The Cambrian Explosion
runs from approximately 635 million years ago to 485 million years ago, which covers two formally recognized periods in Earth's prehistory, the Ediacaran Period and the Cambrian Period. This stretch of time is when almost all the phyla of animals now living first evolved, and it's the famous time when evolution went into nitro-fuel mode, running so fast that creationists claim it couldn't possibly be natural.
Some of the following stuff is from the book, and some of it isn't.
Back some years ago, a scientist named Paul Hoffman made a dramatic claim: far in the Earth's past, before life was more than single-celled critters, the whole damn Earth froze over from pole to pole in a gigantic glaciation that made the Pleistocene ice ages look tropical. He called this the "Snowball Earth" hypothesis. Like an increasing number of scientists, Hoffman is very good at taking his case to the media, but other scientists don't all agree with him about just how
frozen Earth really was, and how many times this happened. There may have been areas of open ocean. In any case, there would still have been deep-sea thermal vents where life could have survived. The one thing everybody agrees upon is that the Earth did indeed suffer through several colossal ice ages, and most of it, if not quite all of it, was very cold. What life there was back then must have had a tough time just surviving. We've even got a decent idea of when
each of these monster ice ages happened.
At any rate, the Ediacaran Period starts just as the last really huge glaciation, the Marinoan, ends, 635 million years ago. There was one more ice age, the Gaskiers glaciation, between 514 and 513 million years ago, but it was milder and less extensive than the full Snowball Earth ones. After that, Snowball Earth conditions ended for good, and life started enjoying some good weather for a change.
The beginning of the next period, the Cambrian Period, has been moved earlier than it used to be. Thus, a lot of rather interesting things that happened back then are now considered to have happened in the early Cambrian instead of the Ediacaran. Back in Darwin's day, and through much of the twentieth century, the start of the Cambrian used to be when the first fossils of any life at all appeared. In fact, that was one of the major challenges to Darwin's theory: how can you say life evolved, when it seems to just poof into existence at this point? Poor Darwin worried quite a bit about it, but thought we might eventually find earlier fossils.
He was right. We have, and you're going to read a lot about them (if I keep writing this). But throughout the Ediacaran and much (not all) of the Cambrian Period, there was no land life -- except maybe microbes living in a microbial crust in some places. The land was just bare rock and basins full of gravel and boulders, though it would have had freshwater streams running across it and emptying into the oceans. No land plants, and certainly no land animals. All multicellular life lived in the oceans. Think about that.Timelines and geological divisions
You can see a lot of divisions of the Cambrian Period at Wikipedia
. No, those aren't just arbitrary chunks of a timeline: each and every one of them is distinct, and exists for a reason.
Geologists have declared that the Cambrian Period officially starts with the appearance of a fossil called Treptichnus pedum
. That's not actually an animal, but the fossil trace of the animal -- its burrow, we think. Since we never find any bits of carapace or skeleton in the burrows, presumably it didn't have any hard parts. For what it's worth, priapulid worms create burrows with a similar shape.
Timeline suggested by the book:
-- Life probably began around 3.5 billion years ago.
-- The earliest eukaryotes (critters either one-celled or multicellular who all have their DNA nicely contained in a nucleus instead of floating around freely in their protoplasm) show up by 1.8 billion years ago.
-- First fossils that everybody agrees are of multicellular life, algae, appear 1.2 billion years ago.
-- Supercontinent Rodinia forms before one billion years ago.
-- It starts to break up around the time of the Sturtian glaciation, 717 million years ago.
-- By Ediacaran time (around 635 million years ago), it's no longer a supercontinent but a series of scattered continents, and oxygen levels are starting to rise.
-- During the first part of the Cambrian Period, called either Stage 1 or the Fortunian, you see some wee tubes, cones, spicules, "shells", and other stuff appear. They get more and more diverse as time goes on; multicellular life is now taking hold and getting complex. These are the "small shelly fauna" (SSF)
; pay attention to that phrase, because it'll pop up again. We also see more worm tubes and other trace fossils. Also, an odd group of sponges, the archaeocyathids, show up and start diversifying; they've got a decent fossil record because they have calcium skeletons that preserve fairly well.
-- Geologists haven't formally defined the start of Stage 2 yet. But SSF's and archaeocyathids keep doing their thing.
-- At the beginning of Stage 3, we see the earliest trilobite fossils yet found, but the book doesn't name a genus. Maybe it was just bits of the exoskeleton? You also see more traces of burrowing in the sea floor, as bigger worms and other critters evolved. The Chengjiang fauna of China dates back to this time; it's solid evidence the famous Cambrian explosion was already underway. By now, the world has changed from the Ediacaran: there's much more oxygen in the atmosphere, and the once-smooth microbial mat on the sea floor no longer exists because the burrowers have eaten it into extinction.
-- Stage 4 starts with the appearance of either Ollenellus
, two trilobite genera.
-- No formal
definitions of the beginning for the remaining stages of the Cambrian Period, alas.