The fossil gaps

     His answer at the time was the 'extreme imperfection of the fossil record, and he consoled himself with the thought that as the geological search continued, enough transitional fossils between one species and another would be found to confirm his theory and allay his doubts.

     About the imperfection he was right. There are about 250,000 different species of fossil plants and animals in the world's museums. This compares with about 1.5 million species known to be alive on Earth today. Given the known rates of evolutionary turnover, it has been estimated that at least 100 times more fossil species have lived than have been discovered.¹

Clearly, Earth has seen a lot of life forms we don't know about, and some of them may have been transitional.

But the curious thing is that there is a consistency about the fossil gaps: the fossils go missing in all the important places.

When you look for links between major groups of animals, they simply aren't there; at least, not in enough numbers to put their status beyond doubt. Either they don't exist at all, or they are so rare that endless argument goes on about whether a particular fossil is, or isn't, or might be, transitional between this group and that.

 

Panel 1 Where are the Pre-Cambrian fossils? Although a few fairly complex fossils have been discovered in The upper levels of the Precambrian, their significant absence in general is a well-established phenomenon, accepted by geologists. Most of the explanations ring changes on the idea that the environment in Pre-Cambrian times was unsuitable for the formation and preservation of fossils. 1.      Fossils were destroyed by changes in the rock structure e.g. melting or compression. Without doubt, much metamorphosis (the geological term for substantial changes in rock structure) took place. Marble is a Pre-Cambrian rock that has been formed through limestone crystallizing under great pressure, so that it cannot contain fossils. But at least ten per cent of Pre-Cambrian rocks are sedimentary, many of them continuing in an unbroken procession through to Cambrian times. 2.      There was little or no calcium in Pre-Cambrian seawater; therefore the creatures were soft-shelled, and their fossils were not preserved. There are a number of objections to this theory. It ignores areas of the Earth where the rocks continue undisturbed (see above). Shells also become hard and strong through the action of other chemicals besides calcium - silicon and chitin particularly. So for the theory to work, you have to assume that all three chemicals were missing from the Pre-Cambrian, and arrived simultaneously by chance in the Cambrian - an improbably tall order.   3.      Pre-Cambrian organisms lived on the seashore, a volatile environment unlikely to preserve fossils.      But even if the seashore explanation is true (and it is entirely hypothetical, for there is no evidence one way or the other) some fossils would still have been formed and preserved. And it is generally conceded that the search has been so thorough that, if they exist, they ought to have come to light by now. 4.      'Sudden' and explosive' are relative terms; ten to twenty million years is more than enough time for life to proliferate on an empty globe, once conditions were right.       This is the most persuasive, as well as the most recent, of all the explanations. There is a general law of growth, which predicts a period of steep acceleration in what is called a sigmoidal (s-shaped) curve. Towards the end, when all the ecological niches are filled, the curve flattens off. Something of this sort can be inferred in early Cambrian times, with the oceans at first containing plenty of space, abundant food, and no competition. New life forms, triggered off by a perhaps quite minor but significant environmental change, rapidly multiplied to take advantage of the situation. Once the point had been reached where the oceans were full of life, evolution slowed down.       But although this may be a plausible outline for what happened in the Cambrian, it still leaves unanswered the crucial question of what sudden event caused the single-celled creatures to develop into highly complex multi-cellular ones; and what evolutionary or biological mechanism there was to permit this to happen. In a sense, it just pushed the problem back earlier in time. The origin of multi-cellularity remains 'the enigma of palaeontological enigmas'.

Yet there are lengthy periods of history when there is every reason to expect plenty of intermediates. At such times, geological strata straddling an evolutionary change hold an abundance of evidence - the fossils are of good quality, and their times pan on Earth is known with a high degree of accuracy.

Museums have, for instance, countless piles of fossils of the early invertebrate sea creatures, and an equally large number of ancient fishes. Between the two, covering a period of about 100 million years, there ought to be cabinets full of intermediates-indeed, one would expect the fossils to blend so gently into one another that it would be difficult to tell where the invertebrates ended and vertebrates began.

But this is not the case. Instead, groups of well-defined, easily classifiable fish jump into the fossil record seemingly from nowhere: mysteriously, suddenly, full formed, and in a most un-Darwinian way. And before them are maddening, illogical gaps where their ancestors should be.

‘Probably most people assume that fossils provide a very important part of the general argument that is made in favour of Darwinian interpretations of the history of life. Unfortunately, this is not strictly true,' according to David M. Raup, curator of one of the world's finest natural history museums, the Field Museum in Chicago.

     Instead of finding the gradual unfolding of life, what geologists of Darwin's time and geologists of the present day actually find is a highly uneven or jerky record; that is, species appear in the sequence very suddenly, show little or no change during their existence in the record, then abruptly go out of the record.²

     They are not negligible gaps. They are periods, in all the major evolutionary transitions, when immense physiological changes had to take place. A couple of examples will do for the moment.³

     One, how did fish become amphibians? The most important body changes here are that fins must turn into feet, and-a pelvis must develop to support the amphibian's weight. (There are many associated changes of enormous complexity and difficulty, such as gills being transformed into lungs, but these, being soft tissue, might not show up as fossils.)      On the basis of gradual Darwinian evolution, you would expect a wealth of transitional forms showing the development of the appropriate fins, the loss of others, and the slow strengthening of the pelvic bones.

 

 

 

 

There are none that show a continuous chain or series. The ‘link’ most often suggested is between rhipidistian crossopterygian fish and an amphibian genus known as Ichthyostega. A similarity of skull pattern, and of vertebrae, can be seen; and the fin-bones in crossopterygian fish are also said to be the forerunners of amphibian limbs. But the creatures are aeons apart, anatomically. The limbs and pelvic girdle of the amphibian are well adapted for walking on land. Yet there is nothing whatsoever in the fossil record to show how they reached this stage. 

Two, how did mammals evolve their jaw? The change between the reptile jaw and the mammal jaw is profound: and if orthodox evolutionary teaching is correct, the first is ancestral to the second. 

All reptiles have at least four bones on each side of the lower jaw, and one bone in each ear. With mammals, on the other hand, the position is almost precisely reversed -every known mammal, alive or extinct, has a single jaw bone and three bones in each ear. 

These bones are readily preserved as fossils. Yet there are    no existing fossils of transitional forms showing, for instance, three or two jaw bones, or two ear bones. No one has put forward an explanation, either, of how a transitional form might have managed to chew while its jaw was half-way through being re-articulated, nor how it would hear while two jaw bones were being absorbed in its ear structure. 

These are severe but by no means untypical conflicts between palaeontology and Darwinism. Professor N. Heribert-Nilsson of Lund University, Sweden, after forty years in the subject, summed up in his book Synthetische Artbildung: 

It is not even possible to make a caricature of evolution out of palaeobiological facts. The fossil material is now so complete that the lack of transitional series cannot be explained by the scarcity of the material. The deficiencies are real, they will never be filled.⁴ 

So Darwinian theory, at least in its original form, runs into apparently crucial difficulties even at the first hurdle. We look to the fossils to show us what happened in the course of evolution, and the key ones are not to be found.

What is the explanation?

First, some non-explanations.

Professor Stephen Jay Gould of Harvard University, a vigorous supporter of Darwin but an opponent of many cherished evolutionary dogmas that have grown up around him, calls fossil gaps 'the trade secret of palaeobiological'. Reading popular or even textbook introductions to evolution, one sees what he means: you might hardly guess that they exist, so glibly and confidently do most authors slide through them. In the absence of fossil evidence, they write what have been termed 'just so' stories. A suitable mutation just happened to take place at the crucial moment, and hey presto, a new stage of evolution was reached. 

Here, for instance, is David Attenborough, a zoologist whose book Life on Earth was based on a popular BBC-TV series, explaining away what happened during the vital 100 million years when fish were evolving. After agreeing that the fossil record permits 'only a brief isolated glimpse of the progress of the invertebrates', he is also confident enough to describe, as if they were demonstrable, the following chain of events: 

The corals arrived and began to build reefs, and the segmented animals developed into forms that soon would leave the sea and establish a bridgehead on land. Important changes also took place among the proto-fish. The slits in the sides of their throats, which had originated as filtering mechanisms, were walled with thin blood vessels so that they also served as gills. Now the pillars of flesh between them were stiffened with bony rods and the first pair of these bones, slowly over the millennia, gradually hinged forward. Muscles developed around them so that the front ends of the rods could be moved up and down. The creatures had acquired jaws. The bony scales in the skin which covered them grew larger and sharper and became teeth. No longer were the backboned creatures of the sea lowly sifters of mud and strainers of water. Now they could bite. Flaps of skin grew out of either side of the lower part of the body, helping to guide them through the water. These eventually became fins. Now they could swim. And so, for the first time, vertebrate hunters began to propel themselves with skill and accuracy through the waters of the sea.⁵ 

Such accounts are really not much more helpful than a line of plausible patter before a conjurer says abracadabra and produces a rabbit (or in the above case, a fish) from his hat. Some writers admit this - for instance N. J. Berril in The Origin of Vertebrates: 

There is no direct proof or evidence that any of the suggested events or changes ever took place; what strength the argument may have comes from whatever wealth of circumstantial detail I have been able to muster. In a sense this account is science fiction, but I have myself found it an interesting and enjoyable venture to speculate concerning the Cambrian and pre-Cambrian happenings that may have led to my own existence.⁶ 

Other writers go even further and take a line of honest bafflement – for instance, F.D. Ommaney in The Fishes: 

How this earliest chordate stock evolved, what stages of development it went through to eventually give rise to truly fishlike creatures, we do not know. Between the Cambrian, when it probably originated, and the Ordovician, when the first fossils of animals with really fishlike characters appeared, there is a gap of perhaps 100 million years which we will probably never be able to fill.⁷ 

Or A. S. Romer in Vertebrate Paleontology, writing about rodents .These animals, the most profuse on Earth today (in number of species, they exceed all other mammals combined), flourish under all conditions. But their origin seems beyond explanation:

 

 

 

 

 

 

 

 

 

 

 

 

 

When they first appear, in the later Palaeocene, in the genus Paramys, we are already dealing with a typical, if rather primitive, true rodent, with the definitive ordinal characters well developed. Presumably, of course, they had arisen from some basal, insectivorous, placental stock; but no transitional forms are known.⁸ 

Or any number of writers puzzling over the explosion of life forms at the beginning of the Cambrian period. Here, in the space of about ten million years, a curtain was raised on a stage teeming with living things. After 3,000 million years in which nothing more complicated than bacteria and slime lived upon our world, came the dawn of life. Billions upon billions of fossils have been found, showing a marine life that suddenly became rich and abundant: clams, snails, octopuses, crustaceans with hard shells and jointed legs, spiny-skinned animals such as starfish, sea urchins and sea lilies. The dominant life form was the now-extinct sea creature known as a trilobite, up to a foot long, with a distinctive head and tail, a body made up of several parts, and a complex respiratory system. 

But although there are many places on Earth where 5,000 feet of sedimentary rock stretch unbroken and uniformly beneath the Cambrian, not a single indisputable multi-celled fossil has been found there. It is ‘the enigma of palaeontological enigmas’ according to Stephen Gould. Darwin himself said he could give ‘no satisfactory answer’ to why no fossils had been discovered (panel 1). Today's scientists are none the wiser: 

A century of intensive search for fossils in the pre-Cambrian period has thrown very little light on this problem.⁹

 

When we turn to examine the Precambrian rocks for the forerunners of these Early Cambrian fossils, they are nowhere to be found.¹⁰

 

The absence of any record whatsoever of a single member of any of the phyla in the Pre-Cambrian rocks remains as inexplicable on orthodox grounds as it was to Darwin.¹¹