When I read about something happening 150 million years ago, I can't remember how that relates to what other things how long ago; so I'll collect together here the fragments that shall, in due course, accumulate to a time-line. For more extensive topic-specific time-lines, consult Wikipedia; for a time-line of global temperature variation, see XKCD. For sources (in so far as I'm able to trace them), follow links.
In the spirit of my page on the scale of things, I'll break this up into chunks by metric quantifier on number of years before the present (nominally 2000; and I habitually use whatever quantifier puts the number in the range from about 0.1 to about 100) in the main list. (I should arguably switch to the Holocene calendar. For events whose timing is known most accurately relative to The Big Bang, I include a separate tail-piece, presently with too few entries to warrant a separate page.) Only three quantifies show up, though: the k yr range tells the tale of humanity's development; the M yr range is the era of fossils; and the G yr range is the time-scale of cosmology. Shorter time-scales I classify as history on a page of their own; and time itself is only meaningful back to the start of the universe, so no longer time-scale is relevant here.
A century ago radio was a novel concept and not much in use. Recorded history stretches back of order ten millennia, to roughly the point where the present ice age (which stretches back at least about a hundred millennia) entered its present interglacial phase (in which only the North polar sea and the South polar continent are ice-locked). The duration of such inter-glacial interludes is generally of order ten millennia. We have archeological records stretching back significantly further than those ten millennia – though only of the things that survive for such time-scales.
Holoceneera – into their own page on history.
Boats tend not to survive in the archeological record, as (prior to the last two centuries) they're made of perishable materials: however, evidence survives of dugout canoes as far back as 8 millennia ago. This is more indicative of how long evidence of past things survives than as evidence of how long folk have been making boats: there is little reason to doubt folk have been making boats for much longer than this. Human settlements on islands (some remote, some separated from their mainlands by strong currents) long before this are hard to explain unless fairly large groups arrived roughly together. Better designs of boat are less durable (for reasons intimately connected to what's better about them): if evidence of some type reaches back as far as such evidence could be expected to survive, it does tell us folk have had them for at least that long – but doesn't tell us that folk didn't have them earlier – possibly much earlier.
I encourage the reader to consider entries below in a similar light; while some may indeed be roughly the earliest instances of what they desccribe, for others there may simply be no surviving evidence of earlier instances. Where time reliably destroys evidence of some human activity, we cannot confidently assert that folk weren't doing it before our oldest proof of them doing it – we can only assert they have been doing it for at least that long.
The Neolithic era, a.k.a. new stone age. Sources vary as to when it starts and ends. Archeologists have found buildings from at least 11 k yr ago. People began living in relatively permanent villages and domesticating crops and (other) animals. In time some villages grew and their people built larger structures that strongly suggest social organisation, although initially without signs of a hiearchy. Around 10.8 k yr ago there was a significant shift in culture accompanying technological innovations, while domesticated species start to show significant differences from their wild-type cousins. Around 8.5 k yr ago, possibly driven by a few centuries of cooler, drier climate in The Middle East, that region's larger settlements broke up in favour of smaller settlements, just as the arrival of pottery paved the way for the eventual invention of writing. This dispersal doesn't appear to have involved a major drop in total population, or the complexity and extent of trade networks in the region.
Neolithic people were honing stone blades on a boulder in Dorset's Valley of Stones.
Çatalhöyük (Çatal = fork (in a path), höyük = mound), in Anatolia, was inhabited, moving from the large mound to the small mound around 8 k yr ago, with a population fluctuating from 3 to 8 k.
Oldest archeological evidence of cats and humans cohabiting. For more solid evidence of domestication, one has to wait another 4.2 k yr. By this time, farm animals and crops show clear signs of domestication, resulting from selective breeding (whether or not the selection was guided by foresight). Then again, domestic cats seem to have arrived in Eastern Europe around 8 k yr ago. It's easy to suspect humans of (at least) colluding in how they got there.
Blinkerwall may be Europe's oldest
of stones and boulders that's suspected of having been used as an aid to
hunting, by constraining the route taken by chased animals. Rising sea levels
have since submerged it in the Baltic.
Evidence of early neolithic agriculture: a variety of fig prospering, that normally wouldn't survive, as it needs human intervention to propagate it from cuttings.
First successful human settlements in the British Isles.
If we look at the set of ancestors of each human alive today, as a function of time, going backwards, there must come a point at which these sets are all identical, known as the identical ancestors point; this is estimated to be between five and fifteen millennia ago.
The first European humans were already painting erotic pictures 40 k yr ago.
grand solar minimum, when we lost much of the protection from cosmic rays that the solar wind gives us. The combination lead to increased 14C (Carbon-14) production in the atmosphere; other results allagedly included changes in the chemical composition of the atmosphere, dramatic expansion of the arctic icesheet in North America, shifts in Pacific rain belts and southern wind belts. It is conjectured that this may have contributed to the demise of our Neanderthal cousins and various larger species of mammal and may have made caves a better place to live. The red ochre hand-prints seen around this time might even be a result of folk using red ochre as a sun-screen.
Modern humans in Russia.
Modern humans in Australia.
Estimated date of the patrilineal most recent common ancestor of all currently living humans, a.k.a. the Y-chromosomal Adam, based on study of the Y chromosomes of men from around the world.
Supervolcano on Sumatra explodes (creating Lake Toba), possibly forcing the human gene-pool through a bottleneck, down to between one and ten thousand breeding pairs.
hobbits– and died out at roughly the time that (full-sized) humans were spreading into that part of the world (see 50 k yr ago, reaching Australia).
A third of a million years is about the same fraction of the Sun's life expectancy (tenish giga years) as the fraction a day is of a life-span of 80ish years.
You run and you run to catch up with The Sun – but it's sinking,
rushing around to come up behind you again.
The Sun is the same, in a relative way, but you're older;
shorter of breath and one day closer to death.
Ancestors of modern domestic cats diverge from those of their surviving wild relatives; they may have been living with humans all that time.
estimated date of the matrilineal most recent common ancestor of all humans, a.k.a. the Mitochondrial Eve, inferred from study of diversity in human mitochondrial DNA.
Fossils of modern humans (H.Sap.) in East Africa's Rift Valley have been dated to 0.16 M yr ago; our species is believed to have arisen 0.2 M yr ago in sub-saharan Africa, and to have migrated out of Africa – in small groups as early as 90 k yr ago and in significantly larger numbers from around 50 k yr ago.
a.k.a. a quarter million years ago, some of the
humans who'd made it to Europe, specifically
plainly using fire. Admittedly, this is probably just a case of
haven’t been able to show it systematically or robustly until now (quoting
Magill, from the article) rather than when humans in Europe first started
cooking socially; given that their ancestors had been using fire for hundreds of
thousands of years by then, they surely brought fire-management skills with them
out of Africa, so what they were doing in Europe was just a continuation of what
they grew up with before they moved there.
Separation of Britain from mainland Europe (a prehistoric Brexit). The North Sea was previously land-locked, but broke out through what is now The Channel, possibly in a single day, gouging a deep trench valley in the process. This Anglian stage glacial era moved the Thames from a more northerly course to its present course, through (the eventual site of) London; and its ending created the white cliffs of Dover. Within this period, there were hominids, likely including Neanderthals, using stone tools, includeing knives up to a light nanometre long.
Earliest human (or hominid ?) attempts at colonising the British Isles. Around this period, (probably less desperate) hominids were cooking food and, later, wearing clothes.
Approximate date of last common ancestor of modern humans and Neanderthals – as long as you ignore the more recent interbreeding between the two groups. Perhaps better described as the last common ancestor of the Neanderthals and modern humans who didn't migrate out of Africa, where their cousins who did (and who make up most of the ancestry of non-African human populations) ended up interbreeding with the Neanderthals and Denisovans.
came down from the trees, our ancestors had to go back up to get bipedal and only after that come back down to make the most of the plains that opened up in their East African jungles.
Last common ancestors of chimps and humans.
Emergence of felines as a separate group of carnivores. Some migrated to the Americas about 8 M yr ago, via the Berring land bridge.
Antarctica and South America separated, allowing an ocean current to circulate round Antarctica, via the Drake Passage, with major impact on Earth's climate – including Antarctica's glaciation.
Early primates with a mutation began to see red – the origin of colour vision.
Earliest evidence of zombie ants, whose behaviour is hijacked by a fungal infection.
Meteor strike in Yucatan, leaving the Chicxulub crater; dinosaurs died out in the aftermath.
The universe appears to be about 13.7 G yr old; our solar system formed four to five G yr ago and the third major planet (our home, Earth) from its star (The Sun) has teemed with life for much of the time since, acquiring its oxygen-rich atmosphere around 2 G yr ago. The Sun is expected to survive for about another four or five G yr.
Australia split off from Gondwanaland, after a slow splitting-up along a rift valley. The oldest known amber-fossil of a bee dates from about the same time.
around 100 M yr ago,
in concert with the flowering plants.
The Permian-Triassic mass-extinction, possibly caused by a meteor strike; the resulting crater, lurking under Antarctic ice, is about 480 km (300 miles) wide. This is also roughly when Gondwanaland (Pangea ?) began breaking up, possibly also thanks to that meteor. Life barely survived, some of it sheltering in coastal waters.
A mass-extinction wiped out plenty of life on Earth.
Phytoplankton evolved strategies for spreading out (and becming spikier, hence harder to eat) when animals came grazing.
The Cryogenian era,
including 0.2 G yr of
the Neoproterozoic era. The period is known to
have seen extensive glaciation, reaching even to the tropics, but (at least) the
era is believed to have seen interglacial warmings.
Odd changes happened to ocean chemistry,
possibly caused by a
polar wander incident – the Earth realigning itself around its
spin axis in the space of a few million years.
Here's a video of the last gigayear's activity of the tectonic plates of Earth.
cyanobacteria (a.k.a. blue-green algae) showed up somewhat earlier, but at this point their environmental pollution became a major part of the atmosphere. It might be arguable that the damage took a few more mega years to entrench itself.
Apex Chert stromatolites in Pilbara, Western Australia, are 3.4 G yr old and some experts maintain that their origin is biological.
Rocks as old as 3.75 G yr can be found in the Hudson Bay area of Canada and in West Greenland. The Hudson Bay rocks reveal high levels of CO2 in the atmosphere at a time when the Sun is believed to have been about 25% less bright than it is today; without the CO2, oceans would have frozen. Rocks allegedly formed earlier, as much as 3.9 G yr ago.
Formation of Earth and Moon. Meanwhile, a collision in the Kuiper built broke up 2003 EL61 (which might otherwise have been bigger than Pluto).
Close study of the cosmic microwave background reveals that the universe is 13.7 gigayears old. The first few gigayears were dominated by an initial explosion and the progression of phases through which the results expanded and cooled; so I describe them below in terms of time after that initial explosion.
This far back, it is quite common to give the red-shift of observed things, which is what's typically actually observed, rather than (or as well as) the time since the present or time after the big bang (either of which is typically estimated based on the red-shift). The red-shift is normally denoted z, with log(1 +z) = b, the the hyperbolic angle describing the relative velocity as v = c.tanh(b) of the light's source. (In so far as Hubble's coefficient, the fractional rate of expansion of space, is constant: it is this hyperbolic angle, b, that has been increasing linearly with time, so 1 +z = exp(b) grows exponentially with the time since the light was emitted. For small z, log(1 +z) = b is well-approximated by z; but this fails for larger z.)
Light from the early universe has reached us by way of a long journey, during which its spectrum, as observed by the gas it was passing through, has steadily been red-shifted. As it passed through gas, the light at the frequencies making up that gas's spectral lines got absorbed, exciting or ionizing the gas; as it continued its journey, the resulting gap in the light's spectrum got red-shifted along with the rest, while some higher frequency part of that spectrum red-shifted down to the relevant spectral line, to be absorbed by gas the light passed through later. This turns each spectral line of the primordial gas (mostly hydrogen) into a succession of absorption lines in the spectrum we observe; thus the light from a distant source carries a pattern of lines that tell us when in its journey it was passing through (relatively dense) patches of neutral hydrogen. (In contrast, ionised hydrogen scatters all frequencies, but less effectively; so it thins the light relatively uniformly across its spectrum, to a less pronounced degree than neutral hydrogen absorbs at its spectral lines.)
A prominent line in hydrogen's spectrum is known as
Lyman α, so
this pattern of lines (or at least the part of it due to this line) is known as
forest. When the light was passing through space well-illuminated by
some nearby star (or galaxy, or whatever), it was atenuated relatively uniformly
(and slowly) by the ionized gas it was passing through (and only a little at the
spectral lines of any transiently neutral gas in the mix); when it was far from
such illumination, (more of) the gas it travelled through was neutral so
absorbed light relatively efficiently, but selectively (at its absorption
spectral lines), cutting a gap in the light's spectrum. The resulting pattern
of lines tells us how prevalent neutral gas was in the space it passed through
during the course of its journey. For more details, see
of the universe. Because the following is in most-recent-first order, its
entries may be better read in reverse order, to give a chronological
End of the
Dark Age; fiat lux.
After a respectable fraction of a gigayear, some clumps of matter had formed,
collapsed into stars and started producing light. This ionized the gas filling
the rest of space (which had been plasma before recombination, below); so this
event is known as
reionization. (Hydrogen has a hyperfine transition
(the electron flipping its spin from matching the nuclear spin to its opposite)
that produces a 21 cm spectral line, in the radio frequency range; such radio
waves from the period of reionization have red-shifted to around 2 metres since
then. This dates to (maybe) c. 0.18 Gyr into the story.) It happened over a
respectable period of time, partly because early stars didn't start everywhere
at the same time (although there likely was a cascade effect, as stars exert
pressure on nearby gas, that tends to make it clump up and form stars) and
partly because the light from stars had to reach all the places where the stars
weren't. Although light now resumed bouncing off charged particles, their
density was vastly reduced (compared to before recombination, below), thanks to
the expansion of space, so we can still see through the gas from this later
Of course, the light ionizing the gas thus got absorption lines carved in
its spectrum by the spectral lines of the neutral gas it ionized; and, as it
red-shifted, the line grew into a trench that only ends when the light got clear
of neutral gas. That, of course, means it had reached a bit of space where some
other early star's light had already ionized the gas. So the light reaches us
trough carved in its spectrum (at high red-shift) by its early history.
The JWST is pushing back the start-date of galaxy formation, but (as at mid-2023) 0.35 Gyr ago is a later-bound.
The density of matter comes down to of order a billion atoms per cubic metre, a mass-density of order a femtogramme per cubic metre. Energy densities get low enough that matter was no longer being excited back into unstable states as fast as it could decay out of them, so matter condensed out and formed atoms (and presumably molecules; it was mostly hydrogen), leaving light to travel on its way, with no charged particles to scatter off and only limited spectral lines of atoms and molecules to be absorbed by.
The resulting (warm orange, 4 kK) sea of photons, initially in thermal equilibrium with the matter, has been expanding and adiabatically cooling ever since. Today, it is observable as a background so far red-shifted that it is microwave radiation. It still retains the form of black body (i.e. thermal) radiation, with a temperature of about 2.7 Kelvin: indeed, it is the most perfect match yet seen to the theoretical model of black body radiation.
Following this event, called
recombination, the universe was dark for
a while, as there was nothing but neutral atoms (and I suppose molecules)
roughly evenly spread throughout the universe; so the period until reionization
is known as the
Dark Age. Before this, light was continually
bouncing off loose charged particles, so we can't see earlier events (dense
plasma is opaque); consequently, red-shifts aren't measured. Everything (below,
i.e.) before this is theoretically extrapolated from what we can see.
garden varietymatter, albeit squashed together with the density of neutronium in a modern neutron star and at a temperature of ten gigakelvin. Up to this point, interactions with the energetic leptons have been keeping the proportions of neutrons and protons roughly equal, but the falling temperature begins to render those interactions weak, leading to the neutron's greater mass tending to bias the mix towards the proton. By the time weak coupling between hadrons and leptons has faded away and the resulting
freeze outhas completed, protons out-number neutrons six to one. The free neutron's instability (albeit with a half-life of order ten minutes) could only push that bias further in the proton's favour – except in so far as the neutrons combined with protons.
Grand Unification Epoch. Random quantum fluctuations within the roiling storm are all the structure there is; the spectacularly high temperature would otherwise make all uniform.
Lineweaver and Patel, in October 2023,
All objects and some questions that includes a log(mass)
vs log(length scale) chart of all bodies in the universe and some interesting
analysis of the evolution of the universe since the big bang.