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Precambrian - From the beginning to 650Ma
The Precambrian isn't really a single unified time period... it makes up roughly seven-eighths of the Earth's history. In other words, most of what Earth did, it's done during the Precambrian. Hence, the most important events in biological history took place during this time. The Earth formed, the first tectonic plates began to move, eukaryotic cells evolved, the atmosphere became enriched in oxygen -- and just before the end of this period, complex multicellular organisms, including the first animals, evolved. So, a lot.
During the first part of the Precambrian - the Archaean - The atmosphere was pretty markedly different from what we know today; it was a soup of methane, ammonia, and other gases which would be toxic to pretty much everything that lives here now. Our oldest fossils date to to the Archean, approx. 3.5 billion years ago, and consist of bacteria microfossils.The second half of the Precambrian was the Proterozoic and this was a particularly hoppin' time. Stable continents began to form over the next billion years, and the first abundant fossils of living organisms ( mostly bacteria) appeared, but by about 1.8 billion years ago eukaryotic cells began to enter the record, which is the first evidence of oxygen in the atmosphere. Oxygen meant life for some, but disaster for the existing inhabitants who happened to like their toxic soup just fine, thank you very much. Sorry, dudes.
Cambrian - From 543 - 490MaPhoto: Dotted Yeti / Shutterstock.com
As you may have already figured out, 'pre' usually comes before something, and in this case, the Precambrian came before the Cambrian.
This was a time when most of the major groups of animals first appear in the fossil record. This event is sometimes called the "Cambrian Explosion", because of the relatively short time over which this diversity of life forms 'exploded'. Once the Earth hit on oxygen as a concept and a party theme, the ball really got rolling.At least, it was rolling in the oceans... the land was completely barren, which makes sense if you think about the fact that dirt as we know it is the product of billions of years of erosion, and Earth hadn't had that kind of time to make any yet. It basically had a microbial 'crud' that acted as a soil crust covering the land. Apart from some minor evidence that a few animals might have made land, the new continents (which had just formed during the breakup of the supercontinent Pannotia) resembled deserts with shallow seas at the margins. The seas were relatively warm, and for most of this era, there was no polar ice at all.
Ordovician - From 490 to 443MaPhoto: AuntSpray / Shutterstock.com
Hey everyone, a new supercontinent!
During the Ordovician, most of the world's land was collected into the southern super-continent Gondwana. Throughout this period, Gondwana shifted towards the South Pole and much of it was submerged underwater. Because of this, the Ordovician is known for its diverse marine invertebrates, including graptolites, trilobites, brachiopods, and the conodonts (early vertebrates). A typical marine community consisted of these animals, plus red and green algae, primitive fish, cephalopods, corals, crinoids, and gastropods. More recently, there has been found evidence of tetrahedral spores that are similar to those of primitive land plants, suggesting that plants may have invaded the land at this time.From the early to mid part of this period, the weather was pleasantly warm and muggy (if you like that sort of thing). However, when Gondwana finally finished moving in on the South Pole at the end of this time, massive glaciers formed causing shallow seas to drain and sea levels to drop. That's bad news for all the things that really liked warm, shallow seas to live in. This is very likely what caused the mass extinctions that characterize the end of the Ordovician, in which 60% of all marine invertebrate genera and 25% of all families went extinct.
Silurian - From 443 - 417MaPhoto: Alena Hovorkova / Shutterstock.com
Thanks to the stabilization of the earth's general climate, the Silurian period ended the previous pattern of erratic climatic craziness. Because of this, the glaciers started to melt, which contributed to a pretty major rise in the levels of the biggest seas. Coral reefs entered the world stage for the first time, and fish evolution and diversity really turned on the gas. The Silurian saw the rapid spread of jawless fish as well as the incredibly important appearances of both freshwater fish and first fish with jaws. Jaws are a big deal when you think about how many creatures on earth have them.The Silurian is also the first period in Earth's history where we have good, solid evidence of life on land, including relatives of spiders and centipedes, and also the earliest fossils of vascular plants.