Can someone explain the Cambrian explosion?

[palebluehuh]

Some point in the evolution of life on earth, it is said that almost all of the twenty-six phyla simply appeared, where once everything was essentially goop (a gross oversimplification, I know, but I just wanted to type the word 'goop'). This point is called the 'Cambrian explosion', and I'm just curious to know more about it. Has anyone come up with a plausible explanation for it, or is it still a big mystery to everyone?

[coquina]

First off, when we speak of the "fossil record", for the most part, the only creatures that are recorded are those with hard enough parts to be fossilized.

 

This link gives a pretty good overall explanation:

http://www.pbs.org/wgbh/evolution/library/03/4/l_034_02.html

 

Have you ever heard of the "Burgess Shale"? If you haven't, find Stephen J. Gould's book "Wonderful Life" and read it.

 

The Burgess Shale is located in the Canadian Rockies - a special set of circumstances preserved an ecosystem of bizarre soft bodied creatures.

 

My personal favorite is opabinia:

http://images.google.com/images?hl=en&lr=&ie=UTF-8&q=opabinia

 

followed closely by hallucigenia:

http://images.google.com/images?hl=en&lr=&ie=UTF-8&q=hallucigenia

 

wiwaxia is also a cute little number:

http://images.google.com/images?hl=en&lr=&ie=UTF-8&q=wiwaxia

 

Anomalocis is the largest - you wouldn't want to meet him in a swamp.

http://images.google.com/images?q=Anomalocaris&hl=en&lr=&ie=UTF-8&sa=N&tab=wi

 

It just so happens that conditions were perfect to preserve these wonderful creatures.

 

There is another reason that the further one goes back in time, the fewer fossils one finds. That is due to the fact that the leading edges of the continental plates are being subducted into the earth's interior and melted. As the recycling continues, new plate material is being formed at the mid ocean ridges, so the fossils are being continually destroyed.

 

Hope you enjoyed the links.

[palebluehuh]

Interesting. I've heard of the Burgess Shale, but I never thought it was that early. Thanks a bunch.

[Martin]

Some point in the evolution of life on earth, it is said that almost all of the twenty-six phyla simply appeared, where once everything was essentially goop (a gross oversimplification, I know, but I just wanted to type the word 'goop'). This point is called the 'Cambrian explosion', and I'm just curious to know more about it. Has anyone come up with a plausible explanation for it, or is it still a big mystery to everyone?

 

Two or three years back I encountered a plausible explanation called

"snowball earth"

googling might find it again

(a lot of these explanations get shot down, that's what they're for maybe, so people can find reasons to rule them out and keep looking, I dont know the current status of "snowball earth")

 

It said that before 600 million, life was mostly soft and/or singlecell

It said there was a huge extinction 600-550 million with like 90 percent of species dying off and open a LOT OF NICHES

It said that mostly all they accomplished up to that time was merely to get an oxygen atmosphere, it was this mass extinction that cleared the way and kicked off the Cambrian explosion

then it asked WHY this extinction

 

and it said "albedo" reflectivity. Ice has a high albedo so there is a positive feedback instability-----if you have a big enough iceage then the ice reflects away the sunlight and not much sunlight is absorbed and the earth gets still colder and you get more ice-----it's a runaway freeze. The oceans freeze over down to the equator.

 

Now the question is how do you get out of that? (Have you read the same article or been thru this?) the answer is VOLCANOS. over several 10s of millions of years volcanos belch out a lot of CO2, and there are no algae to reduce the CO2 to oxygen so the CO2 just keeps building up, until the greenhouse is so strong it melts the ice

 

again there is this positive feedback, once the ice starts melting the earth's albedo is reduced and it absorbs more sunlight and warms up and that melts more ice

 

so volcanic activity is like a kitchen timer. you can have only so many years of snowball earth

 

then suddenly you have this warm earth with lots of CO2 in the atmosphere (in addition to oxygen) and multicelled complicated animals and plants

appear, lots of business opportunities are created in a hurry and it all proliferates in mad profusion like the internet economy

 

get someone to criticise this scenario before you take it seriously---I dont even qualify as an amateur

 

I think they found some "ice-oh-tope" abundances of some elements that suggested to them that the earth had frozen over some 600 million year ago, but not sure about this. good thread idea

[palebluehuh]

Very fascinating, Martin. Out of sheer curiosity, I looked the theory up, and here are a few links for the interested:

 

http://www-eps.harvard.edu/people/faculty/hoffman/snowball_paper.html—A detailed explanation of the 'Snowball Earth' theory, with evidence and studies to support it

 

http://news.bbc.co.uk/2/hi/uk_news/scotland/1857545.stm—BBC report on the theory's refutation, but it doesn't seem to be a strong one (i.e., it still doesn't seem to completely rule out the possibility of massive pre-Cambrian glaciation; just makes it unlikely for such glaciation to cover all the Earth)

 

http://en.wikipedia.org/wiki/Snowball_Earth—the Wikipedia article on the 'Snowball Earth Theory'

 

http://www.sciam.com/article.cfm?articleID=00027B74-C59A-1C75-9B81809EC588EF21The Scientific American article on it

 

I've also emailed the geology staff of my university for an answer; maybe they know something.

[NavajoEverclear]

wow thats a really cool part of evolutionary history that I didn't even know about, and I passed the AP Bio test with a 4! (not a 5, but it made me happy, and surprised everyone else since i failed the last term of the class for simply not turning in my work.) Speaking of the AP test, i hear in the East US you take the course of two years, here its crammed into one. Nice to get it done faster, but we probably dont learn as much in detail.

 

Oh yeah, my reply to the thread. Well i probably should go look in my AP cliffnotes to be exact on my history, but didn't the cambrian take a couple billion years? The older the period, the longer it was, i bet you've seen that circle chart where it shows what portion of history each era took, and the entirity of human existence is near about immeasurably small.

 

A Richard Dawkins book called River out of Eden makes a good point about generalizations we make about how undramatic the arisal of seperate kingdoms really was. If you really think about it the original sepration between one organism and another was so gradual, that for a long time, that which would evolve into one kingdom and that which would evolve into another, were indistinguishable from one another, and even reproductively compatable should they meet, except for the fact of geographical seperation.----- of coarse his wording of it was much better than mine, but you get the point

[Martin]

 

http://news.bbc.co.uk/2/hi/uk_news/scotland/1857545.stm—BBC report on the theory's refutation' date=' .[/quote']

 

how people think, they set up a theory, get it to predict something, check the prediction, find it doesnt check, refute the theory, get a new theory---good show gradually accumulate better understanding

 

so they refuted snowball earth! (or almost, maybe it still standing on shaky leg)

 

paleblue, could you write down some numbers so I dont have to go looking for the basics. just for this thread: when was the Cambrian explosion?

 

if there was a big die-off just before it, then what do they estimate was the percentage of species that went extinct? was it as large as 90 percent?

(I am remembering from a longago article and find that incredibly large)

was that basically when multicell life was invented, so recently?

somebody (everclear, paleblue, other...) please thumbnail the basic facts of the Cambrian explosion

[coquina]

has a great geology site:

 

http://www.ucmp.berkeley.edu/exhibit/geology.html

 

and a great "Geologic Time Machine"

http://www.ucmp.berkeley.edu/help/timeform.html

 

The Cambrian period lasted from 543 to 490 MYA:

http://www.ucmp.berkeley.edu/cambrian/camb.html

 

But there are fossils from the Vendian Period, 650 to 543 MYA :

http://www.ucmp.berkeley.edu/vendian/vendian.html

 

At work, will post more later...

[palebluehuh]

Okay, the professor I emailed basically said he was a skeptic of the 'Snowball Earth Theory', which he said was more like a hypothesis, not having much proof to back it up. He stated that we don't have really any knowledge of the topography of the time, thence, we can't really know where mountain ranges lay. He went on to say that we have glaciers, even at the equator, in high-altitudes like Kilimanjaro in Africa, and that could account for the evidence of glaciation in low latitudes.

 

So, I guess it's basically settled.

 

Martin—http://en.wikipedia.org/wiki/Geologic_timescale—here's a basic timeline for the era I'm talking about. According to this, the Cambrian explosion occured between 542 million and 488 million years ago. The chart also says that 'Snowball Earth' may have occured 630 million years ago, in what is appropriately called the 'Cryogenian Period'. But the hypothesis remains tentative, as far as I can tell (I'm not a geologist or any other sort of -ist, so I'm largely clueless).

[Martin]

Thank you both!

It looks like Coquina is on the trail of some information on which to base a guess as to the size of the extinction prior to the C. explosion.

(if the experts believe there was one)

 

Remembering back to when I read some about this, there is (or was at the time) a puzzle as to why it didnt happen sooner.

 

Singlecell life apparently got started by 3 billion years ago (or even more!)

so why did life remain "stuck" in singlecell mode for so long and only recently (half a billion) start to exploit the possibilities of multicell.

 

----may wish to skip this because off topic---

this was connected at least in the minds of some with a "Fermi Question" issue----could habitable planets abound and even singlecell life be fairly common but could perhaps Earth be one of the rare cases in our galaxy which (because it doesnt seem to happen automatically) enjoyed a Cambrian explosion?

 

unless we know what triggered it, it must be nearly impossible to estimate the liklihood of it having happened in other places

----back to business----

[coquina]

It may appear that my posting is drifting as much as the continents, but hang with me, I have a point to make. Probably most of you have some basic knowledge of "plate tectonics" - that continents drift around the globe floating on a sea of magma. There have been periods of geologic time when they have all been jammed together, and others when they have been spread apart.

 

Their relative positions have a great deal to do with what the climate of the entire earth was like, because they affect the formation of ocean currents. When the continents are spread out, as they are now, warm and cold currents has the opportunity to circulate among them and affect the temperature much more.

 

When all the continents are jammed together, the interior of the land mass is unaffected by the stabilizing influence of the ocean. Snow accumulates on the land masses, and sea level falls. The continental shelves are exposed as dry land which eliminates a prime breeding ground for life, shallow water through which light and warmth can penetrate.

 

For a very good background in continental drift, ocean formation and demise, check out James Madison University's site on the "Wilson Cycle"

http://csmres.jmu.edu/geollab/Fichter/Wilson/Wilson.html

 

There is detailed, step by step information there, but this page is the "cut to the chase" version.

http://csmres.jmu.edu/geollab/Fichter/Wilson/wilsonsimp.html

 

IMHO, the other site that provides invaluable information of past times is the Paleomap project:

http://www.scotese.com/earth.htm

 

Check it out - one can see how the continents were placed over time. (There are ways that this is deduced, if anyone wants to discuss it, I suggest you start a separate thread.) There is also a section on "paleoclimate".

 

So - lets concentrate on the Precambrian and the Cambrian to see what was going on:

 

http://www.scotese.com/precambr.htm

As you can see, all of the continents were jammed together with one very large ocean surrounding them. There was a southern icecap, but Antarctica wasn't the land beneath it. The land that is now North and South America was.

There is not a separate climate map for the Precambrian, but the authors refer to the period as an "ice house world".

 

Onward to the Cambrian:

http://www.scotese.com/newpage12.htm

 

You can see that the continents are starting to drift apart again.

 

Climate:

http://www.scotese.com/ecambcli.htm

It is thought that it had warmed up considerably and was a temperate world. As the "ice house" melted, sea level rose. There would have been a lot of shallow seas around the edges of the continents that would have allowed sun to penetrate to the bottom. For the most part, the seas were connected. Most, if not all, of the Cambrian critters were marine; ocean currents would have spread them around and allowed them to proliferate. I think this would have provided ample ammunition for the "Cambrian Explosion".

 

There were probably other contributing factors too. There is a lot more to climate than continental drift - the earth's orbit varies from almost circular to eccentric. The earth's "wobble" changes too. There are times when the two coincide to make a very warm earth. Google "Milankovitch Cycle"

 

I will check back this evening - today - I am going shopping for my trip across the pond. Hope you enjoy the links. L8R.

[Martin]

I guess coquina is off to UK on vacation

I will add this link to what she's assembled already

http://palaeo.gly.bris.ac.uk/Palaeofiles/Cambrian/Index.html

 

coquina, please let us know when you get back from the trip

 

 

people, notice this oxygen thing:

being multicelled requires some mechanism to get the oxygen to the insiders, unless the concentration of O2 is high already

 

"A prolonged glaciation event also occured during the late Precambrian and could have caused a period of oceanic upwelling, increasing primary production and consequently the atmospheric oxygen level. It is thought that a critical level of oxygen may have been reached, firstly enabling animals to construct large bodies without oxygen diffusion becoming a constraint, and secondly allowing the synthesis of biomineralized skeletons"

http://palaeo.gly.bris.ac.uk/Palaeofiles/Cambrian/triggers/extrinsic/extrinsic.html

[coquina]

Great link, Martin.

 

I said I was going shopping - been there, done that, maxed the card

 

I'll leave here on 9/28 and drive to daughter's house, who lives in DC area, fly from Dulles to Heathrow on 9/29, fly back on 10/6, spend the night with daughter and return 10/7.

 

My cousin has the internet, so I may be able to check in from time to time while there. Keep those geology threads rollin' folks. We can roll right on up through the Holocene if ya wanna.

[Dov]

The Cambrian Explosion is explained very clearly and lucidly in the October 2004 issue

of Scientific American (http://www.sciam.com) :

 

"The Hidden Genetic Program of Complex Organisms"

(Biologists assumed that proteins alone regulate the genes of humans and other complex organisms. But an overlooked regulatory system based on RNA may hold the keys to development and evolution)

By John S. Mattick