Cambrian Explosion and Evolution

[HansMustermann]

No one's denying that complex animals radiated fairly close to the Cambrian. However, the Ediacara has a rich fossil record (well, relatively) of complex animals well before the Cambrian. Also, as I stated before, the issue is one of preservation. Until organisms evolved hard parts there isn't much chance of being fossilized; it's been argued quite effectively that the C.E. is really the diversification of shell forms. And as I said earlier, we just don't know what kinds of soft-bodied organisms there were prior to the Cambrian. There could have been all sorts of nectic soft-bodied things that we either haven't found or which never fossilizied in the first place.

Characterizing Precambrian animals as simple tube-things lying on the ground is a mischaracterization of the fauna. The only thing we can really say is "We don't know what animals were like". And you still haven't addressed the trace fossils of the Precambrian, which argue for complex, mobile organisms relatively early.

Yes, from what age? How many millions of years before the start of Cambrian are we talking about?

Of course those complex forms I mentioned didn't just pop out of nowhere, directly with eyes and fins and armor and everything. I _know_ that that's not how evolution works.

But I'm saying that right before the Cambrian (right before at those scales meaning still millions of years), it's where most of that crap starts appearing. It's in less than 10 million years before the border we assigned for Cambrian that you see it going from the likes of Namacalathus, barely a sorta cup on a leg, to those more complex creatures that you mention. Yes, technically they were still in the Ediacaran, but it's still squashed against its final days. (Ok, millions of years.) It's not like they're spread evenly across the the 90+ million years that the Ediacaran lasted.

 

[Dinwar]

Fairly old, as it turns out.

Here's an article about the ongoing controversy regarding Precambrian trace fossils.

Besides, your argument necessarily boils down to "We don't have evidence of complex soft bodied organisms, therefore they didn't exist." Fossils of soft bodied complex organism don't exist from the Pleistocene--it's silly to expect them from the Precambrian. The seas could have been teeming with jellyfish, nectic worms, etc. without leaving any traces. The real question is why the crap started appearing. Was it because the phyla just evolved? Or was it because the preservational environment was more suitable? Or was it because the animals evolved parts that are more ammenable to preservation? Considering the fact that the seas were relatively firm-bottomed for much of the past (as opposed to today's soft-bottomed seas), this is still an open question.

 

[Skeptic Ginger]

I think it is better to think of organism stability or instability more in terms of a continuum. Mutation moves along at about the same rate, as does gene mixing in sexual reproduction. But there are many influences on natural selection and that results in some genetic models being more stable, with intra-species diversity accumulating, and some being less stable giving periods of rapid inter-species diversity.

A most fascinating new thing was discovered recently about bacteria. They have a genetic repair mechanism that shuts down when the organism is exposed to a toxin. The result is the mutation rate increases. Evolution is so much more complex than Darwin ever imagined.

 

[I Ratant]

it would be easier if there was one

.
There is.... well, there's a lot of "The answer" versions around.
Few of which agree with each other, or reality.
My aero prof used to tell us that "Thinking is painful", which is why so few people actually indulge, and prefer "the answer", which are the original TOEs for the masses.

 

[I Ratant]

Yes, I know about those, but those seem to be

A) exactly the kinds of things I'm talking about. They were much simpler things with apparently not much mobility and most of which just lied on the ocean floor. There just isn't as much of a pressure to evolve in radically different ways when you're still a tube or frond lying on the ocean floor filtering whatever comes by, and any other creatures that can chase you around also didn't exist.

B) the more complex ones are actually right before Cambrian.

.
But we got those complex guys, good!
Whatever stopped them and started us, good on it!

 

[HansMustermann]

Fairly old, as it turns out.

Here's an article about the ongoing controversy regarding Precambrian trace fossils.

Thanks for the links but, basically, bingo.

Besides, your argument necessarily boils down to "We don't have evidence of complex soft bodied organisms, therefore they didn't exist." Fossils of soft bodied complex organism don't exist from the Pleistocene--it's silly to expect them from the Precambrian. The seas could have been teeming with jellyfish, nectic worms, etc. without leaving any traces.

Actually, it turns out, we do have fossils of some soft-bodied organisms. Sure, not as good as of something with a carapace, but now and then they do turn up.

But anyway, it seems to me like that's how science works. We base theories on the evidence we have, not on lack of evidence to the contrary. I mean, equally it could turn out that there's some trivial trick to FTL travel, but in the meantime we go by what we did measure.

The real question is why the crap started appearing. Was it because the phyla just evolved? Or was it because the preservational environment was more suitable? Or was it because the animals evolved parts that are more ammenable to preservation? Considering the fact that the seas were relatively firm-bottomed for much of the past (as opposed to today's soft-bottomed seas), this is still an open question.

That is a very good question, of course.

 

[Dinwar]

Thanks for the links but, basically, bingo.

Those were two found while downloading a report--hardly an exhaustive study.

Actually, it turns out, we do have fossils of some soft-bodied organisms. Sure, not as good as of something with a carapace, but now and then they do turn up.

Yeah, I know. I've seen fossil jelly fish. My point is that they're increadibly rare, even in comparatively recent sediments--it's not unexpected to not find fossils of squishy things in sediment that's over half a billion years old.

But anyway, it seems to me like that's how science works. We base theories on the evidence we have, not on lack of evidence to the contrary. I mean, equally it could turn out that there's some trivial trick to FTL travel, but in the meantime we go by what we did measure.

The problem is, paleontology always works with a bad dataset. There have been many, many arguments over the years about what can be said regarding a lack of fossil evidence. The concensus, as far as I can tell, is that a lack of fossils which are typically rare means nothing (a lack of common fossils is another issue entirely). Saying "This thing, which rarely fossilizes, isn't present in these really old rocks, therefore it didn't exist" is always going to put you in the middle of a rather intense debate. And it's not like we're violating any laws of....well, anything in saying that complex animals may have existed, the way your FTL travel example does. All people like me are saying is that you cannot say that the lack of evidence for fossils which rarely occur anyway means that the animals didn't exist.

Your stance is perfectly justifiable as well. I'm just on the other side of this debate.

 

[HansMustermann]

I'm not saying they necessarily and absolutely didn't exist. Nobody in any science will make such an absolute statement. Even when we say that the galaxy does this and that, or there's this or that impossibility for FTL, there is an implicit and well understood "until more data proves us wrong".

I'm just saying that the data we do have becomes increasingly more diverse and complex in the last 10 million years or so before the Cambrian, and then basically that trend accelerates a bit. (Though maybe not as fast as the "explosion" was originally believed.)

Sure, it's a crap data set, no doubt, but that's the available evidence.

But if anyone finds something with fins and eyes from earlier than the Cambrian, I'm sure that the theories will be adjusted accordingly.

 

[Dinwar]

How many fossilized worm eyes do we have?

How many fossilized sea cucumber fins?

 

[Dancing David]

No one's denying that complex animals radiated fairly close to the Cambrian. However, the Ediacara has a rich fossil record (well, relatively) of complex animals well before the Cambrian. Also, as I stated before, the issue is one of preservation. Until organisms evolved hard parts there isn't much chance of being fossilized; it's been argued quite effectively that the C.E. is really the diversification of shell forms.

Exactly, preservation is the key!

No Magdelenian revolution either.

 

[jasonpatterson]

The real question with the CE is why only one or two phyla evolved after it. We should have expected something similar when animals invaded land, and we didn't see it (though to be fair, Arthropoda is polyphyletic). Chordates are still chordates. Mollusks are still mollusks. Why the dirth of novel body plans?

I don't know that we really should have expected a great diversification of body plans once the move to land occurred. The relatively few phyla that colonized the land already had successful body plans that had been tested over millions of years in the oceans. Major changes to this body plan are unlikely to have given a particular survival advantage and they very quickly snapped up all of the good real estate.

A bigger problem might be that the concept of a phylum is necessarily subjective to some degree. We have to decide when to insert a break between phyla. A good example is the arthropods. You state that they are polyphyletic, but I've also read arguments suggesting that arthropoda is too narrowly defined and that it should include things like water bears. I'm not saying you're wrong, just that there is a difference of opinion, and the very fact that a difference of opinion even exists in something that should be objective is problematic. Why should limbed vertebrates not be their own phylum, separate from limbless vertebrates. It's a major body plan change, but we've decided that it's insufficiently large a change to qualify as a new phylum.

Cladistics helps with this, I think, because it can show where all of these new features came from. Many people get the impression that the CE happened all at once, with all of these new phyla appearing more or less simultaneously from some root stock of animal life. (I realize that Linnaean taxonomy doesn't say anything like that, of course.) Cladistics shows how some innovations (bilateral body plan, let's say) happened earlier than others (protostome vs deuterostome development) and each of these divisions is objective.

Here is a nice cladistic tree of life that tends to point toward what I'm saying. It has a time scale at the bottom (the second pink band from the left is the Cambrian, for those who can't read 6 point font easily...) It shows that a whole lot of action was indeed going on in the Cambrian, but it didn't happen all at once or in some crazy, unexplainable fashion, and that there was diversification both before and after it as well.

 

[Dinwar]

The relatively few phyla that colonized the land already had successful body plans that had been tested over millions of years in the oceans.

The reason proposed for the C.E. in this thread (well, the one that's not biogeochemical) is the opening of new niches. The move to land necessarily would have opened new niches--MANY new niches, far more than the change from benthic to nectic would have.l ook at plant evolution for exmaples--the entire kingdom arose from the transition. No similar event occurs with animals. Chordates are chordates, be they swimmers or flyers or crawlers. Arthropods are arthropods, and if you study lobsters you can make sense of cockroaches fairly easily. Annalids are annalids, platihelmenties are platihelmenties, etc., despite having vast new niches open up AND being exposed to completely novel selective pressures. It's odd.

You state that they are polyphyletic, but I've also read arguments suggesting that arthropoda is too narrowly defined and that it should include things like water bears. I'm not saying you're wrong, just that there is a difference of opinion, and the very fact that a difference of opinion even exists in something that should be objective is problematic.

Oh believe me, I know the difference of opinion on this matter! In fact, that's one reason I use it--it shows just how bad systematics is as a field of study. It's based on a completely erronious definition of "species" (and an entirley Platonistic one at that), and has little to nothing to do with evolution (it was developed long before Darwin). It also has an insidious tendency to channelize our thinking about things. Whenever we see an organism in the fossil record we attempt to put it in one of the known phyla (SOP for systematic studies). This has lead to many, many arguments which amount to "But that phyla doesn't exist today!" when discussing ancient organisms, particularly of the CE.

Cladistics helps, but the problem is that it's not terribly useful. Museums need to put things in drawers, and we need a way to discuss things that's at least SOMEWHAT stable. Systematics provides that. Cladistics is vastly more accurate, but complicates discussions to such a degree that many paleontologists have argued that it would render discussions of classification impossible. And cladistics is far more open to interpretation than people think. I mean, it's assumed that each node will have two branches coming out of it in any caldistic analysis. However, there's no reason to think that any one species will give rise to only two daughter species at any given time. Subpopulations of a single species can theoretically produce multiple new species contemporaneously, and cladistic analysis has no way to differentiate between such an event and situations where you just don't have enough data to say what came from what. Cladistics may be a step in the right direction, but it's no where near a full step yet.

 

[Soapy Sam]

In any discussion of this topic, the matter of what went before the Cambrian must be considered- particularly the extremes of climate often referred to as "Snowball Earth". If there were, as we now strongly suspect at least three periods of all but absolute global ice age in the late precambrian, we would expect to see rapid expansion after they ended into the range of new habitats formed.

As for further back- much further back- I suspect this is a question of improved technological identification of micro- and even nano-fossils, which I expect exist in abundance if we only knew what to look for.

Of course, the older rocks are, the more chance they have been metamorphosed at the sort of pressure and temperature that erases all fossil and mineralogical evidence of life, or even lifelike chemistry. But there are a lot of very old, low grade metamorphics. It's possible we will develop the ability to spot life signatures in these.

I think explosions are , like car chases, of interest to adolescents.

Grown ups prefer a good, slow, detective story.

 

[Dinwar]

As for further back- much further back- I suspect this is a question of improved technological identification of micro- and even nano-fossils, which I expect exist in abundance if we only knew what to look for.

What you're looking for are chemical trace fossils. Things like REALLY low delta-C13 (indicative of methane production), or certain sulfur ratios, things like that. We can find them (well, other people can--I can interpret the results but it's not my thing, I prefer chordates), and in fact are very good at finding them. Trouble is, they don't tell you what the organism that made them IS the majority of the time. You know methane was produced, but can't tell the difference between a cow fart and bacterial waste.

Some of them may even survive metamorphism. Remember, a rock can only utilize the minerals in or near the rock--meaning that if you start out with a rediculously low amount of C13 you'll end with a rediculously low amount of C13. Harder to interpret, but not impossible. The real issue is outcrops--old stuff tends to be buried, so we don't see as much of it. Check out the criticisms to Sepkouski's Curve to get a better idea of the issues involved with that little.....barrel of worms? Keg? Perhaps megatanker....

 

[jasonpatterson]

The reason proposed for the C.E. in this thread (well, the one that's not biogeochemical) is the opening of new niches. The move to land necessarily would have opened new niches--MANY new niches, far more than the change from benthic to nectic would have.l ook at plant evolution for exmaples--the entire kingdom arose from the transition. No similar event occurs with animals. Chordates are chordates, be they swimmers or flyers or crawlers. Arthropods are arthropods, and if you study lobsters you can make sense of cockroaches fairly easily. Annalids are annalids, platihelmenties are platihelmenties, etc., despite having vast new niches open up AND being exposed to completely novel selective pressures. It's odd.

It's the creatures that filled those new niches that makes the difference though. In the case of the CE the new niches were able to be exploited due to a few fairly revolutionary advances. It basically went from sponges and jelly-critters to a whole slew of new plans. Virtually any change to a blob of slime or a gutless worm can result in a radically new body plan, and in competition with blobs of slime or gutless worms, a large number of alterations were able to take hold and diversify. In the case of the move to land, the creatures that came on shore were already good competitors, and the type of radical change that would merit reclassifying them as a new phylum after that move just wouldn't happen. Honestly, I have a hard time imagining a physiological change to tetrapod that would merit classifying it as a new phylum.

We might have tried to distinguish among amphibians/reptiles and birds/mammals based on their differences, but we've already classified chordata as a phylum. Once you've got that in your mind, you're pretty much stuck. If the notochord is the primary identifier of the chordates, tetrapods would essentially always be chordates regardless of what they turn into (assuming that they don't lose their notochord at some point, which would be a neat trick now, but even then, presumably whatever species lost its notochord would retain many of the other features indicative of chordates...) That's a big part of why I like cladistics, it just doesn't have this problem.

 

[Dinwar]

In the case of the move to land, the creatures that came on shore were already good competitors, and the type of radical change that would merit reclassifying them as a new phylum after that move just wouldn't happen. Honestly, I have a hard time imagining a physiological change to tetrapod that would merit classifying it as a new phylum.

There's to counterpoints here: First, it wasn't just tetrapods that came on land, and second, WHY was there so little change in body plan?

The first is fairly obvious--worms, arthropods, and the rest made it onto land, and any potential changes that could re-classify those into a new phylum in water would be identical on land.

The second is the real question here. The thing is, in the Cambrian there was a great deal more than just sponges and blobs and slime bits--we know there were relatively complex critters floating and swimming around already. Then something happened, and we got msot of the modern phyla. What was that something? We don't know yet--it's still being debated. But I see no reason why goinig from an aquatic to a terrestrial environment would be LESS extreme than any change that could have potentailly caused the C.E., and therefore you'd expect similar amounts of change in form. We don't see that.

The concept we're dancing around here is channelization of body plans. We know it happened, we just don't have a good handle on why. And that's what I'm asking, essentially.

We might have tried to distinguish among amphibians/reptiles and birds/mammals based on their differences, but we've already classified chordata as a phylum. Once you've got that in your mind, you're pretty much stuck.

There's also the human tendency to want to put roughly equal amounts of things into roughly equal catagories. I've heard people argue that phylum Arthropoda should be broken up solely because it includes too much diversity (I laughed a bit). And the simple fact is that there's not a lot of diversity in chordates. We've all got more or less the same body plan. But arthropods? Those things should have gone NUTS when they came on land. Same with worms. Plants did. Something was different with animals in the Cambrian.

That's a big part of why I like cladistics, it just doesn't have this problem.

Yeah, it replaces that problem with about five hundred OTHER problems.

 

[Soapy Sam]

As always, I'd warn against confusing taxonomy with reality. Map isn't territory.
We might debate the actual existence of "species" as anything other than a taxonomic file; Phylla don't evolve, because they don't exist outside academic institutions. Beasties evolve. Or points in a hyperspace of potential bodyplans are (however briefly) occupied before being abandoned. We can look at it in a number of ways- but no human visualisation of evolution is more than an approximation of reality. (Albeit a useful one).

On the body plan shortage business- my guess is this is a matter of genetic inertia and competition.

Consider the electric car. It seems a grand idea, but it can't evolve from an internal combustion car and that's all we have- so it needs to be invented from scratch. But as a new kid on the block it must also compete with IC cars with a range of 500miles and top speeds of 150mph. It can't. It will only succeed if it has some killer app- which may be "greenness"- but it's far more likely it will replace IC cars only after the mass extinction thereof due to oil prices hitting $X per gallon.

The natural selection analog of that process requires a similar clearin g of the competition. But even the end Permian extinction wasn't thorough enough. There were always enough of the tried and tested designs around to suppress the invention of new ones. Compare the explosion in personal computer design between 1970 and 1980; the winners were Apple and the generally crappier, but workable IBM PC. And that's how it stayed until phones started moving into computer habitats.






Once you have a few body plans that work, any radical departure therefrom would need a massive natural advantage, just to compete with what already exists. Air breathing out of water is one, flight another. Both enabled their possessors to move into an environment with little competition. (WW1 warplanes would have stood little chance against WW2 monoplanes- but there were no Spitfires around and they could fly, which infantry couldn't).

 

[Dinwar]

but no human visualisation of evolution is more than an approximation of reality. (Albeit a useful one).

And systematics didnt' even try--it was a way to group things into comprehensible categories, long before the idea of evolution took hold.

On the body plan shortage business- my guess is this is a matter of genetic inertia and competition.

Those are two of the leading theories. Some have argued that the HOX genes are no longer flexible enough to allow for the development of new body plans, and therefore we don't get them. This is not entirely true (fruit fly research into genetics shows it to be at least partially possible to develop novel body plans), but considering the difficulty with changing body plans of, say, a vertebrate it's also nto entirely false. Others have argued that our view of natural selection as a driving force in evolution are flawed, and that a better way to view natural selection is as a conservative force--organisms always occupy local highs in fitness space, meaning that any change minor enough to not outright kill the organism ("hopeful monsters") is less well suited to the local environment. This would necessarily channelize bodyplans, but it doesn't explain why when new environments are opened new body plans don't arise (fitness space is a function of competition, predation, and food availability as much as the environmental setting).

The real answer is probably a combination of those, along with some stuff we don't know yet.

 

[HansMustermann]

Actually, I'd say it (also) has something to do with _how_ we evolve.

In terms of programming, God seems to favour spaghetti code, basically. The DNA code mostly just grows over time, and it's a lot of proteins (genes) which modify the way earlier modifications work, which in turn were modifying modifications to modifications.

Think basically about the dictum, "ontogeny recapitulates phylogeny". It's not entirely true, but it is what I'm talking about. You get body type A, and then some genes appear that basically say "and then this and that must happen" and they encode body plan B, and then some new genes appear which add some more stuff to the end of the differenttiation to B and you get C, then more changes and you get D, and so on.

I'm not sure it's easy to get a viable modification that goes all the way to A, since by now waay too many other genes depend on those that encoded body A. One modification in one of those probably creates hundreds of unpredictable and not necessarily wanted changes in how other stuff works in that spaghetti code.

In a way, it's a real case of irreducible complexity. A change in A would need also some changes in the genes for B, and in the genes for C, and in the genes for D, to even be viable. And the probability of all that happening at the same time is really low.

In programming terms, we're basically at a point where probably 90% of the code in you would be easier to just throw away and rewrite from scratch than to change. Except there doesn't seem to be a God to do that rewrite, or at least he doesn't seem interested in it.

 

[DC]

God seems to

what has this guy to do with it?