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Introduction
Anti-evolutionary apologists have long claimed that the lack of transitional forms between higher taxonomic ranks is evidence of creation. Gish (1993, p. 135) writes:
"The fossil record is powerful, positive evidence for creation, and every new discovery strengthens the case for creation and exposes additional difficulties for evolutionary theory. Evolutionary theory is, of course, dead, as long as the two huge gaps between single-celled organisms and the complex invertebrates and between complex invertebrates and fishes continue to exist. The total failure to reduce these gaps, let alone close them, in spite of an intense search by thousand of paleontologists during more than 125 years, establishes beyond doubt that the required transitional forms will never be found. The fact that the gaps between all higher taxa, such as families, orders, classes, and phyla, are systematic and almost always large, is simply additional confirmatory evidence for creation."
Phillip Johnson (1998) writes:
“The "Cambrian Explosion" is the sudden appearance of the major animal groups (phyla) in the rocks of the Cambrian era, without apparent ancestors. As Dawkins himself has put it, "It is as though they were just planted there, without any evolutionary history." Of course, Dawkins and all other Darwinists believe that this appearance is an illusion caused by the incompleteness of the record, and that a complete fossil record would show a universe of transitional forms and side branches, all having evolved by tiny steps from a single common ancestor. Gould raises a radically different possibility. He explains that there are two possible explanations for the absence of Precambrian ancestors: "the artifact theory (they did exist, but the fossil record hasn’t preserved them), and the fast-transition theory (they really didn’t exist, at least as complex invertebrates easily linked to their descendants)."
Stephen Robinson (1998, p. 3) writes:
“The sudden profusion of marine life in the Lower Cambrian is commonly referred to as ‘the Cambrian Explosion’. In the space of about 5 million years, most extant phyla and classes of marine invertebrates as well as others that did not survive even the Cambrian suddenly appeared out of nowhere.”
This view is widespread among young and old earth creationists. But is it true in light of modern data? What I will attempt to show in this note is that the ‘gaps’ in the evolution of Cambrian life forms are now being filled in by data obtained over the last 15 years; much of this data coming in within the past 2 years. While there are older interpretations of the Precambrian animals as being totally unrelated to modern animals(such as Seilacher's Vendozoan hypothesis), the newer data is showing evidence of relationships between the Ediacaran fauna and that of the early Cambrian. We will look at the evolutionary sequences which lead from Precambrian worm-like creatures to the arthropods, molluscs, brachiopods, and annelids, four different phyla.
Secondly, I will show that it is absolutely false that the majority of animal phyla appeared in the Cambrian. While this is widely stated by apologists and paleontologists, it is actually an assumption not borne out by the data.
Definition
Before we can discuss phylum level evolution, we must know what a phylum is. A phylum is a group of animals sharing a similar body plan. Gilbert states(1991, p. 831):
“Only about 33 animal body plans are presently being used on this planet (Margulis and Schwartz, 1988). These constitute the animal phyla.”
There are a couple of cautions about the use of the term phyla. A phylum is assigned to a given creature based upon its having the shared characteristic body plan. Occasionally, however, different body plans are not assigned different phyla and this creates an appearance that phyla can't evolve. A case in point concerns a deep-sea sponge which is classed with the Porifera in spite of the fact that it has an entirely different body plan. Vacelet and Boury-Esnault (1995, p. 335) relate:
“Our results raise fundamental questions about the validity of
characteristics used to distinguish the phyla of lower invertebrates. A sponge is defined
as a ‘sedentary, filter-feeding metazoan which utilizes a single layer of flagellated
cells (choanocytes) to pump a unidirectional water current through its body. Except for
being sedentary, the cave Asbestopluma and presumably all Cladorhizidae lack these basic
sponge attributes. In an extreme environment where active filter-feeding has a low yield,
cladorhizids have developed a mode of life roughly similar to that of foraminiferans or
cnidarians. Their feeding mechanism relies on passive capture of living prey and on transfer
of nutrients into the body through intense cell migrations, the analogue of cytoplasmic
streaming in foraminiferan pseudopodia. This may be compared to the emergence of macrophagy
in abyssal tunicates, also accompanied by a reduction of the filtering system although in
Cladorhizidae the result is more extreme, with a main body plan different from Porifera
and resembling no other modern anatomical design.”
“Such a unique body plan would deserve recognition
as a distinct phylum, if these animals were not so evidently close relatives of Porifera.
Their siliceous spicules show clear similarities to several families of poecilosclerid Demospongiae.”
In cases like that above, the lack attribution of phylum rank for these 'sponges' hides the fact that the Porifera may very well have given rise to an independent phyla. The only real connection between the two groups are the spicules which act as evidence of common descent. If the Cladorhizid sponges were to lose the spicules, the connection between the two groups would be lost. Body plans are obviously more of a continuum and difficult to separate than the simplistic concept of phyla espoused by anti-evolutionists would imply.
Phyla were defined more than a century ago based upon descriptions of modern fauna. Using this schema forces ancient animals to be included in one of the modern phyla and ignores the possibility of phyla-level evolution. The problem is similar to that of describing a tree only by examining the very tips of the branches and not looking at the trunk. This makes the concept of a phyla time-reversed. It is forcing present concepts back into the past, which is not the way that evolution was supposed to have occurred.
One further definition. It must be made clear that we are discussing multicellular animal life. There is evidence for multicellular algae going back much further in time.
The Setting
The Cambrian/Precambrian boundary is no longer considered as the place where life suddenly appears. There is a continuum of life across this boundary. Grotzinger et al (1995, p. 603-604) write:
"Once held as the position in the rock record where the major invertebrate groups first appeared, the Precambrian-Cambrian boundary now serves more as a convenient reference point within an evolutionary continuum. Skeletalized organisms, including Cambrian-aspect shelly fossils, first appear below the boundary and then show strong diversification during the Early Cambrian. Similarly, trace fossils also appear first in the Vendian, exhibit a progression to more complex geometries across the boundary, and then parallel the dramatic radiation displayed by body fossils."
Evidences of macroscopic life forms are now found as early as 680 myr ago in the form of worm burrows (Pagel, 1999, p. 881). And several modern phyla are now claimed to appear in the Precambrian and thus are not part of the supposed 'Cambrian Explosion.' These are:
Phylum Porifera (Sponges Brasier, Green and Shields, 1997, p. 303)
Phylum Mollusca (Fedonkin and Waggoner, 1997, p 868)
(This may be a proto-Mollusc rather than a true mollusc--Campbell 2001)
Phylum Annelida (Cloud and Glaessner, 1982, p. 788)
Phylum Cnidaria (Conway Morris, 1998, p. 29)
Phylum Arthropoda (Waggoner, 1996, p. 190)
For a chronology of the events surrounding the Cambrian/Precambrian boundary see:
Cambrian/Precambrian Chronology
One final issue must be discussed before examining the paleontological evidence. What is a transitional form? Anti-evolutionists say they don't exist but they often don't define what it is that doesn't exist. A transitional form is an animal that will share traits between the two different groups. For instance, an animal that is transitional between worms and arthropods has both worm-like and arthropod-like traits. It is very difficult to classify such an animal because it doesn't quite fit the definition of either group. So, it is unfair to insist that the animal be classified in one or the other groups and then claim that there are no transitional forms. Due to the vagaries of fossilization, it is not expected that we will ever find animals which are on the direct line between the two groups, and if we do find an animal on the direct line, it is not clear that we would recognize it as such. So what we have in the fossil record are animals that are close to the line of descent. But even these animals will show many of the traits that were involved in the actual line of descent between the two phyla. Some might claim that this is cheating. It isn't. Even if we found the individual animal that is on the direct line of descent between two groups, we would not have any way of knowing that. Thus, we must treat all transitional forms as if they are near to the actual line of descent.
Worms
In the late Precambrian, the Vendian Period (620-540
myr ago) to be exact, worm burrows become increasingly numerous. Indeed,
recently, larvae of segmented worms have been found in the Doushantuo formation
of China (570-520 myr; Bengtson and Zhao, 1997, p. 1645). Since just prior
to the Vendian the only evidence for macroscopic life was that of the worms,
if the various phyla were to evolve, it must have been from the worms.
Arthropods
The first in the line leading from worms to arthropods is an animal similar to the lobopod, Aysheaia. It can be seen at
http://nmnhwww.si.edu/paleo/images/dayshia.gif
and
http://nmnhgoph.si.edu/paleo/images/fayshia.gif
This animal is basically a 'worm' with ‘legs’, which are lobopods. A lobopod consists of muscles surrounding a blood-filled cavity. Lobopods are soft and pliable but they accomplish the purpose of moving the animal along. Each lobopod has a couple of claws at its end and the front two have spikes which apparently serve the purpose of grasping prey. Lobopods will be very important in showing the evolutionary affinities leading to the arthropods. But there is nothing arthropodian about this animal.
Anomalocaris is similar to the animal believed to be in the line leading to arthropods. It can be seen in reconstruction and actual fossils at:
http://www.science.uwaterloo.ca/earth/waton/anomalo.html
http://www.geo.ucalgary.ca/~macrae/Burgess_Shale/Anomalocaris.gif
Anomalocaris shows its relation to the lobopodians by having lobopods for legs. (Conway Morris, 1998, p. 183 and p.196 n. 24 and 26 cites Budd for this.). Budd, apparently published after Conway Morris' book went to press. Budd writes of Pambdelurion, an anomalocarid found in the Sirius Passet fauna of Greenland (Budd, 1998, p. 204):
"Although anomalocaridid-like in its overall morphology, possessing a large body with lateral flaps, a single pair of cephalic appendages and a 'peytoia'-like mouth apparatus, it differs profoundly in the possession of lobopod-like limbs in both the cephalic and trunk regions."
But Anomalocaris also has one characteristic feature of an arthropod, two arthropod ‘legs’ are attached to the front, by the mouth and act as grabbers for catching prey. These spikes are similar to those seen on the front legs of Aysheaia This can be seen at (look at the legs on the upper right of the photo being served from):
http://www.nmnh.si.edu/paleo/images/fayshia.gif
In addition it has a tail fan and flaplike projections that perform several purposes: protection, propulsion, and respiration. Conway Morris (1998, p. 184) says,
“Of these, perhaps the last was the most important. Imagine that in due course the flaps seen in Anomalocaris were modified. The leading edge now formed an elongate bar and behind it the structure was transformed in a series of trailing filaments. This, of course, would be reminiscent of the gills that arise above the walking legs of many arthropods. Thus, if the lobopod limbs were transformed into jointed appendages and the flaps into gills, then one could envisage, at least in broad outline, the transformation between an animal similar to Anomalocaris and a fully fledged arthropod.”
Thus Anomalocaris has traits that link it to the lobopods as well as to the arthropods. But it also has traits that link it to even more ancient ancestors. Its mouth is wormlike. Collins (1996, p. 291) notes that Anomalocarids have
" 'circum-oral' sclerites'[radiating teeth] indicate that the anomalocarids 'were related to aschelminth worms rather than to arthropods'. Studying other specimens of the same Chengjiang species, Ramskold described the abdominal appendages in greater detail as 'three or more [postoral] limb pairs modified into large gnathobases which form part of the masticatory apparatus', and 'trunk limbs' with 'small, jointed clawed endopods' and exopods...modified into large lateral flaps'. "
Being related to worms is not inconsistent with being related to arthropods IF the animal is in the process of evolving from worms into arthropods. The mouth of Anomalocaris is seen below:
http://senckenberg.uni-frankfurt.de/burgess/burgess22.jpg
There are other links between the Precambrian and the Cambrian arthropods. Bomakellia, a Vendian creature shows amazing similarities to trilobites, yet it is a precambrian creature. It can be seen below.
Here is a picture of Bomakellia kelleri from http://goniagnostus.homestead.com/files/Bomakellia_colorsm.jpg
And Spriggina floundersi, another Vendian animal also shows similarities to the trilobite in the head region. It can be seen at
http://www.paleobase.com/gallery/metas/Spriggina1.jpg
For comparison with a trilobite see:
http://www.flash.net/~mortongr/trilobite.jpg
For creationists to claim that there is no evidence of phyla level transitional forms, is wrong. There is evidence. When Walter Bradley wrote in 1999 (p. 219-220), “The Cambrian Explosion of all of the major animal phyla in a very short five-million-year time period remains one of the greatest mysteries of origins research today.” all of the data cited above was available. The same can be said for what Robert C. Newman (1999, p. 116) wrote in the same book. “Rather, new types of plants and animals regularly seem to show up without any record of close predecessors. This is especially true of the so-called ‘Cambrian explosion,’ Where all the major body plans (phyla) of the animals appear in just five or ten million years (more than five hundred million years ago), with nothing comparable having happened before or since. This phenomena look more like Gordon Mills’ proposal that somehow God has added new information to the genes, or perhaps Robert DeHaan’s suggestion that new genetic programs were turned on. It is possible that most living things are descended from one or a few common ancestors, but if so, the transitions look too abrupt to be purely natural phenomena.” Christians need to be honest with their readers and tell them of the evidence.
So, what we find in the fossil record are animals
with all stages of the transition between worms and arthropods.
We have
1. worms
2. lobopods
3. Anomalocaris with both lobopods and arthropod
appendages and possible incipient arthropod gills
4. arthropods.
Brachiopods
The evolutionary lineage of the Brachiopods appears to begin with a creature called Halkieria evangelista from the Sirius Passet formation of Greenland which is about 525 million years ago. This animal has three broad zones of hollow scales. These scales are hollow and are inserted into the animal's body in such a fashion as to make a chainmail-like armor covering the body of the metazoan. These small scales overlap and make a flexible armor with which to defend against attack. The back of the animal has palm-shaped scales, the sides have knife-shaped of scales and the edges of the belly have sickle-shaped scales. This will become important later when we talk about an animal transitional to the annelids. The bottom of Halkieria was soft, without scales and could best be described as being like a snail's bottom. It enabled the animal to glide across the ocean floor. This fact will become important when we talk about annelids and molluscs. Below is Halkieria
The shell at the ends of Halkieria is very similar to the shells of the brachiopods seen in the earliest Cambrian strata, like Obolella (Moore, Lalicker and Fisher, 1952 p. 224) Compare the shell with http://www.flash.net/~mortongr/obolella.jpg
What is believed to have taken place is that the two shells at each end of Halkieria were used by this animal as protection devices. The animal would curl up between the plates when threatened. (Conway Morris, 1998, p. 192-194). There are other facts that support this interpretation. The edges of brachiopod shells have chitinous bristles, called setae, which extend away from the shell. The microscopic structure of the setae is identical to that of the chaetae of polychaete worms and we will see that these evolved from the scales of Halkieriids. In some of the brachiopods the setae are segmented as is the case with Halkierian scales. And finally, "Neocrania", a modern brachiopod begins life as mobile animal that moves across the ocean floor, bearing the setae of a brachiopod but lacking the shell. Eventually it settles down, folds itself in the middle, secretes a shell and remains fixed like others of their kind. This is the type of action Conway Morris postulates as the steps leading from Halkieria to brachiopods.
Molluscs
Once again the path to molluscs and annelids goes through Halkieria. Halkieria had the snail-like undersides of molluscs and thus moved across the ocean floor in a similar fashion. Another animal which probably evolved from the Halkieriids and shared some of the same traits with them was Wiwaxia. The connection between the two was that both had the muscular snail-like foot, they shared the same microscopic structure in their scales which were hollow in both species. Wiwaxia possessed something amazing, the characteristic mouth and feeding mechanism of a mollusc which is called a radula. (Conway Morris, 1998, p. 186-187) It is very easy to see how Wiwaxia could become a mollusc simply by losing the scales and dermal plates and then adding a different type of shell. Wiwaxia can be seen at:
http://www.nmnh.si.edu/paleo/images/dwiwax.gif
As can be seen, Wiwaxia has the three zones of different scales. The protruding sclerites are merely cultrate sclerites that have been elongated.
Annelid Polychaetes
Wiwaxia appears to be on the way to becoming a polychaete annelid. Given the way Wiwaxia and a polychaete look, it might be difficult to make such a claim. (Compare the polychaete Canadia below with Wiwaxia)
http://www.ucmp.berkeley.edu/annelida/canadia.gif
What connects these two very different looking creatures?
The fact that the microscopic structure of the notochaetae (the curved
spines seen above) of the polychaete is identical to the microstructure
of Wiwaxia's scales, proving a connection between the two (Conway Morris,
1998, p. 187).
One other connection between these two creatures, and the halkieriids, firms up the suggested phylogeny. At least one specimen of Wiwaxia (USNM199936) had a structure on one end resembling a brachiopod shell, which Conway Morris and Peel (1995, p. 332) interpreted as a vestigial shell. This is reminiscent of Halkieria. But the polychaete, Canadia shows that it also has a posterior plate made of chitin. Conway Morris and Peel (1995, p. 342) state:
“In any event, the similarities of the shield with the posterior halkieriid shell may not be convergent, and the possibility of sternaspids being primitive should be investigated. In particular, although the evidence is slender there is quite a strong similarity between the sternaspid plate and the putative shell in Wiwaxia.”
Thus, an animal very similar to Wiwaxia probably gave rise both to Annelids and to Molluscs.
What we have seen is the evidence for phyla level evolution. Evidence that the anti-evolutionists of all stripes say doesn't exist. So when Christian apologists tell you:
“Another notable example of discontinuity in the history of life is the ‘Cambrian explosion,’ some 570 million yeas before present, when the major phyla appear in the fossil record without obvious precursors. Complex, multicellular organisms such as the trilobites, corals, and crustaceans, appear fully formed in an approximately ten-million-year window of geologic time that has been called the ‘Big Bang’ in the history of life.” (Davis, 1999, p. 229)
Don't believe it. There is evidence of precursors--they just don't tell you about it. Christian apologists need to do better than this.
Why is there an idea of a Cambrian explosion?
Given the data above, why do apologists still treat the Cambrian as an explosion? Basically, it is because they have not kept up with the increased knowledge of this time period over the past 50 years. Lazarus J. Salop (1983, p. v) wrote:
“Progress in Precambrian geology has been exceptionally great, indeed, quite striking for geologists of the older generation; only some 30-40 years ago the Precambrian appeared as an uncertain and even mystic prelude to geologic evolution. Even the very name-Precambrian-means some indivisible unit in the early history of the Earth, the beginning of which is poorly known.”
In general it is my belief that apologists have not really studied the Precambrian in any detail or have merely re-stated what others have erroneously written. The first red-herring to be corrected is the false idea that virtually all phyla appear in the Cambrian. Ray Bohlin writes:
“Nowhere is the problem of sudden appearance better demonstrated than in the Burgess Shale found in the Canadian Rockies. The Burgess Shale illustrates that in the Cambrian period (which evolutionists estimate as being over 540 million years ago) nearly all of the basic body plans (phyla) of animals existing on earth came into existence in a geological instant (defined as only 5 to 10 million years). No new phyla have appeared since that time (the Bryazoa [sic—grm] are one possible exception, but many paleontologists believe they will eventually be found to originate in the Cambrian period)." (Bohlin, 2000, p. 23)
This simply isn't true. Berkeley has posted an interesting display of when the various phyla appear. It can be found at
http://www.ucmp.berkeley.edu/phyla/metazoafr.html
When one analyses the first appearance data of that chart, they find the following (with one addition of Cycliophora which isn't on their list and the moving of 3 additional definite Vendian phyla from the Cambrian to the Vendian as noted earlier on this page):
Period
# phyla which appear in period
Recent
12
If one considers the Vendian/Cambrian animals as constituting the Cambrian Explosion, then we have 13 phyla appearing in the Cambrian Explosion and 20 AFTER the Cambrian Explosion. While one can assume that the 13 phyla which have no fossil record arose in the Cambrian, assumptions are NOT data. The plain fact is that the Cambrian Explosion doesn't even represent the majority of the phyla. Will these other phyla be found in the Cambrian? Maybe. But one can't rationally assume what the future holds in order to argue to his case.
And if one adds the plant phyla which appear after the Cambrian (why plant phyla should be excluded as Ray seems to imply is beyond me. They ARE phyla after all (Bohlin, 2001, p. 138)), one gets the following chart.
Period # total phyla which appear in period
Recent
13
Eocene
2
Cretaceous
2
Jurassic 1
Triassic 3
Carboniferous 5
Devonian 4
Silurian 1
Ordovician 1
Cambrian 9
Vendian 4
(same note as above concerning phyla in the Vendian)
This yields Cambrian Explosion 13, Post-Cambrian 32! Sounds like a football
score! And given that 13 phyla first appear within the past 10,000 years (having
no fossil record) one could, if one wanted, claim that we are in another
explosion. I wouldn't make that claim but it would fit within the data. To claim
that all or even the majority of animal phyla appear in the Cambrian is
demonstrably FALSE yet the claim is blindly made being repeated endlessly by
apologist to apologist with no one even questioning the validity of the
statement.
So why did so many phyla ‘suddenly’ appear in Cambrian Seas? Basically, it was because of the evolution of hard shells that gave an animal a much greater probability for preservation. Hard shells are more easily fossilized than soft flesh. But why did the animals of the Cambrian evolve shells? There are several schools of thought; I will mention only a few of them. Probably all of the suggested causes had some influence on the development of hard skeletons. Bengston and Zhao (1992, p. 367) write:
“The evolutionary mechanisms behind the origin of mineralized skeletons in animals at the Precambrian-Cambrian transition about 550 million years ago have been vigorously debated. One school holds that skeletal biomineralization began as a detoxification process, another that it evolved mainly to promote biomechanically efficient constructions for locomotion, feeding, and so on, yet another that it arose as a response to predation.”
Detox theories
The evidence for detoxification consists of the fact that phosphorous became common in the Cambrian seas. Conway Morris, 1998, p. 162) writes:
“It is certainly not immediately clear how this break-up of the super-continent might change the balance and distribution of food supply in the oceans. But perhaps there are some clues. The interval marked by the Cambrian 'explosion' is also a time when quite unusual amounts of sedimentary phosphate were being deposited in the shallow shelf seas that rimmed continents. These regions are now found in such places as Australia, South China, and Kazakhstan. Precise estimates of the total volume of phosphate that accumulated in the geological interval are not easy to obtain. Nevertheless, the fact that some of the world's most important mines that extract this phosphate, largely to provide agricultural fertilizer, are situated in rocks of Cambrian age gives a crude indication of the massive quantities of phosphorus that must have been deposited.”
The detox theory says that phosphorous was absorbed by the animals and they then needed to get rid of it because of toxic effects. Life solved the problem by depositing it in the form of shells. Supporting evidence is that Cambrian exoskeletons appear to be unique. Lipps and Signor (1992,p. 341) write:
“Some Cambrian arthropods had cuticles which were unique in the history of the phylum: they were composed of calcium phosphate. This is of particular interest in view of the common occurrence of phosphatic shells in other phyla in the Cambrian. There are no Recent arthropods with a cuticle composed entirely of calcium phosphate, although a phosphatic component is present in some cuticles (e.g., the crab Cancer), especially in their outer part.”
Robert Carroll (1988, p. 23) notes that there are variations on the theme of skeletons forming because of the metabolic handling of calcium and phosphorous,
"Tarlo argued that bone originally developed not for protection, but as a phosphate reserve. Phosphate is both a vital component for energy storage and transfer in all vertebrates and a substance that may be available in fluctuating amounts in the natural environment. The amount of phosphate is frequently a limiting factor in population growth. Superficial bone in some Paleozoic fish shows changes in its extent that are thought to be seasonal and may correspond to the periodic deposition and resorption of phosphate. Bone also acts as a calcium reserve in all vertebrates."
Biomechanical locomotion theories
The line of logic behind this view is short and simple. If muscles have something hard against which to exert their force, they can be more efficient. Prior to the development of skeletons animals moved very slowly in a wormlike, sluglike or centipede like fashion. None of these means of locomotion allows for speed or efficiency. Skeletons allowed animals to move much more rapidly.
Predation theories
This is a relatively old view. Albert Gaudry (1883) suggested that animals developed skeletons to enable them to ward off predators. He wrote:
‘The majority of animals found in the Paleozoic, and especially in the Silurian, appear to have been better suited to defense than to attack, as if, in the early days of the world, these creatures (which are rare today) had had a greater need to be protected.”
And apparently they did have a need to be protected. The earliest known evidence of predation occurs in the very latest Precambrian in the form of boreholes drilled into calcified tube of Cloudina (Bengtson and Zhao, 1992). From that time on, animals seemed to change their morphology and behavior in manners consistent with the idea that they were trying to avoid predators. Worm burrows changed from shallow and horizontal to vertical as the animals sought protection from the predators by burying themselves in the ocean floor(Conway Morris 1998, p. 158-159). Other animals retreated behind 'stone' walls, such are shells, and the shells rapidly grew thick across many taxa allowing the animals to survive. This had an unintentional by-product in allowing an animal to become fossilized. This fossilization is what led earlier researchers to remark on the ‘sudden’ appearance of animals-an appearance which is now known to be a very gradual appearance of more and more complex animal life forms extending from 700 million years ago to 500 million years. And it wasn't just metazoans that became skeletonized at this time. Appearently microscopic foraminifera became skeletonized also. Culver argues that this fact is inconsistent with the above explanations of the Cambrian explosion and is only consistent with predation as a cause of the sudden skeletalization of major parts of the biosphere. Culver (1991, p. 689) writes:
“Discussions concerning the appearance of skeletonization near the base of the Cambrian [about 550 Ma (million years ago)] are often restricted to metazoans and take little account of the acquisition of hard parts by protists at the same time. For example, hypotheses relating the evolution of skeletonization to increases in body size and to detoxification of excess calcium in metazoans do not apply to protists and hence are weakened by the appearance of testate protists in the Early Cambrian. However, this appearance is not inconsistent with the hypothesis, applicable to both metazoans and protists, that the initial function of skeletons was to protect the organism, primarily against predation. The presence of agglutinated foraminifera in the Lower Cambrian, probably Atdabanian Stage-equivalent strata, of the Taoudeni Basin, West Africa is reported here. These specimens extend considerably the known geologic range of several genera, they represent the earliest known unequivocal foraminifera, and they further remind us that protists as well as metazoans should be considered in accounting for the origin of skeletalization.”
Biogeographic theories
During the last stage of the Precambrian, a global continent, called Rodinia, began to rift and break apart. As Hagadorn and Waggoner (2000, p.357) state,
"Rifting of the supercontinent Rodinia occurred in the late Proterozoic, separatijng the Cordilleran margin of Laurentia from east Gondwanaland. This rifting may have begun 150-200 million years before the CAmbrian, or possibly much later, int he Vendian."
The change of environment caused by this event may have played a role in the Cambrian explosion.
One final comment. As we have seen above, the diversification of animals began in the late Precambrian and continued into the Cambrian. The entire period of this diversification took over 50 million years. It is true that the most rapid part of the diversification took around 10 million years, but that ten million years was part of a longer period of diversification. It is inconsistent for Christians to pay attention to one part of this period as if it is the creation event, when, in fact, species were appearing over a much longer period of time. It is also time for Christian apologists to catch up with the new information.
Acknowledgements: I would like to thank Andrew MacRae, David Campbell, and Jon Wolfe for their helpful comments. The errors are mine, not theirs.
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References
Bengtson, Stefan, and Yue Zhao, 1992 “Predatorial Borings in Late Precambrian Mineralized Exoskeletons,” Science, 257:367-369.
Bengtson, Stefan, and Yue Zhao 1997. "Fossilized Metazoan Embryos from the Earliest Cambrian," Science, 277:1645
Bohlin, Ray, 2000.“The Five Crises in Evolutionary Theory,” in Ray Bohlin, ed., Creation, Evolution, & Modern Science, (Grand Rapids: Kregel Publications)
Bohlin, Ray, 2001.“A Response to Morton's Critical Review of Creation, Evolution & Modern Science,” Perspectives on Science and Christian Faith, 53:2:137-139
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Last Revised 7-7-2001
