The Cambrian “Explosion”, Part 4

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February 8, 2011 Tags: History of Life

Today's entry was written by Keith Miller. Please note the views expressed here are those of the author, not necessarily of The BioLogos Foundation. You can read more about what we believe here.

The Cambrian “Explosion”, Part 4

This is part four in a series by Keith Miller (earlier parts can be found on the sidebar). It is an updated extension of Miller and Campbell's 2003 essay “The ‘Cambrian explosion’: A challenge to evolutionary theory?” from the book Perspectives on an Evolving Creation: Grand Rapids, and it coincides with our Question, "Does the Cambrian Explosion pose a challenge to evolution?". A pdf version of Miller's full paper can be found here.

Before the “Explosion”: What went bang?

A very important question is what organisms existed before the Cambrian “explosion.” Were there Precambrian precursors, or did the Cambrian explosion really happen in a biological vacuum? Many critics of evolution claim that the Precambrian is devoid of fossils that could represent body plans ancestral to those of the Cambrian invertebrates.

The words of Darwin are often cited as evidence of the seriousness of the problem for evolution.

"There is another and allied difficulty, which is much more serious, I allude to the manner in which species belonging to several of the main divisions of the animal kingdom suddenly appear in the lowest known fossiliferous rocks. Most of the arguments which have convinced me that all the existing species of the same group are descended from a single progenitor, apply with equal force to the earliest known species."1

When Darwin published his model of descent with modification by means of natural selection, knowledge of the fossil record was in its infancy. In particular, the Precambrian and Early Cambrian fossil record was virtually unknown. Even the fossils of the now famous Burgess Shale and similar units were as yet undiscovered. After more than a century of paleontological work, the situation has changed dramatically. In keeping with evolutionary expectations, fossils are now known from the late Precambrian and early Cambrian that record several dramatic transitions in the history of life.

The presence of Late Precambrian animals was recognized in the 1950s and became widely publicized by the early 1970s. These are the famous Ediacaran fossils named for fossil-rich beds in the Ediacaran Hills of South Australia and now recognized at sites throughout the world. These organisms are typically preserved as impressions in sandstones and siltstones. Associated with these fossils are trails (Figure 1) and simple burrows of organisms that show a limited increase in complexity and diversity toward the Cambrian.

The record of life actually extends far beyond the Ediacaran fossils (~565-545 My) into the deep geologic past. Fossils of algae, protists, and bacteria (Figure 2) are present throughout much of the precambrian. The earliest convincing fossils of bacteria are recognized in rocks 3.5 billion years old, and chemical signatures point to the presence of life even earlier. Finely layered mounds (called stromatolites) produced by the activity of mat-building bacteria and algae appear at about this time and become relatively abundant by around 2.7 billion years ago. Evidence of eukaryotic algae, possessing membrane-bounded nuclei and internal organelles, dates to about 1500 million years ago, or earlier if chemical evidence is accepted. Multicellularity had appeared by 1000 million years ago in the form of diverse and relatively advanced seaweeds. The earliest fossils of metazoans (multi-celled animals) may be represented by simple disk-shaped fossils found in rocks 610-600 million years old.2

The earliest unambiguous indication of the rise of metazoan life is preserved in the spectacular phosphorite deposits of the Doushantuo Formation of China dating to at least 580 million years ago. Phosphate can preserve organisms and tissues in such great detail that individual cells can often be recognized. Where environmental conditions are ideal for this type of preservation, extraordinary fossil deposits may result. In the case of the Doushantuo, phosphatization has preserved not only a variety of algal remains, but also the cellular tissues of sponges and millimeter-sized tubes that might represent stem cnidarians.3 However, even more spectacular is the preservation of metazoan eggs and early embryos. These embryos (see Figure 3) are of uncertain affinities but may represent cnidarians or even bilaterians (animals with bilateral symmetry).4

The Ediacaran fossils provide the next window into the rise of metazoans. These fossil-bearing units span from about 575 million years to the base of the Cambrian, and are found in south Australia, Namibia, the White Sea coast of Russia, and Newfoundland. The enigmatic soft-bodied organisms were preserved as impressions, or molds, on the surfaces of sandstone and siltstone layers. These sediment layers accumulated in shallow-marine environments where the seafloor was covered by firm microbial algal mats. The microbial mats covering the seafloor appear to have been important in determining the lifestyles of the Ediacaran organisms, as well as their unique mode of preservation.5

Most soft-bodied impressions of the Ediacaran (or Vendian) can roughly be placed into three general groups -- disks, fronds, and flat-bodied, bilaterally-symmetric forms. The biological affinity of these fossils is very difficult to determine and highly debated.6 Disks are the earliest appearing, and most common, Ediacaran fossils. They have often been identified as medusoids (“jellyfish”) but many appear to have been attached to the bottom, and none bear clear structures that would place them in a living group. Some do clearly possess tentacles around their margins suggesting a stem or sister group relationship to the cnidarians. Some sack-shaped fossils might even be stem anthozoans (the cnidarian group that includes anemones and corals).7

A few disk-shaped fossils may be related to other living phyla. One such form appears to be a sponge that might be assignable to the modern class of hexactinellids.8 Another is a small disk that has a raised center with five radial grooves that has been interpreted as a stem echinoderm (the phylum that includes modern starfish and sea urchins) that lacked the characteristic porous calcareous plates and other diagnostic features of true echinoderms.9

The frond-shaped forms include organisms that were attached to the bottom by a stalk, and others that appear to have been free lying. These fossils have also been assigned by some workers to a group of modern cnidarians (the “sea pens”) or to ctenophores. However, like the disks, the fronds are fairly diverse and some may be unrelated to living phyla.10 Others, although likely not able to be placed into a living cnidarian group, may be stem cnidarians, or even stem anthozoans. The discovery of better preserved fronds in the Cambrian that closely resemble some of the Ediacaran fossils would seem to support this interpretation.11

The bilaterally-symmetric forms of the Ediacaran are the most diverse and most enigmatic fossils of the late Precambrian. Some of these fossils may represent early experiments on the pathway to the living phyla.12 For example, Dickinsonia (Figure 4) and the similar Yorgia are fairly large flat highly-segmented forms that some workers have interpreted as annelids or stem annelids, while others have seen resemblances to other worm phyla or even chordates. These organisms do appear to have been able to move about the bottom as seen by associated crawling and resting traces. Even if not members of a living phylum, these organisms appear to at least be mobile bilateral metazoans (or bilaterians). Another bilateral form that has been the subject of much recent attention is Kimberella (Figure 5). This 555 million year old fossil has been interpreted as a stem mollusk.13 Scratch marks found associated with Kimberella indicate that it had some form of feeding structure (though probably not a true mollusk radula) that enabled it to graze the abundant algal mats. Other bilateral fossils have been interpreted to bear similarities to arthropods, although these interpretations are disputed.

An important, but less attention-getting, component of the Ediacaran fossil record is the presence of trace fossils such as trails, burrows and feeding traces. Except in the few cases mentioned above, there are no body fossils preserved of the organisms that made these traces. These traces tend to be small unbranched sediment-filled burrows that run horizontally along the sediment surface or under the microbial algal mats. Somewhat more complex burrows appear toward the base of the Cambrian including irregularly branching burrows and shallow vertical burrows.14 These traces are important because they point to the existence of small worm-like organisms that were probably feeding on and in the algal mats that covered extensive areas of the seafloor. The biological identity of these organisms is unknown, although they were clearly bilateria.

There is one more set of fossils that are known from the late Ediacaran (550-543 million years) that reveal yet another aspect of the metazoan diversity before the Cambrian. These fossils include tiny calcified or phosphatized tubes, cones and goblet-shaped structures that record the presence of animals capable of producing mineralized skeletons. They are commonly embedded within algal buildups that formed reef-like structures, and may be quite abundant.15 These algal-metazoan reefs foreshadow the later algal reefs of the Cambrian. The very peculiar cm-sized goblet-shaped Namacalathus (found as calcified fossils) lived attached to the algal mounds by stalks (Figure 6). Although the preserved shape of these fossils is consistent with that of cnidarians, their biology is uncertain.

The cone-in-cone structures of Cloudina (Figure 7) , and the more tubular Sinotubulites could have been produced by various types of worms such as serpulids. However, as with the trace fossils, the identity of the actual tube formers remains unknown. A significant observation of the Cloudina fossils is that many of them are perforated by borings. These borings provide the first clear evidence of predation before the Cambrian.

It is clear from the above discussion of the latest Precambrian, that the Cambrian explosion did not occur in a biological vacuum. Although many of the fossil specimens are enigmatic and difficult to classify, they nonetheless show significant biological diversity. Furthermore, at least a few living phyla had already appeared by the beginning of the Cambrian, and other forms likely represented stem groups related to later-evolving phyla.

Notes

1. C. Darwin, 1872, On the Origin of Species by Means of Natural Selection, 6th ed., p 234-255.
2. Summaries of the early fossil record of life can be found in Schopf, J.W. (ed,), 1983, Earth’s Early Biosphere: Its Origin and Evolution, Princeton University Press; and Knoll, A.H., 2003, Life on a Young Planet: The First Three Billion Years of Evolution on Earth, Princeton University Press. During the writing of this essay, a new fossil discovery from Australia has indicated the presence of possible sponge-grade metazoans in rocks 640-650 million years ago. See Maloof, A.C., et al., 2010, “Possible animal-body fossils in pre-Marinoan limestones from South Australia,” Nature Geoscience doi:10.1038/ngeo934.
3. Li, C-W., J-Y Chen, and T-E Hua, 1998, “Precambrian sponges with cellular structures,” Science 279: 879-882. Xiao, S, X. Yuan, and A.H. Knoll, 2000, “Eumetazoan fossils in terminal Proterozoic phosphorites?,” Proceedings of the National Academy of Science 97(25): 13684-13689.
4. J-Y Chen, et al., 2000, “Precambrian animal diversity: Putative phosphatized embryos from the Doushanto Formation of China,” Proceedings of the National Academy of Science 97 (9): 4457-4462. Xiao, S., and A.H, Knoll, 2000, “Phosphatized animal embryos from the Neoproterozoic Doushantuo Formation at Weng’an, Guizhou, south China,” Journal of Paleontology 74 (5): 767-788. J-Y Chen et al., 2006, “Phosphatized polar lobe-forming embryos from the Precambrian of southwest China,” Science 312: 163-165. Xiao, S., J.W. Hagadorn, C. Zhou, and X. Yuan, 2007, “Rare helical spheroidal fossils from the Doushantuo lagerstatte: Ediacaran animal embryos come of age?,” Geology 35 (2): 115-118.
5. Seilacher, A., 1999, “Biomat-related lifestyles in the Precambrian,” Palaios 14: 86-93.
6. Fedonkin, M.A., 1992, “Vendian faunas and the early evolution of metazoa,” IN, J.H. Lipps and P.W. Signor (eds.), Origin and Early Evolution of the Metazoa, Plenum Press, New York, p.87-129. Jenkins, R.J.F., 1992, “Functional and ecological aspects of Ediacaran assemblages,” IN, J.H. Lipps and P.W. Signor (eds.), Origin and Early Evolution of the Metazoa, Plenum Press, New York, p.131-176.
7. Gehling, J.G., 1988, “A cnidarian of actinian-grade from the Ediacaran Pound Subgroup, South Australia,” Alcheringa 12:299-314.
8. Gehling, J. G., and Rigby, K., 1996, “Long expected sponges from the Neoproterozoic Ediacara fauna of South Australia,” Journal of Paleontology 70(2): 185-195.
9. Gehling, J.G., 1987, “Earliest known echinoderm -- a new Ediacaran fossil from the Pound Subgroup of South Australia,” Alcheringa 11:337-345.
10. Narbbonne, G.M., M. Laflamme, C. Greentree, and P. Trusler, 2009, “Reconstructing a lost world: Ediacaran rangeomorphs from Spaniard’s Bay, Newfoundland,” Journal of Paleontology 83(4): 503-523.
11. Conway Morris, S., 1993, “Ediacaran-like fossils in Cambrian Burgess Shale-type faunas of North America,” Palaeontology 36(3): 593-635.
12. Dzik, J., 2003, “Anatomical information content in the Ediacaran fossils and their possible zoological affinities,” Integrative and Comparative Biology 43: 114-126. Fedonkin, M.A., 2003, “The origin of the Metazoa in light of the Proterozoic fossil record,” Paleontological Research 7(1): 9-41.
13. Fedonkin, M.A., and Waggoner, B.M., 1997, “The later Precambrian fossil Kimberella is a mollusc-like bilaterian organism,” Nature 388:868-871.
14. Crimes, T.P., 1992, “The record of trace fossils across the Proterozoic-Cambrian boundary,” IN, J.H. Lipps and P.W. Signor (eds.), Origin and Early Evolution of the Metazoa, Plenum Press, New York, p.177-202. Zhu, M., 1997, “Precambrian-Cambrian trace fossils from eastern Yunnan, China: Implications for Cambrian explosion,” IN Junyuan Chen, Yen-nien Cheng, and H.V. Iten (eds.), The Cambrian Explosion and the Fossil Record, Bulletin of the National Museum of Natural Science No. 10 (Taichung, Taiwan, China), p. 275-312.
15. Chen, Z., S. Bengtson, C-M. Zhou, H. Hua, and Z. Yue., 2008, Tube structure and original composition of Sinotubulities: Shelly fossils from the late Neoproterozoic in southern Shaanxi, China, Lethaia 41: 37-45. Hofmann, H.J., and E.W. Mountjoy, 2001, Namacalathus-Cloudina assemblage in Neoproterozoic Miette Group (Byng Formation), British Columbia: Canada’s oldest shelly fossils, Geology 29: 1091-1094. Grotzinger, J.P., W.A. Watters, and A.H. Knoll, 2000, Calcified metazoans in thrombolite-stromatolite reefs of the terminal Proterozoic Nama Group, Namibia, Paleobiology 26(3): 334-359.


Keith Miller is research assistant professor of geology at Kansas State University in the United States. He is editor of Perspectives on an Evolving Creation (Eerdmans, 2003), an anthology of essays by prominent evangelical Christian scientists who accept theistic evolution. He is also a member of the executive committee of the American Scientific Affiliation, an association of Christians in the sciences, and a board member of Kansas Citizens for Science, a not-for-profit educational organization that promotes a better understanding of science.

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Mike Gene - #50530

February 9th 2011

The earliest convincing fossils of bacteria are recognized in rocks 3.5 billion years old, and chemical signatures point to the presence of life even earlier.

The last time I checked, Schopf’s fossils were being seriously challenged in a very detailed study by Martin Brasier and colleagues.  Brasier argued that the fossils are not remnants of living things, but represent the activity of ancient and exotic geochemical processes.  Perhaps Stephen Moorbath, from the Department of Earth Sciences at Oxford University, says it best:

For the time being, the many claims for life in the first 2.0-2.5 billion years of Earth’s history are once again being vigorously debated: true consensus for life’s existence seems to be reached only with the bacterial fossils of the 1.9-billion-year-old Gunflint Formation of Ontario. -  Moorbath, S. 2005. Dating earliest life. Nature 434: 155-156.

Has the situation changed?


freetoken - #50563

February 9th 2011

@Mike Gene - #50530

I take it you mean this Martin Brasier?:

http://www.youtube.com/watch?v=bRJXEEvvBac

In that video he discusses the search for Precambrian fossils.


Mike Gene - #50674

February 10th 2011

Hi Darrel,

That second link is really quite amazing.  Yes, the evidence seemed to be growing for archaea as highly derived bacteria.  But along comes this extensive look to reassert the more traditional view that both are equally ancient (!).  The relationship between these two domains is quite the enigma.  For those unfamiliar with the situation, suffice it to say that archaea look like standard bacteria and are organized like standard bacteria.  But the machinery they use to copy their DNA, make RNA, make proteins, fold proteins, eliminate proteins, and generate ATP, look like something eukaryotes (like mold or amoeba) use.  They are almost like eukaryotes in a bacterial skin.


freetoken - #50566

February 9th 2011

This is a nicely compacted overview of Precambrian life, and combined with the other three parts of this series makes for a quick read on the subject.


Darrel Falk - #50586

February 9th 2011

Hi Mike,

Thanks for your question and pointing out the ongoing discussion about the details.  Here are two freely accessible reviews on the topic, as well as a BioLogos article by Dr. Mike Tice.


http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1578727/?tool=pubmed

http://gbe.oxfordjournals.org/content/2/379.long

http://biologos.org/blog/oxygen-and-co-creation/

Blessings,
Darrel


Keith Miller - #50631

February 9th 2011

Mike:

The rocks most often referenced with regard to the 3.5 billion year date for the first evidence of life are those of the Warrawoona Group in Western Australia.  These rocks contain both domal structures interpreted as stromatolites (microbially constructed laminated buildups on the seafloor) and microstructures interpreted as fossils of bacteria.  As you state, the biological origin of both of these features have been strongly contested.  However, isotopic evidence is still suggestive of photosynthesis.

Another area with rocks of equivalent age to the Warrawoona Group is the Barberton of South Africa.  The Barberton also includes cherts that contain stromatolite-like structures and spherical and filamentous “microfossils.”  These structures are more convincing than those of the Warrawoona and still preserve organic matter. 

It should also be kept in mind that any organisms present at this early time may not be cyanobacteria, or representative of any living microbial group.  Anaerobic photosynthesis or sulfate reduction are possible metabolic pathways for these early organisms.

Keith


Mike Gene - #50670

February 10th 2011

Hi Keith,

Thanks for the info.  I have long embraced the 3.5 billion date.  But I’m trying to get a feel for just how solid this date is.  For example, from a review back in 2002:

In general, the most ancient microfossils are dated as follows: (a) the indisputable remains of cyanobacteria found in the Belcher Supergroup are about 2.0 billion years old; (b) the possible microfossils of early eukaryotes found in the Negaunee Iron Formation are slightly older than 1.9 billion years; (c) the very probable remains of cyanobacteria found in the Transvaal Supergroup are 2.52–2.55 billion years old; (d) possible remains of cyanobacteria and heterotrophic bacteria are 2.69–2.76 billion years old (the Fortescue Series) or even 3.4–3.5 billion years old (the Onverwacht, Fig Tree, and Warrawoon Series).

From:  Sergeev VN, Gerasimenko LM, Zavarzin GA. 2002. Proterozoic history and present state of cyanobacteria.  Microbiology 71: 623-637.

And then there is Moorbath’s observation:  “true consensus for life’s existence seems to be reached only with the bacterial fossils of the 1.9-billion-year-old Gunflint Formation of Ontario.”


Mike Gene - #50671

February 10th 2011

But then you say, “The Barberton also includes cherts that contain stromatolite-like structures and spherical and filamentous “microfossils.”  These structures are more convincing than those of the Warrawoona and still preserve organic matter.”

So perhaps it would help if I asked two questions:

1.  What is the consensus view about the earliest reliable date for life?  As an outsider looking in, there seems to be a lot of opinion. 

2.  On a scale of 1-10, where 10 is the strongest possible scientific case for X and 0 is no evidence to decide in favor or against X, where would you score the Barberton of South Africa as evidence of life?

I’m not trying to be contentious here; I’m just trying to get a better grasp on the situation.


Mike Gene - #50673

February 10th 2011

BTW, let me add one more thought experiment for fun.  Let’s say physicists develop a very expensive rudimentary time machine.  It can be used to send a small collecting vial into the past and then can be brought back into the present for analysis of its contents.  But the technology is such that the machine works only one time and it would take another decade (at least) to build a new one after it is run.

So here is the question for anyone out there.  If you are trying to study the origin of life, what time in the past do you send the capsule to?  Let’s ignore the “where?” to make it easier.  If you send it too late, life might look too “modern” to be all that interesting.  If you send it too early, there might not be any life.  You got one shot.  And you send the vial ______ years into the past.  Why?


Headless Unicorn Guy - #50707

February 10th 2011

So it seems there were precursors in the late Precambrian and the Cambrian was just when everything reached critical mass.

P.S.  Figure 6 keeps reminding me of H.P.Lovecraft—a sensor stalk from the Great Race of Yith?


Keith Miller - #50719

February 10th 2011

Mike asked “What is the consensus view about the earliest reliable date for life?  As an outsider looking in, there seems to be a lot of opinion.”

Part of the question is what is meant by “consensus.”  There is very little in science that would have unanimous agreement.  I think more in terms of what is the dominant view.

My take is that the dominant view among workers in Archaen paleobiology is that life in some form was present by 3.5 billion years ago.  The disagreements arise over what particular piece of evidence is strongest in making that case.  Even in the article by Brasier and others (“A fresh look at the fossil evidence for early Archaean cellular life,” Phil. Trans. Royal Soc. B, (2006) 361, 887-902) critiquing the biogenicity of proposed “fossil bacteria” in 3.0-3.5 by rocks, they still argue for the likelihood of anaerobic and hyperthermophilic endolithic microbes in those same rocks.

My comments need to be understood as those of an interested follower of the literature but not a specialist in Archean paleobiology.

Keith


pds - #50732

February 10th 2011

The Ediacara fauna appeared suddenly in their own explosion as discussed here:

http://thedesignspectrum.wordpress.com/2010/07/15/the-ediacara-explosion/

Then they persisted in stasis right up to the Cambrian.  Then another explosion.

There is no good record of any gradual transitions to new Cambrian body plans, even though the Ediacara fauna were soft-bodied and left a pretty good fossil record.

I find this article very one-sided.


Keith Miller - #50812

February 11th 2011

Response to pds:

The Ediacaran biota did not appear suddenly as a diverse assemblage of organisms.  The first Ediacarans were simple disks that first appear around 610 my ago.  Considerably later the low diversity “Avalon type” Ediacarans, characterized by the rangeomorphs, first appear around 575 my ago.  Latter still are the more diverse “Ediacaran-type” and “Nama-type” forms.  Also trace fossils begin as very simple trails and increase in diversity toward the base of the Cambrian with simple branching traces and short vertical burrows.  Mineralized forms such as Cloudinia first appear as part of the Ediacaran faunas around 550 my.  The different Ediacaran assemblages also likely reflect different environments.  To summarize, the Ediacaran is not some uniform set of fossils that all appear in the geological record within a short time, and then go extinct.

Examples of fossil transitions to various living phyla will be discussed in a latter post.

Keith


pds - #50819

February 11th 2011

Keith,

Maybe you can comment on the Virginia Tech study I linked to.  There seems to have been an explosion of body plans:

>>>Surprisingly, however, as shown by Shen and colleagues, these earliest Ediacara life forms already occupied a full morphological range of body plans that would ever be realized through the entire history of Ediacara organisms. “In other words, major types of Ediacara organisms appeared at the dawn of their history, during the Avalon Explosion,” Dong said. “Subsequently, Ediacara organisms diversified in White Sea time and then declined in Nama time. But, despite this notable waxing and waning in the number of species, the morphological range of the Avalon organisms were never exceeded through the subsequent history of Ediacara.”<<

<

http://thedesignspectrum.wordpress.com/2010/07/15/the-ediacara-explosion/


unapologetic catholic - #50822

February 11th 2011

“Avalon Explosion”

It’s safe to say “explosion” means something different to a mining engineer than it does to a geologist or paleontologist. 

This inability to understand that the explosions being discussed are mere figures of speech and took place over tens or hundreds of millions of years leads to muddled thinking and inaccurate conclusions.


Keith Miller - #50918

February 12th 2011

The article referred to in the post by pds is:  Bing Shen, Lin Dong, Shuhai Xiao, and Michal Kowalewski, 2008, “The Avalon Explosion: Evolution of Ediacaran Morphospace,” Science, vol. 319, p. 81-84.

The above paper is not an analysis of temporal trends in the origin of phyla, or of the origin and diversification of taxa at any scale.  Rather it is an analysis of morphospace.  The concept of morphospace attempts to construct a theoretical description of the possible range of geometries or shapes using a particular set of variables.  Then this theoretical space is compared to the shapes actually seen in a particular groups of organisms.  Morphospace does not have any necessary relationship to taxonomic categories.  Members of a single taxonomic group may occupy a range of morphospace, and several taxonomic groups may occupy the same morphospace.


Keith Miller - #50919

February 12th 2011

Morphospace - part 2

David Raup was one of the first to develop the idea of morphospace.  He constructed a three dimensional cube that mathematically described the possible range of coiled shell shapes. The article is: David Raup, 1966, “Geometric analysis of shell coiling: general problems,” Journal of Paleontology, 40: 1178-1190.  His shell morphospace is also discussed in the book,  David Raup & Steven Stanley, 1978, Principles of Paleontology.

In a relatively recent publication, Roger Thomas and others developed a broad morphospace model for basic types of invertebrate skeletons and then applied it to the Cambrian skeletonized animals.  They found that most of the space had been occupied by the end of the Cambrian.  See: R.D.K. Thomas, R.M. Shearman, & G.W. Stewart, 2000, “Evolutionary exploitation of design options by the first animals with hard skeletons,” Science 288: 1239-1242.


Keith Miller - #50920

February 12th 2011

Morphospace - part 3

The paper by Shen and others is a similar approach to the Ediacaran using a very differently defined morphospace appropriate for Ediacaran soft bodied assemblages.  They restricted their analysis to “classical” Ediacaran forms and excluded certain types of preservation.  They analyzed three Ediacaran assemblages from three different times. What they concluded was that while taxonomic diversity increased from the Avalon assemblages to the younger assemblages, the range of morphospace occupied by these “classical” forms did not.  “Thus, changes in taxonomic diversity that occurred through time while morphospace range remained relatively constant should affect the internal structure of morphospace.”  As a result, the morphospace became more crowded over time. 

This is actually a pattern that has been recognized in other evolutionary radiations (including the Cambrian)—particularly those that followed extinction events.  When there is a large amount of available niche space, then the first phase of evolutionary diversification tends to move into these open spaces.  Subsequently, organisms diversify within those spaces.  This does not contradict anything that I stated above.


Robert Byers - #51133

February 15th 2011

A common YEC question on this would be this.
If the geology upon which the biological contentions here is based, was wrong would the biology be wrong?
It seems it would.
So if the whole demands geological presumptions then does not this mean the whole Cambrian issue is not a biological issue? Its not based on actual biological research.
So how can great conclusions about biology come from studies not dealing with biology?
This is a flaw in evolution foundations.
Evolution doesn’t examine biology by biology but rather biology by geology.
If evolution was wrong this would be the pregnant flaw in retrospect.


pds - #51149

February 15th 2011

Keith,

You said,

“This does not contradict anything that I stated above.”

My point was not that it “contradicted” what you said.  My point was that the explosion-stasis-explosion pattern we see in the fossil record is not very well explained by evolutionary theory.  It is a serious problem.  Your post was simply one-sided.


Keith Miller - #51305

February 16th 2011

The power of common descent as a unifying scientific theory is that it makes coherent sense of a very wide range of completely independent observations.  It also generates testable hypothesis in a wide range of scientific disciplines.  The construction of such theories is the goal of the scientific enterprise.

Science is largely the discovery and interpretation of patterns observed in our observations of the natural world.  Common descent is consistent with the patterns observed in the fossil record (both temporal and spatial), genomic data, organism morphology (cladistics), biogeography, etc.  It is this congruence of diverse observations that gives the scientific community its confidence in the validity of descent with modification from common ancestors.

Keith


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