapiens have domesticated dogs in Kirkuk, Iraq
10,000 years ago
The first Homo sapiens sapiens settlements pop up.
10,000 years ago
Homo sapiens sapiens learn to cast copper and harden pottery
6,000 years ago
Writing developed in Sumeria
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