A few fragments of prehistory

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 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.

k yr, a.k.a. millennium

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.

10ish to 5ish k yr ago
The development of writing and, as a result, history. From this point on, written records give us far more detailed information about events – and the archeological record is vastly more detailed, too – so we know more about the last few millennia than about everything that came before. As a result, I separate out the last ten millennia into their own page on history.
8 k yr ago

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. 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.

11.5 to 8 k yr ago

The Neolithic era, a.k.a. new stone age. People began living in relatively permanent villages and domesticating crops and (other) animals. Archeologists have found buildings from as much as 11 k yr ago.

9.4 to 7.7 k yr ago

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.

9.5 k yr ago

Oldest archeological evidence of cats and humans cohabiting. For more solid evidence of domestication, one has to wait another 4.2 k yr.

12 to 11 k yr ago

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.

Around the same time, the Natufian culture is known to have domesticated dogs in the middle East (mentioned here).

12 k yr ago

First successful human settlements in the British Isles.

10ish k yr ago onwards: the Holocene era
12.9 k yr ago
Comet impact maybe provoked climate change and disrupted ecologies in the northern hemisphere.
5 to 15 k yr ago

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.

at least 14 k yr ago
The invention of pottery appears to have happened in East Asia – and took something like six thousand years to spread West to the Middle East and, thence, Africa and Europe.
15 k yr ago
Rice cultivation in East Asia (mentioned here, from Korean research).
10 to 18 k yr ago
The Magdalenian culture, ranging at least from the Iberian peninsula to what is now Poland, made tents and hunted large animals including reindeer; they made tools of flint, bone and antlers. Their resettlement of Europe came just as the ice age was taking a break for the present interglacial.
35 to 24 k yr ago
The end of the Neanderthals
40 to 10 k yr ago: upper Paleolithic

The first European humans were already painting erotic pictures 40 k yr ago.

39 k yr ago
Heinrich event 4, a 2 k yr period of sluggish circulation and falling temperatures in the Atlantic, associated with a desert period in Iberia at the end of the Neanderthals' era.
40 k yr ago
The invention of tallies, physical objects on which to make marks representing numbers – of cattle, slaves and trade-goods, for example. This long pre-dates (other kinds of) writing.
45 to 42 k yr ago

Modern humans in Russia.

c. 50 k yr ago

Modern humans in Australia.

c. 60 k yr ago

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.

70 to 75 k yr ago

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.

M yr

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.

Pink Floyd

0.13 M yr ago

Ancestors of modern domestic cats diverge from those of their surviving wild relatives; they may have been living with humans all that time.

0.14 M yr ago

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.

0.16 M yr ago

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.

0.4 to 0.2 M yr ago

Separation of Britain from mainland Europe. 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.

0.7 to 0.5 M yr ago

Earliest human (or hominid ?) attempts at colonising the British Isles.

6.3 to 5 M yr ago

Last common ancestors of chimps and humans.

11 M yr ago

Emergence of felines as a separate group of carnivores. Some migrated to the Americas about 8 M yr ago, via the Berring land bridge.

41 to 34 M yr ago

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.

40 M yr ago

Early primates with a mutation began to see red – the origin of colour vision.

48 M yr ago

Earliest evidence of zombie ants, whose behaviour is hijacked by a fungal infection.

65 M yr ago

Meteor strike in Yucatan, leaving the Chicxulub crater; dinosaurs died out in the aftermath.

G yrLife's increasing complexity: starting c. 3.9 G yr ago, bacteria c. 3.5 G
yr ago, photosynthesis 2.5 G yr ago, eucaryotes 1.5 G yr ago and the Cambrian
explosion 0.5 G yr ago

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.

0.1 G yr ago

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.

0.168 to 0.14 G yr ago: Origin of the ants

A lesson for us all subsequently diversifying around 100 M yr ago, in concert with the flowering plants.

0.251 G yr ago

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.

0.29 to 0.248 G yr ago
The Permian era, during which the world's land-masses were all fused together as Pangea, stretching from pole to pole. Ferns begin being displaced by seed-bearing plants; and conifers emerge.
0.4 to 0.45 G yr ago
Evolutionary divergence of the arachnids (spiders, scorpions, mites and ticks).
0.45 G yr ago

A mass-extinction wiped out plenty of life on Earth.

0.85 to 0.544 G yr ago

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.

0.8 G yr ago

Odd changes happened to ocean chemistry, possibly caused by a true polar wander incident – the Earth realigning itself around its spin axis in the space of a few million years.

2.7 to 2.3 G yr ago: Oxygen atmosphere

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.

3.4 G yr ago: Early life

Stomatolites in Pilbara, Western Australia, are 3.4 G yr old and some experts maintain that their origin is biological.

3.9 to 3.75 G yr ago: Oldest rocks

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.

4.3 G yr ago
Earth (possibly) cool enough to have liquid water.
4.6 or 4.5 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).

4.56 G yr ago: Iapetus formed
Saturn's moon Iapetus has a ridge round its equator; one theory to explain this involves it forming quite early in the Solar System's history.
13.7 G yr ago: The Big Bang

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.

The first Gigayear

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 the Lyman-alpha 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.

0.15 to 1 G yr: new light (12.4 > z > 10.2; 2.6 > b > 2.4)

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. 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 time.

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 with a Gunn-Peterson trough carved in its spectrum (at high red-shift) by its early history.

0.38 or 0.37 M yr: matter and light decouple (z = 1089, b = 7)

Energy densities got 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 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.

five minutes
Atomic nuclei begin to freeze out from the hadron-lepton plasma, initiating the transition to an era dominated by magnetohydrodynamics.
1 second
Exotic matter has almost all decayed into garden variety matter.
10 µs
Phase transition from quark-gluon liquid to hadron-lepton plasma.

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