Thyme for Time

My sister knows I make a lot of bad jokes. One of my favorites is "Thyme? I prefer Space!" whenever she happens to be cooking something and I see the jar of thyme leaves in her spice cabinet (...I told you they were bad). Unfortunately, thyme isn't very exciting, so let's talk about time instead.

Time, as is to be expected, is a very old concept. Time measurements have undergone very little change in the years since the Sumerians created them with their whacky base-60 numerical system and they were subsequently stolen by the Babylonians. Interestingly, it's called a second because it's the second division of the hour by 60 (sixty minutes in an hour, sixty seconds in a minute). The hour was historically defined as 1/12 of the time between sunrise and sunset (again, due to the fancy base-60 system). Today we use a 1/24 division of the mean solar day (a solar day being the time from noon on one day to noon on the next day). Days and years are both related to the sun (one for the Earth's revolution, the other for its orbit) - months and weeks are more lunar-related time measurements.

The year is a bit older, and stems from the creation of calendars and astrology, although most early civilizations reckoned years in regnal time (i.e. the duration of a monarch's reign). Years are bunched into decades, scores, centuries and millennia - but it gets a bit hazy after that (agricultural civilization only has twelve millennia under its belt, after all). After a millennium we've got non-standard geologic divisions - epochs, periods, eras, and eons.

And since the history of all time would make this post far too long (especially since my picture is to scale), let's talk about Earth's geologic time.

The most recent history of the Earth saw the rise of mammals and birds in the Cenozoic era (and humans specifically in the Quarternary period). Unfortunately the easily remembered term "Tertiary" for the penultimate part of the Cenozoic portion of the geologic timescale has been replaced by a few harder to remember (and harder to pronounce) periods. Because geologists hate people.

This relatively recent time is also when we had cool dinosaurs, and less exciting conifer trees. This period saw an explosion of life forms - from crocodiles and jellyfish to trilobites and crazy jawless fish (and a bunch of annoying insects in the Permian period). This is also the time period people usually associate with supercontinents like Pangaea.

In contrast to the Phanerozoic eon - the period of diverse, complex life on Earth - there's the Precambrian supereon (the vertical black line in the picture). The last portion of the Precambrian was the Neoproterozoic era, which was marked by the first appearance of animals, as well as an abundance of glaciation (a completely ice-covered 'snowball Earth' is thought to have occured during the Cryogenian period).

The Mesoproterozoic era is probably far more interesting for many people, since sexual reproduction (and much less exciting cell specialization) first appeared then. Geologically these periods saw lots of orogeny (mountain building). Algae has a field day (field period?) during the Ectasian period - with green algae forming oceanic colonies and the emergence of new and improved red algae. The supercontinent no one cares about - Rodinia - formed during the end of this era (in the Stenian period).

The Paleoproterozoic era has a lot of interesting happenings - unfortunately none of them appear interesting enough to merit a song by Bloodhound Gang. During the Statherian period the supercontinent Columbia formed (early complex single-celled organisms are rushed to the hospital ODed). In an effort to quell the mounting mountain deficit, the Earth creates a bunch of mountains during the Orosirian period (in a striking change from usual naming conventions this one is relatively normal, from the Greek for 'mountain range').

Mountains tend to be formed either from volcanoes or plate tectonics. Mountains on land are the result of two continental plates colliding at a convergent plate boundary (see the Himalayas for a modern example). The process takes millions of years, but eventually the mountains are built, only to be taken down by the natural forces of erosion.

This era saw the development of chloroplasts and normal photosynthesis as a development from algae's earlier foray into making deadly poisons. For millions of years the Earth's atmosphere had minimal amounts of oxygen. Early life produced oxygen as a byproduct, and inadvertently pumped out large amounts of a poisonous chemical. Oxygen is a very deadly element (flammable, reactive, and originally poisonous to all existent forms of life). This mass-production of oxygen resulted in fine layers of rust in rocks from the Rhyacian period. The oxygen bound with iron dissolved in seawater to form insoluble iron-oxide (also known as rust), which settled on the ocean floor and eventually became part of a rock layer.

The large amounts of this new-fangled oxygen also resulted in the major extinction which defined the Siderian period. Many geologic periods are bound with palentology and biology, in that the boundaries correspond to major extinction events. The K-T extinction is usually the most famous of these (K for Cretaceous period, T for the now defunct Tertiary period - take that easy understandability for future generations!) for killing off everyone's beloved dinosaurs. The most devastating extinction event happened at the end of the Permian period (the P-T or Permian-Triassic extinction), which killed off nearly 90% of all species on Earth. Nobody cares about it though, because it's hard to love a trilobite...plus it killed off a bunch of insects, and I think we can all do without squirrel-sized mosquitoes.

The Archean eon (or eon of origin) featured the development of simple singled-celled organisms and bacteria. The Neoarchean era here is the time when life started pumping out oxygen into the atmosphere to kill off their descendants. On a completely unrelated note, the lack of oxygen during this era should result in a dead Captain Picard in some random episode of Star Trek. The Hadean and Archean eons are usually divided by the occurrence of abiogenesis (the origin of life). The bacteria often left small fossilized imprints in rocks, which allows us to date the origins of life - but not yet definitively how it formed. The large collections of bacteria also resulted in some interesting features called microbial mats and stromatolites, which are cool looking little bulges of bacteria and accretioned material (...cooler and less disgusting in fossilized form). Many of these formed during the Mesoarchean era, but have become increasingly less common throughout Earth's history due to increases in the number predators (although they can still be found in a living form in a few special environments). Most of the oldest rocks on Earth date to the Paleoarchean and Eoarchean eras.

During late Hadean and early Archean eons (especially the Eoarchean era), the oceans began to form. The Earth's atmosphere - like much of the matter in the early solar system - was probably made of hydrogen and helium. These elements easily escaped Earth's gravitational pull and left the planet's surface bare. The subsequent atmosphere would be created from volcanoes expelling gases trapped during the solidification of the molten planet. Volcanoes expelled mostly water vapor and carbon dioxide (and they still do), which provided a basis for the atmosphere and oceans. Oceans developed from rain and condensation in cloud cover along with comet and meteorite collisions over millions of years (the moon is also theorized to be a result of a large collision). Most comets and meteorites hold an appreciable amount of ice, which likely added substantially to the amount of water in the oceans.

At the time there were far more things flying around the solar system. This is one of the reasons most of the impact craters in the solar system date back 3.5 to 4.0 billion years. Before that time they would impact molten Earth. Due to erosion and geologic processes, most craters don't remain visible on Earth like they do on the moon and other planetary bodies without an atmosphere (...and acne-ridden teenagers). Heavy elements like iron and nickel sank to the center of the molten Earth, while lighter elements like silicon and carbon (and compounds like sodium chloride - salt) drifted to the surface.

The proto-Earth was likely created by a nearby supernova which expelled a tremendous amount of hydrogen and helium, along with trace amounts of other elements (trace being a relative term, considering a vast majority of the Earth is made out of them).

And there we have an abbreviated history of the Earth. It turns out dinosaurs aren't important enough to warrant more than a sentence in the whole affair. This is unfortunate, because dinosaurs are sweet.

The Quarternary is the only part of my picture that's appreciably off; it's slightly too big - it should be 1/40 of the first 100 million years (or about 1.25 pixels tall). The image is 3456 pixels tall, for reference.

...Yes, I did waste a lot of time making that picture.