The Cambrian Explosion
Charles Darwin admitted that one of the great unresolved problems of his theory was the fact that there was no fossil record evidence of animals before a time which geologists call the Cambrian period. The Cambrian period lasted from 541 million to 485 million years ago. He knew that in the Cambrian rock strata were many different forms of animals but that there were none at all seen before (during his lifetime). He said in The Origin of Species:
“If my theory be true, it is indisputable that before the lowest Silurian stratum was deposited (now called the Cambrian), long periods elapsed, as long as, or probably far longer than the whole interval from the Silurian age to the present day; and that during these vast, yet quite unknown periods of time, the world swarmed with living creatures.”
In the sixth edition of his great work he wrote:
“To the question why we do not find rich fossiliferous deposits belonging to these assumed earliest periods prior to the Cambrian system, I can give no satisfactory answer.”
He believed, quite rightly, that if his theory was to stand, the fossil record would one day reveal the host of animals that were the precursors to the rich Cambrian fauna. His theory rests on the fact that organisms have evolved gradually over long stretches of time and that to achieve such complex creatures as those of the Cambrian period would require a clear, earlier, succession of slowly changing fossils. Darwin therefore, very honestly, put forward a test of his theory, which if proved negative would be fatal to it. This, as philosopher Karl Popper has explained, is the true mark of a scientific theory. A theory must be potentially falsifiable. There must be clear tests to a theory, which if failed, would reject that theory. One such test is that proposed by Darwin – that future discoveries would be bound to uncover the myriad of beasts that led gradually to the Cambrian fauna.
We now know that when the rocks of the Cambrian were deposited on the Earth from 541 million years ago about 85% of the Earth’s history had already passed. The first organisms on earth were bacteria, appearing about 3.8 billion years ago. These non-nucleated, asexually dividing bacteria, the prokaryotes, were the only creatures around until about 2 billion years ago when the eukaryotic microbes appear as chemical fossils. Eukaryotic cells are much larger than prokaryotes, have their DNA in nuclei, they have organelles such as mitochondria within the cell (or chloroplasts for photosynthesis in plant cells) and they reproduce sexually. All animal, plant, fungal and algae cells are eukaryotic. Following the appearance of the eukaryotic cell it would be about 1.5 billion years before any multi-cellular animals came on the scene in the Cambrian period.
In 1909 Charles Doolittle Walcott, secretary of the Smithsonian Institution, was looking for fossils in a remote area of British Columbia and came across an amazing collection. They were dated as being from the mid-Cambrian period, 520 million years ago. The preservation of these fossils was superb and they included many soft-body structures of animals. It is rare indeed for soft parts of animals to be fossilized. This collection was very complete and gave a wonderful window into the nature of the Cambrian, sea-bottom dwelling creatures.
Walcott collected furiously, year after year, and he came across a host of animals that were very hard to classify. He tried very hard to fit them into the existing known phyla. A phylum is a particular unique body plan and all animals are classified into approximately 35 phyla. We are part of the chordate phylum. However the Burgess creatures include a large number of body plans that are unique and that do not seem at all represented in modern animals. Walcott in fact tried to ‘shoe-horn’ all his finds into existing phyla. It is believed that he did this because to do otherwise was to flout the existing understanding of evolution. According to Darwin, the tree of life had one or two roots and gradually diversified over time to produce many branches. It had the shape of an inverted cone (point downwards). Anyone who suggested that at the very beginning of multi-cellular life there were more phyla than at present, would have been guilty of heresy. It would mean that the cone (or tree) would have to be turned the other way up. The very process of evolution, according to Darwinian theory, including modern Darwinism, requires very gradual diversification. However the Burgess Shale animals reveal the very opposite, as I shall describe here.
In the 1970s a group of British and Irish palaeontologists began to look in depth at the range of specimens from the Burgess Shale. They began to examine systematically the thousands of specimens that were then available. What they discovered sent shock waves throughout the palaeontological community. They described animals that were bizarre and quite alien in appearance. Nothing like them had ever been seen before. Alongside more familiar body plans were some that were more like hallucinations than specimens – hence the wonderful name Hallucigenia for one beast which had seven pairs of spines on one side, seven tentacles on the other and was so novel in shape that even now we do not know for sure which is the front or back. There was Opabinia with five eyes and a frontal ‘nozzle’. There was Wiwaxia, a flattened, oval creature covered in plates and spines. Many others of these small sea creatures were of totally unclassified body plans. All are now extinct.
In fact there are between 15 and 20 sea animals in the Burgess fauna that have anatomies quite unlike any known phylum and which probably represent entirely separate phyla. What is more, virtually our modern phyla are also represented in the Cambrian explosion and those that are not appear relatively soon after in the geological strata. Even our own phylum, the chordates, is represented by the Burgess Shale species called Pikaia.
The bulk of the entirely new body plans appear over a period of just 10 million years – a mere blink of the eye in evolutionary terms. One of these is the arthropod known as the Trilobite. Many fossils of this beautiful creature are present. We see in the Trilobite the very first eyes in the fossil record - see link. These are not ‘primitive’ but are in fact superbly engineered and advanced compound eyes which appear abruptly with absolutely no precursor forms in the fossil record.
There are fossils that do immediately precede the Cambrian Period, known as the Ediacaran fauna. They are seen worldwide and are soft bodied but bear no resemblance to any of the host of Cambrian animals. The Ediacaran fauna are a puzzle and are very hard to categorize. Most resemble jellyfish like discs, quilted pillows or mud-filled bags. What can be said is that they do not in any way show links with the Cambrian fauna.
And so what we see in the Cambrian explosion are many phyla (body plans) including those that we would call modern. In taxonomy (the classification of life forms) the amount of variation is called disparity. There is a very large disparity in phyla in the Burgess Shale (as well as the fossils from China and Greenland). Within each of the phyla however there are relatively few species compared to modern phyla. The huge question is: how can one explain such an enormous disparity of body plans so early in the ‘evolution’ of life. It seems to turn Darwin’s evolutionary tree literally on its head. In Darwinian theory there should be less range of body plans at the beginning and gradual diversification over time. For the past 20 years I have endeavored to keep abreast of the palaeontological evidence but have not seen any answers or new specimens that show why, in Darwinian evolutionary terms, there is:
a) A sudden explosion of new forms without precursors.
b) Such a wide disparity in body plans at the very beginning of multi-cellular life.
The scientific community have put forward a number of possible ‘triggers’ for the explosion. The idea is that dramatic environmental changes may have sparked off the evolutionary exploitation of the available habitats. These include:
A very cold period (‘snowball earth’).
Increased oxygen in the atmosphere (but this has now been shown to have happened gradually).
Increase in calcium in the oceans.
Increase in plankton in oceans.
An ‘arms race’ of predators when vision appeared amongst the animals. Read Andrew Parker’s book on this – I find it very unconvincing.
It is never made clear why such environmental changes (which have occurred at other times as well) could influence the specific mutational DNA changes needed to form brand new body plans as complex as any today, all at once, never to happen again.
I will leave the last word about the Cambrian Explosion to the late Stephen Jay Gould (former Professor of Zoology and Professor of Geology at Harvard University):
“Darwin invoked his standard argument to resolve this uncomfortable problem: the fossil record is so imperfect that we do not have the evidence for most of life’s history. But even Darwin acknowledged that his favourite ploy was wearing a bit thin in this case…….the problem of the Cambrian explosion has remained as stubborn as ever – if not more so, since our confusion now rests on knowledge, rather than ignorance about the nature of Precambrian life.”
Antony Latham 2017
 Darwin, Charles. (1859). The Origin of Specie by Means of Natural Selection. John Murray
 Darwin, Charles R. (1876). The Origin of Species by Means of Natural Selection (6th ed.). p. 286
 Popper, Karl. (1934). Scientific Method. From Popper selections. Princeton Univ. Press.
 Gould, S.J. (1989). Wonderful Life. The Burgess Shale and the Nature of History. New York. Vintage.
 Conway Morris, Simon. (1998). The Crucible of Creation. The Burgess Shale and the Rise of Animals. Oxford Univ. Pres.
 Moore, Janet. (2001). An Introduction to the Invertebrates. Cambridge Univ. Press.
 A good overview is in: Meyer, Stephen C. (2013). Darwin’s Doubt. The explosive origin of animal life and the case for intelligent design. Harper Collins.
. PMID 18174439. doi:10.1126/science Shen, B.; Dong, L.; Xiao, S.; Kowalewski, M. (January 2008). "The Avalon Explosion: Evolution of Ediacara Morphospace". Science (abstract). 319 (5859): 81–84. Bibcode:2008Sci...319...81S.1150279
 Sperling, E. A. et al. (2015). Nature 523, 451–45
 Parker, Andrew (2003). In the blink of an eye: the cause of the most dramatic event in the history of life. Free Press.
 See discussion on this in: Latham, Antony. (2005). The Naked Emperor. Darwinism Exposed. P. 43. Janus Publishing
 Gould, S.J. (1989). Wonderful Life. P.57.
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Dickinsonia - Wikipedia Commons
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