Mystery of Cambrian Explosion - Solved?

[Broomstick]

I'm re-reading In the Blink of an Eye by one Andrew Parker, which is an excellent book for those of us who are evolution supporters and enthusiasts. (Also, thought it was time for an evolution thread unrelated to screaming fundies)

For any of you who don't know (and if you don't, you need to learn), the "Cambrian Explosion" is one of the long-standing mysteries of the history of life. Before the Cambrian there were very few fossils of multi-celled animals, and none with fossilizing hard parts until just a teeny little bit before the Cambrian era. From the Cambrian forward, starting about 543 million years ago, we get the fully glory of diverse animal life, including hard parts like skeletons and shells. The diversity of life "exploded" as it never has before or since, and the big question was always "why"?

Darwin hypothesized that the "explosion" may be an illusion and it was just a matter of finding the right fossil beds. Well, a century later, with many more fossils dug up, it looked like something drastic had happened on the PreCambrian/Cambrian border. It wasn't an illusion - life DID change incredibly 543 million years ago (give or take a million)

And no one was able to come up with the why of it - WHY did life change, what happened?

Andrew Parker thinks he has the answer. After reading the book, I think he may well be right - his explanation is supported by evidence, and it makes logical sense.

And while I was going to delve more into this, I have to run off to work (darn life interfering with hobbies again!) and will resume later...

... but meanwhile, who has heard of Dr. Parker's theory? Does anyone know the "shorthand" name for it? And even if you've never heard of Dr. Parker, what's YOUR best guess for the cause of the Cambrian Explosion?

[Lusankya]

Sight? I remember reading something along those lines for something, but it may have been the cause of a mass extinction at some stage, so I might be thinking about a different period.

[Anguirus]

I just did a report on the Cambrian Explosion for Bio, but I am not acqainted with Dr. Parker's work.

As far as I know, it's still very mysterious. What does he say?

[AMX]

*google*
Apparently, he says the driving force was the development of a working eye.

[Ariphaos]

Didn't chitin also evolve then?

[GrandMasterTerwynn]

Didn't chitin also evolve then?

Yes, hard external bits evolved then. Parker theorizes this was because there was suddenly a reason to develop armor (as the predators suddenly got much more efficient, on the account of being able to see better than they did before.) What it doesn't quite answer is what triggered the evolution of a camera-type eye . . . after all, it seems that light-sensitive patches have been present on species for a hundred million years before the Cambrian, which begs the question of what the delay was.

Though, this might be answered by looking at what went on at the transition between the pre-Cambrian time periods, and the Cambrian.(1) At that time, Earth was emerging from a fairly significant glaciation event, so sea levels were rising quickly, opening up a lot of new shallow marine habitats. Also, this was apparently the first time that Earth's oceans became oxygenated in any real quantity. (Though there was plenty of oxygen in Earth's atmosphere, the oceans were highly efficient at locking away any that entered the water. At the Cambrian, the efficiency dropped off enough for the oceans to become oxygenated.)

The significance of these two factors is that there were new niches for life to colonize, and it became metabolically favorable for life-forms to support such energetically expensive processes as shell formation, and the care and feeding of a nervous system sophisticated enough to process visual input.

[Broomstick]

Dr. Parker's theory is sometimes called the "lightswitch theory", although he makes it very clear that light has always existed in many earthly environments, it's vision that is key here. He theorizes that the Cambrian explosion occured in a 5 million year window and it was the evolution of practical vision that spurred the evolutionary event.

Sight? I remember reading something along those lines for something, but it may have been the cause of a mass extinction at some stage, so I might be thinking about a different period.

Yes, the evolution of sight would have caused a mass extinction among pre-cambrian life forms that had no defenses against a sighted predator, meaning pre-Cambrian lifeforms represented by the Ediacara formation (as one example) didn't last through the transition.

Part of his theory is that it was the trilobites that first evolved vision, it was at that point they took on a predatory role, and this accounts for the sudden blossoming of trilobites. It was predatory pressure on other life forms that spurred the development of sight in other arthropods in the Cambrian (the only phylum with sight in this period) and armor and motility in other life forms. Presumably the first camoflague would also arise during the Cambrian, but it is not possible to prove that through fossils.

Didn't chitin also evolve then?

Yes. Chitin, various types of armor and skeletonization, jaws, spines, legs, antennae, among other things. Like eyes.

Before the Cambrian body types were mostly sponges, jellies, anemones, and lots of wormy-looking things. After the Cambrian pretty much everything else suddenly appears, including the first representatives of phylums such as the Chordates (that's our phylum). All within 10 million or so years of the p-C/C division.

Just before the earliest Cambrian strata there are some fossils of what seems to be a soft-bodied trilobite which includes markings that aren't eyes (trilobite eyes are calcite, a crystal, very hard for animal bodyparts, and fossilize readily) but are in the correct place and are the correct shape to be eyes - or proto-eyes.

Another interesting aspect of his theory is that eyes evolved not once but at least six times, eyes can evolve from simple light detectors to a fully functional camera-eye in 200,000 years, and the intermediate forms between actually have use - which he backs up by example from living creatures. Some bacteria, for instance, have "eyespots" (ironically, he photographs some of those along the path the Burgess Shale quarry), as do things like some jelly fish, which are simple light/no light detectors. Jelly fish and some shellfish have "cup eyes", which are a more refined light detector - the provide directional information as well as light/no light but can't be called true eyes and they don't provide true vision. You can then move into "pin hole camera" eyes, which are basically a more refined cup that now provide true vision, but without using a lens - this is found in the chambered Nautilus. Then we have camara eyes with lenses, which can evolve from either exoskelton or skin (the lens in our eyes, along with the whites of our eyes, are derived from skin cells in the early fetus). There are routes for several types of compound eyes, all of which are found in nature, in living creatures.

In other words, despite being a very complex and biologically expensive sense, evolving vision isn't that difficult once living things are under selection pressures to adapt. Rather like flight - another expensive and complex biological trait - has evolved multiple times (insects, dinosaurs, birds, and mammals)

He also points out that in a lighted environment even creatures without eyes must adjust to visual predators - thus, blind critters evolve camoflague or warning colors; spikes and armor; and even types of behavior that make them less visible although the creatures have no vision whatsoever. Once someone evolves vision it starts affecting everything living around them. He also talks about cave creatures living in total darkness, and in some instances emerging back into lighted environments.

I found it a very methodically laid out book, and it covered a lot of territory, including new techniques that give some indication of the colors of creatures, even 500 million years ago. He also seems pretty careful to distinguish between hard facts and speculation.

As I said, if you're an evolution enthusiast it would be worth your while to pick up a copy of the book. It helps if you're already familar with the era in question, but it's not vital as background.

[Broomstick]

Parker theorizes this was because there was suddenly a reason to develop armor (as the predators suddenly got much more efficient, on the account of being able to see better than they did before.) What it doesn't quite answer is what triggered the evolution of a camera-type eye . . . after all, it seems that light-sensitive patches have been present on species for a hundred million years before the Cambrian, which begs the question of what the delay was.

Parker does address some of those questions in his book, while being careful to state he hadn't sought the evidence for reason for the evolution of vision, he had been trying to produce evidence that it was vision that was the trigger for the Cambrian event.

Though, this might be answered by looking at what went on at the transition between the pre-Cambrian time periods, and the Cambrian.(1) At that time, Earth was emerging from a fairly significant glaciation event

He does discuss this, and feels the 50 million year gap between the end of the ice age and the Cambrian explosion event to be too lengthy a time period to account for it, although there is ample room for discussion on this point and he doesn't seem wedded to that position. More a matter of "show me the evidence" rather than a complete dismissal.

Also, this was apparently the first time that Earth's oceans became oxygenated in any real quantity. (Though there was plenty of oxygen in Earth's atmosphere, the oceans were highly efficient at locking away any that entered the water. At the Cambrian, the efficiency dropped off enough for the oceans to become oxygenated.)

That's another position under considerable debate, and not just by Parker.

The significance of these two factors is that there were new niches for life to colonize

There are other theories, too, including levels of light penetrating to the upper ocean levels with a variety of possible causes ranging from erosion to cosmic dust clouds. Problem is, we're talking about half a billion years ago - that's a long time! Evidence becomes scare and deteriorates, and our computer models and such become more and more uncertain the farther back we go.

Suffice it to say, if the "lightswitch" theory becomes commonly accepted then the next big question for that era is what lead to the sudden appearance of vision.

it became metabolically favorable for life-forms to support such energetically expensive processes as shell formation, and the care and feeding of a nervous system sophisticated enough to process visual input.

The nervous system to support vision is actually more of an issue.

Although the "box jelly" defies all expectation - it's a jellyfish, for goodness sake! No central nervous system at all, no brain... it's largely a thin double-sheet of tissue wrapped around a bag of goo. But it's got a functional camera eye! Which pretty much makes everyone go

But aside from that little enigma everything else that sees has a certain minimum eye size and a certain minimal sophistication of nervous system. How does the complex nerve net required for vision come to be?

Well, one answer is that it involved appropriately nerves originally used for other purposes. In humans that are blind from birth, parts of the brain normally assigned to vision are reassigned to touch and sound. Likewise, in people born deaf brain areas that normally deal with sound may be reassigned to vision. This makes it plausible that the reverse occured - nerves and brain power originally used for touch or other senses were "re-assigned" to vision.

Something not mentioned by Parker, but which I have long found interesting, is that our eyes are formed from the same tissues are skin is. The color of your retina is directly related to the color of your skin, because in a sense it is skin, a highly modified form of skin.

Anyhow, it's food for thought.

[Broomstick]

What the heck, I supplied some positive, here's a helping of negative:

A hostile review of Dr. Parker's book by Simon Conway Morris: http://www.americanscientist.org/templa ... ttpI6wswR8

Conway Morris was a graduate student at the time he was part of the Cambridge team that re-opened the Burgess Shale Quarry, which is one of the world's best sources of early Cambrian fossils, in the early 1980's, which triggered quite a bit of revision in theories about the early Cambrian. Just thought I'd add it for a contrary view.

(Personally, I'm not a fan of Dr. Morris, as he always struck me as one of those conceited types who's convinced everyone is wrong but himself. Nevermind that he's been wrong a time or two. As an example, he raked Stephen J. Gould over the coals for his use of an early reconstruction of hallucigenia to support his (Gould's) theory of Punctuated Equilibrium, with particular scorn for his use a "flawed" reconstruction. And who's "flawed" reconstruction did Gould use? The one originally published by Simon Conway Morris.... At least Gould had the class to admit his error in print when new evidence came to light)

[Anguirus]

Like I said, I haven't read the book under discussion, but Conway Morris makes quite a few excellent points in that review. The section he quotes with "Let there be light" does sound as if it borders on the naive. And he doesn't exactly get hostile...at least not to the extent that's routine on this website.

[Broomstick]

Well, no, the Conway Morris letter I linked to appeared in a respectable scientific journal which is unlikely to descend to levels of overt viciousness seen on this message board.

Parker's idea is intriguing - and not yet perfect, as he mentions several times in the book. And, as you said, Morris raises some good objections. This is how science is done.

It rather reminds me of the initial proposal of the impact-theory for the end of the dinosaurs which was not well received at all at first, and there were a lot of calls for "where's the crater?" for a couple decades. Well, they think they've found the crater under the Yucatan. This theory is probably the most widely accepted as explanation for what happened to to trigger that mass extinction (which affected far more than the dinosaurs) but still has dissenters and objectors and unanswered questions. Not too different from the "punctuated equilibrium" theory of evolution (of which Gould was one of the originators) which, again, has both supporters and dissenters.

Part of the reason I put this up is that the Fundies like use this "dissent" among scientists as amunition in their arguments. Thing is, these bickering scientists do not disagree that evoluation occured, what they are disagreeing on is the details. The other thing is that, while the Fundies believe they have all the answers the scientists know they do not - but to the Fundies, it ain't a good description of the world unless it does have "all the answers".

Parker's theory, if true, also blows a hole is a pet argument of the Fundies - that the eye is so wonderous and complex it couldn't have evolved by chance even once. If Parker is right, though, eyes evolved at least six times. He even describes a possible pathway to get to vision. Huh. Well, it may be complex, but apparently not so hard to assemble out of raw parts.

Another thing - the "Cambrian explosion" has also been a Fundy favorite for a long time. Evolutionists couldn't come up with an explanation, so the Fundies see an opening for divine intervention. But if Parker does have the correct answer (and the jury is still out on that) then no divine intervention is required and we have a reasonable, unmiraculous explanation.