Global History
Here is an overview of our universal history starting
from the big bang, as science could discover.
Most of this information was extracted from various Wikipedia
articles (before noticing there is a short
and a long
summary
of the history of life on Earth; something about cosmic
inflation is from my understanding of general relativity).
The Universe is about 13.75 billion
years old, starting with the "Big Bang" (a period of
expansion starting by initially extreme and decreasing densities and
temperatures). Current theories cannot account for any exact origin
of time, but give a good description of what happened after a very
short time (small fraction of a second) after it.
Cosmic Inflation: the first
process of observable importance, that swept away all observable
traces of what could happen before. In this period, most of the
(very high) energy density (or mass density, which is the same by
E=mc2) was of a form characterized by a high negative pressure. This
negative pressure has a gravitational effect of accelerating the
universal expansion in a roughly exponential manner, according to
general relativity, multiplying the size of the universe by large
numbers.
(The naive idea that, without gravitation, a positive pressure
accelerates expansion like in an explosion, while a negative one
would slow it down, only applies to a limited system surrounded by
void, with its boundaries accelerating inwards or outwards by the
difference between the internal pressure and the external void; such
an effect cannot apply to the big bang because of the uniformity of
the universe that has no such orientation of inwards and outwards;
instead, only gravitation applies, as it is not a force but a
determination of the space-time curvature).
This form of energy has the property of keeping its density during
expansion, rather than diluting it (just as an elastic gathers
potential energy during its extension, or a bubble keeps its surface
density of capillar energy while expanding; this energy needs not
come from anywhere because the conservation of energy only applies
locally, while the universal expansion is a global process).
The precise field/particle responsible for the inflation has not
been identified yet by particle physicists; still, predictions made
out of known physical principles applied to the inflation hypothesis
have already been verified by different observations. This explains
the approximate uniformity of the universe at the largest scales
(that became too far away for causally connecting and regularizing
later), and the small inhomogeneities at the origin of the large
scale structures of the cosmos (collapse of matter into galaxies and
galaxy clusters).
Inflation ends when all that energy converts into "ordinary"
particles and forms of energy, with positive pressure (photons and
speedy particles), thus slowing down again the expansion and
diluting the energy density faster than the density of ordinary
particles (photons lose their energy by Doppler effect, while speedy
particles reduce their speed by the relativity of speed between
different regions).
Big Bang nucleosynthesis. Starting 3 minutes after
the big bang (when the temperature was low enough to not
immediately break away any composite nucleus), lasting 17 minutes
(until the temperature was too low for fusion to occur - to be
compared with the nearly 15 minutes of mean lifetime of free
neutrons): Nuclei heavier than hydrogen formed, leaving about
1/4 th of the number of
nucleons (or mass of
ordinary matter if we forget
other forms of energy : dark matter, photons, neutrinos, and
kinetic energy) into Helium-4,
while about 3/4 remained as Hydrogen-1.
Recombination (380,000
years after the big bang): The temperature of 3000 K is cool
enough to let electrons and nuclei form atoms. The space becomes
transparent to radiation, releasing what is now the Cosmic
Microwave Background (whose temperature decreased to the current
value of 2.725 K by Doppler effect during expansion). Ordinary
matter at that time (hydrogen and helium) was about 4×10-22
times the mass density of water, that is about one atom per 5 mm3.
Reionization (400 million
years after Big Bang). New sources of energy appear and break
again atoms into nuclei and electrons, but the density is now
low enough to leave the space rather transparent. These can be
quasars (matter falling into galactic black holes) and/or
Population III stars (very massive stars, thus with
short lifetime, that were the
only ones which could be formed first, in the absence of the
heavy atoms).
Oldest known star of
the Milky Way: 13.2 billion years ago (500
million years after the Big Bang).
Globular clusters formed
about 12.7 billion years ago (1 billion years after the big bang).
Thin Disk of the Milky
Way (8.8 ± 1.7 billion years ago)
Formation of the Solar System
began 4.57 billion years ago, from a big molecular cloud, after one
or more massive star(s), with thus short lifetime, first formed and
exploded in supernovae, giving heavy elements and compressing the
region of the cloud, making possible the creation of the solar
system.
Moon's formation 4.527 ± 0.010 billion years
ago, probably by a giant impact between the Earth and a Mars-size
planet, that also inclined the rotation axis of the Earth and gave
the Earth a very fast rotation (a day of 6 hours instead of 24).
This rotation later slowed down by transmission to the Moon's orbit
(initially close to the Earth) through tidal interaction.
Late Heavy Bombardment:
period of intense meteorite impacts (began about 4.1 Ga, and
concluded around 3.8 Ga). From Wikipedia: "no consensus yet exists
as to its cause. One popular theory postulates that the gas giant
planets migrated in orbit at this time, causing objects in the
asteroid belt and/or Kuiper belt to be put onto eccentric orbits
that reached the terrestrial planets.
Note that the Sun's luminosity progressively increases in time. It
is now 30% brighter than it previously was. This needs to be
balanced by other processes, mainly a decrease of atmospheric
concentrations of CO2 and other greenhouse gases (which were
initially abundant), to make it possible for water to keep existing
in liquid form meanwhile. This will mainly take place in 2 ways:
deposit of calcium carbonate (CaCO3) in the oceans, and
photosynthesis.
Last universal common
ancestor between all current living forms on Earth (Bacteria,
Archaeas, animals and plants; except viruses): some 3.5 to 3.8
billion years ago
Photosynthesis started about
3.5 billion years ago
Cyanobacteria making
oxygenic photosynthesis (producing O2), may have appeared 3 billion
years ago (or between 2.8 and 3.7 billion years ago).
Great Oxygenation Event (2.4 billion years ago): The oxygen
produced by cyanobacteria, could finally remain in the atmosphere,
after organic matter and dissolved iron were saturated and could no
more capture it. This resulted in a massive extinction of anaerobic
organisms for which oxygen was toxic; but also in the appearance of
an ozone layer that would open the possibility for life outside the
ocean; and the possibility to get more energy for organisms able to
use O2 in their metabolism.
Huronian glaciation (2.4 to
2.1 billion years ago): the Earth was covered with ice, which may be
due to the disappearing of methane (consumed with oxygen).
Eukariotic cells appeared
1.7-2 billion years ago by integrating bacteria that could use O2
for metabolism, in the role of mitochondria.
First multicellular organisms
(1 billion years ago) while the lineages of animals, fungi and
plants were separated; molecular evidence suggests that fungi
colonized land at that time (while procariotes had done it already
around 2.6 billion years ago). Plants started photosynthesis by
integrating cyanobacteria in the role of chloroplasts.
(Ma = million years ago)
More Snowball Earth periods
(intense glaciations) would have occured around the times of 750,
710 and 640 M y
Cambrian explosion : life seemed to complexify a lot around
530 Ma. This would include the development of complex eyes, shells,
skeletons (with the emergence of vertebrates) and exoskeletons -
unless these were what made possible a better preservation of
fossils, that our ability to detect the presence of life diversity
depends on. The earliest fossil crustaceans date from about 513 million years ago
Oldest fossils of land fungi and
plants date to 480–460 M y
Arthropods on land around
530-450 M y. (Arthropods were well pre-adapted to colonize land,
because their existing jointed exoskeletons provided protection
against desiccation, support against gravity and a means of
locomotion that was not dependent on water).
Ordovician–Silurian extinction event (End Ordovician or O-S)
(450-440 M y, at the Ordovician-Silurian transition). Two events
occurred that killed off 27% of all families and 57% of all genera.
Together they are ranked by many scientists as the second largest of
the five major extinctions in Earth's history in terms of percentage
of genera that went extinct.
The first tetrapods evolved
from fish (380 to 375 M y)
Late Devonian extinction (360-375 M y) near the
Devonian-Carboniferous transition. A prolonged series of extinctions
eliminated about 19% of all families, 50% of all genera and 70% of
all species.
Plants evolved seeds (360
M y) which dramatically accelerated their spread on land.
The Karoo Ice Age (360 to
260 M y, named after the glacial tills found in the Karoo region of
South Africa where evidence for this ice age was first clearly
identified). The Earth during this time was covered with an immense
degree of vegetation compared to earlier times, causing a long term
increase in planetary oxygen levels and reduction of CO2
levels that resulted in this ice age.
The amniotic egg
evolved (340 M y), which could be laid on land, giving a survival
advantage to tetrapod embryos. This resulted in the divergence of
amniotes (most terrestrial vertebrates) from amphibians.
Divergence of amniotes,
between the Synapsids
(ancestors of mammals, also called "mammal-like repliles"), which
started to dominate, and the Sauropsids (other reptiles) 310 M y.
Supercontinent Pangea formed
300 Ma (?) (after a long story of continental drifts alternating
supercontinents and separations of continents).
Sauropsids
split: as for those still existing today, the ancestors of
turtoises diverged first, then quite later (at the end of the
Permian period) came a split between Lepidosauromorpha (ancestors of
lizards and snakes) and archosaurs.
Marine reptiles from
different origins started developing and will take an important
place until the Cretaceous extinction (65 M y).
Permian–Triassic extinction event (End Permian - 250 M y at
the Permian-Triassic transition). Earth's largest extinction killed
57% of all families and 83% of all genera (53% of marine families,
84% of marine genera, about 96% of all marine species and an
estimated 70% of land species). On land, it ended the primacy of
Synapsids. The cause of this extinction remains unclear.
Archosaurs split soon after,
between Avemetatarsalia (ancestors of pterosaurs and
dinosaurs, and thus of birds) and Crurotarsi (ancestors of
crocodiles).
Dinosaurs (230 M y) appeared
by diverging from other Archosaurs.
Pterosaurs (220 M y), earliest vertebrates known to have
evolved powered flight, appeared. Pterosaur fossils have been found
on every continent. At least 60 genera of pterosaurs have been found
to date, ranging from the size of a small bird to wingspans in
excess of 10 metres (33 ft).
Triassic–Jurassic extinction event (End Triassic) - 205 M y at
the Triassic-Jurassic transition. About 23% of all families and 48%
of all genera (20% of marine families and 55% of marine genera) went
extinct. Many of the dinosaurs were spared and soon became dominant
among the vertebrates, as most of the other groups of early
archosaurs (like aetosaurs, ornithosuchids, phytosaurs, and
rauisuchians) were killed. These losses left behind a land fauna
mainly made of crocodylomorphs, dinosaurs, mammals, pterosaurians,
and turtles. Mammalian were small, but their lines began to separate.
Pangaea broke up (180 - 200
M y) into Laurasia on the north, and Gondwana on the south.
Gondwana broke up (167 M y)
into East Gondwana and West Gondwana.
Archaeopteryx, a dinosaur
traditionally considered one of the first birds (probably close to
their ancestors but not among them), lived around 150 M y
Flowering plants : first
evidence to 132 M y
Eutherians (ancestors of
placental mammals, which is the main branch of mammals) diverged
from metatherians (ancestors of marsupials), while prototherians
(ancestors of monotremes) had already diverged before. The earliest
known fossil eutherian, was found in Asia, and is dated to about
125 M y.
Confuciusornis (125 to 120
M y) is a genus of primitive crow-sized birds, more advanced than
Archaeopteryx (same remark).
West Gondwana split (130 -
110 M y) into South America and Africa, opening the South Atlantic
Ocean
East Gondwana split between
(India-Madagascar-Seychelles) that began to move northward, and
(Australia-Antarctica-New Zealand), but some connections with Africa
will still exist later.
Later, competition with birds drove many pterosaurs to extinction
and the dinosaurs were probably already in decline, when came:
Cretaceous–Tertiary extinction event (End Cretaceous or K-T
extinction, 65 M y), which may have been caused by the impactor that
created Chicxulub Crater
on the Yucatán Peninsula. About 17% of all
families, 50% of all genera and
75% of species went extinct, including Pterosaurs, all non-avian
dinosaurs, most avian dinosaurs, and many other animals.
This left the space for mammals to diversify and grow larger.
Last common ancestor of primates:
63 M y, as Strepsirrhini (which are the only primates of Madagascar,
also present in South-East Asia and Africa) split from the main
branch.
Laurasia split (about 60
M y), separating Eurasia from (Greenland + North America).
Indian plate collided with Asia
(45 M y) while Australia separated from Antarctica
Split between humans and
chimpanzees at 5 or 7 M y
The Quaternary glaciation, or
current ice age, marked the start of the Quaternary period,
about 2.58 million years ago when the spread of ice sheets in the
Northern Hemisphere began. Since then, the world has seen cycles of
glaciation with ice sheets advancing and retreating on 40,000- and
100,000-year time scales.
Homo genus appeared then
(while the ancestors of the common chimpanzee and the bonobo split from each
other).
Quaternary extinction and Holocene extinction: the
Quaternary period saw the extinctions of numerous predominantly
larger species (megafauna), many of which occurred during the
transition from the Pleistocene to the Holocene epoch (around
12,000 years ago). Among the main causes hypothesized by
paleontologists are natural climate change and overkill by humans.
Homo erectus migrated from
Africa around 2.0 million years ago, and dispersed throughout much
of the Old World, especially in Asia, until they probably went
extinct about 70,000 years ago.
Homo neanderthalensis lived
in Europe from about 400,000 to 30,000 years ago. They were strong
hunters, mainly (but not completely) carnivorous, thus more
depending on fauna for their subsistence than homo sapiens.
The last glacial period (most recent glacial period within
the current ice age) started approximately 110,000 years ago.
The Toba supereruption occurred between 69,000 and 77,000
years ago at Lake Toba (Sumatra, Indonesia), and it is recognized as
one of the earth's largest known eruptions. This supervolcanic event
may have plunged the planet into a 6-to-10-year volcanic winter
A bottleneck in human evolution
probably resulted from this eruption. The homo sapiens population
was reduced to a group of 10,000 or even a mere 1,000 breeding pairs
in East Africa. An important final step of the cultural development
of homo sapiens, as seen in the more sophisticated technology and
artwork, happened in that period (between 100,000 and 50,000 years
ago).
Homo sapiens conquered the world
in several waves quickly after this bottleneck, causing an
extinction of Neanderthals on their way (except for a little
interbreeding between roughly 80,000 and 50,000 years ago in the
Middle East, resulting in 1–4% of the genome of people from Eurasia
having been contributed by Neanderthals).
End of the last glacial period
10,000 years ago.
Other works : foundations of mathematics and physics, other topics - metaphysics