A favourite citation of those who do not understand evolution is that the fossil record does not show any transitional forms (like the now famous fictional Crocoduck that Kirk Cameron proposed), and that if evolution were “true” then we would expect to see an evolutionary continuum between the first single-celled bacterium and all modern creatures.
This “argument” is wrong for several reasons, firstly and fore mostly, nobody has ever said evolution was “true,” scientists say that evolution is a theory, it is not even a scientific FACT because a fact is something that is observed outright, such as the FACT that the sky is up, Truth is a philosophical idea and should not be used in a scientific context. It can be true that someone loves another person, but the fact of love is that a series of chemical reactions are going on that cause this – often irrational – attraction.
The top image shows the ideal fossil record in the form of a tree of life, this is what people want when they say "where are the transitional fossils". The lower image is an apporximation of what the same tree looks like in reality, showing the paucity of the fossil record.
The image on the left shows the ideal state of the fossil record at the top, and the real state of the fossil record on the bottom. It is not a brilliant nor is it an accurate representation of the tree of life, but it illustrates the point and I defy anyone to give the lower image to 10 people and have them all give you the same (join the dots) tree. In comparison Palaeontologists are studying an incomprehensibly large, fragmented and poorly resolved tree of life, some idea of it might be to attempt to read this page of text if only every 150th character were printed in place, with the rest missing, and those characters were not spaced evenly within the document… in fact, you may have noticed the red letters In the first two paragraphs of text, imagine only having those letters to read, and trying to piece together the whole two paragraphs; that’s what studying the fossil record is like.
There are a wide variety of reasons why the fossil record is so poor, and I will endeavour to explain some of them here.
The Remoteness (or Improbability) of Fossilisation
Firstly let’s talk about the fossilisation of different forms of tissue. Animals and Plants are made up of a variety of types of tissues, in all there are 6 major groups to consider: Mineralised (Bony/Shelly), Cartilaginous (The stuff your nose is made of), Lignified Cellulose (wood), Cellulose (other plant, e.g.: Leaves), Chitin (hair/horn/insect carapace) and Volatile Tissues (soft body parts/organs/skin). Each one of these groups of tissues has a different preservation potential, that is to say that different tissues have different likelihoods of being fossilised under the same conditions. Mineralised Tissues such as bones for example, are made up of a large proportion of mineral in the first instance, this is why it is so difficult to break a bone, and how your skeleton is able to hold up your weight (as well as considerably more – as any hiker or boy-scout will tell you). This mineral content is the reason for the high preservation potential, as the fossilisation process only has a relatively small amount of the tissue that it “has” to replace before the process is completed, by analogy you can compare it to making a house from a cave rather than from scratch. Compare this to the other end of the scale, which is volatile tissues, which have a non-zero but minute mineral content, to fossilise this kind of material the process of fossilisation requires a huge surplus of chemical to precipitate to form the fossil, as compared to a bone.
This image shows the preservation potential of the main types of biological tissues, the least preserbable are at the bottom left. These must be preserved very rapidly if they are not to be lost to the fossil record. (source: Harding 2009, Lecture, Original source unknown)
Sometimes you may find a fossil which seems to have miraculously appeared from out of no-where in Geological terms, in one stratum you find it but in strata below you do not, nor do you find any evolutionary predecessors… How can this be? Well first off I can assure you there is no need for magic… there is a rational explanation, and that is evolution that occurs elsewhere. If evolution of a species occurs in one region of the globe, cut off from the progenitor (parent) species by some form of barrier which is later removed. The evolved species can become the dominant one in the lands inhabited by the progenitor species, and when these creatures fossilise, there will be no evolutionary process visible in that location. This lack of evolutionary intermediates will make it look as though the species appears from no-where, but this is not the case.
Hostile Geological Circumstances (Environments)
Another, slightly related reason for the scarcity of the fossil record is the plethora of geological environments in existence compared to each environment’s preservation potential. To take three examples of geological environments that will hopefully illustrate this point, we will discuss a Terrestrial Desert Environment, a Shallow Marine Shelf Environment and a Deep Sea Anoxic Zone.
The Terrestrial Desert is a very hot environment where there are fewer animals; this will aid preservation in that scavengers are less likely to disassemble the skeleton. This environment also lacks several important factors that would greatly aid fossilisation, the main missing parts being a surplus of chemicals to precipitate (chemistry is facilitated by water, so the lack of it is vital here) and the protection from the elements that would otherwise erode the organism (Deserts often do not deposit sediment quickly enough to prevent erosion). The end result of this is that there is a significantly lowered preservation potential compared to most geological environments. Another way to look at it is that a desert is an erosive environment, so the majority of things in it will not be lithified in the first place.
A Marine Shelf environment by contrast, is a lot less erosive, and in most places is depositional (that is to say, forms sediments). This environment is also beneath the sea, and as such has an enormous supply of chemicals that can precipitate out to form fossils (sea-water contains 3.5% ‘salt’ – including six chemical compounds at greater than 0.35 grams per litre); deposition of sediment in this environment is often rapid enough to preserve the most resistant tissues. Whilst there are more predators and scavengers in this environment, the significantly increased biology of this environment increases the number of dead organisms with the possibility to fossilise. The cumulative result of these factors is that this environment has a higher preservation potential than the desert environment previously discussed.
The final environment that we will consider is a Deep Sea Anoxic Zone, in this environment there is a huge quantity of available chemical compounds, few predators or other animals to disturb the dead organisms or the sediments onto which they come to rest. The environment is depositional but in all cases at a slower rate than most environments. One final significant benefit is that this environment will (depending upon the degree of anoxia) prevent the decomposition of the organism’s softer, more volatile tissues, allowing for exceptional preservation of tissues that would otherwise not fossilise. There is however one major caveat to these benefits; and that is that if the ocean is “too” deep, or the Calcium compensation depth is too high in the water column, the hard parts of the organism (shell/cartilage/bone) will all dissolve fairly rapidly.
One poignant example of this latter style of environment is the world famous Burgess Shale Deposits of British Columbia. This enhanced the scientific picture of the environment in which the fauna (animals) were fossilised. Before the discovery, only the hard parts of the fauna (mineralised tissues) were found.
image showing the Burgess Shale Fauna; most of these animals were originally soft bodied, so would not have been known about were it not for the Burgess Shale. (Source: Harding (2009a) lecture, original source: "The Fossils of the Burgess Shales", by Briggs, Erwin and Collier, 1994)
After the discovery of the Burgess shale, the deposit produced a huge quantity of soft-bodied animals that would otherwise not have been known. Comparisons of these two pictures of hte environment show that the average non-hard-part dauna that palaeontology normally misses far out-weighs that which we know – which in itself demonstrates how depleted the fossil record is (Conway Morris, 1997).
Geological Unconformities (Time Gaps)
The geological record is full of unconformities, which put simply are gaps in time. Each unconformity can represent a gap of millions of years, during which time many species could have evolved and become extinct, but no organisms would be preserved because no sediments are being deposited. In point of fact most modern environments that we as humans come into contact with are non-depositional, and it is these areas that will produce future unconformities.
Unconformities allow fossils of completely different geological ages to be juxtaposed one stratum atop the other, so you might see a Triassic rock stratum lying directly above a Silurian stratum where the intervening strata (Devonian, Carboniferous and Permian) are completely missing. Any organisms from this intervening age will not have been fossilised and so there would be a gap in the observed tree of life.
Denudation (Erosion and Weathering)
Along a similar vein to geological unconformities, denudation – or the weathering and erosion of existing rock strata – removes a significant portion of the fossil record, so that any fossils that existed in these removed strata will no longer be there to find, and may indeed have been re-worked and deposited into younger strata, thankfully this is fairly easy to spot when looking at a new fossil, and the re-working doesn’t fool a palaeontologist for long.
Though this process is responsible for the loss of huge amounts of the fossil record, it is also a blessing in disguise because if it wasn’t for the erosive nature of many of the earth’s environments, most fossils would never be found, as they would remain in the rock, hidden forever from human eyes.
Plate Tectonics and Metamorphism
Plate Tectonics and Metamorphism both remove huge amounts of the fossil record over time. Taking these processes one at a time; Plate Tectonics works at what is (on a human timescale at least) a perilously slow pace, however it is the reason that so much of the earth’s early fossil record is missing. The earth’s crust is constantly recycling itself, perpetually being created at Mid-ocean ridges and volcanic regions, and then destroyed at plate margins like those off of the Japanese and Chilean coasts; the oldest ocean crust is approximately 200 million years old. No ocean crust older than this exists beneath the waves, the only places where it does are where it has been obducted (or raised) onto and incorporated into the continents. The continents so contain older rocks up to approximately 4.3 Billion Years old; but these parts are small and rarely in their original state due to the second of these forces: Metamorphism.
Metamorphism is a fossil destroyer in its own right, the process of heating and compressing the rocks involved can deform, crush, and even obliterate any fossils within the rock, and whilst this is not always the case (a friend of mine discovered a trace-fossil in the metamorphic rocks of Anglesey) exceptions to this rule are rare. Combining the extreme age of an ancient rock (say circa 4 billion years), with it’s almost inevitably small size, and the all but inescapable repeated metamorphism of that rock, and the chances of a fossil being preserved, let alone found, are hideously small. Certainly not a fossil in the traditional sense of the word, Biomarkers are a possibility, but that’s another story
Destructive Biology (Predation, Bacteria and Decomposers)
Consider a world without any form of predation or biological decay, putting aside all the severe detriments that this world would have, one group of people would be extremely happy… yep, you guessed it, the Palaeontologists. Just think about it for a moment, if there are no predators, no bacteria and no decomposers, the number of organisms that can be fossilised increases immensely, because a large number of animals that at present are decomposed or disturbed would not be. The carcasses would be left to allow the mostly physical and chemical processes of fossilisation to occur.
This imaginary world does not exist however, and most dead organisms are predated, scavenged, and eventually decompose, and a number of the environments involved do not induce rapid fossilisation, so in most cases by the time fossilisation begins, there is nothing left of the creature to preserve.
Yet another reason why the fossil record is so… empty…
Human Fallibility (Not Knowing Where/How to Look)
Well now we come to the last factor I intend to cover in this essay; Human Fallibility. Humans (Homo Sapiens Sapiens) have many wonderful characteristics; one is their innate curiosity, without which Science, Religion, Technology, and Society itself would never have developed. That said, however good our brains are at many things, they are flawed because of the evolutionary way in which they have come into being… like everything evolved, there are problems. Scientifically speaking the biggest are Bias, over-extrapolation and over-interpretation.
Often scientists have been accused of creating forgeries of fossils; however in many cases we have now found modern analogues or corroborating fossils… not to mention how is it possible for someone to forge something we did not know existed?
Once we get an idea in our heads, it is extremely difficult to change it, so when we are taught that something is the case; it is very hard on us (and our egos) to find we are wrong or that there is a better model out there. For example when the Gunflint Cherts of Southern Canada were first found, no-one knew what they were, and when it was proposed that they were organic in origin, the rocks were so old that the scientific establishment balked at the idea. Since then there has come to light ample evidence to back the hypothesis and we even have modern analogues – the Stromatolites of Shark Bay, Australia.
It takes real initiative to look for fossils in places where the rest of palaeontology considers it impossible to find a fossil… however this has repeatedly shown that there are things that we do not know; Microfossils, Biomarkers, the Ediacaran Hills Sandstones… again and again new, exciting fields open up and the world learns something new. The same applies to methods, electron microscopy, chemical analysis, polarised light (recent studies have shown diffraction gratings on fossils (Parker 1998) including insect carapaces and even feathers; allowing us for the first time to accurately colour in some of our reconstructions of the ancient world); even the re-cataloguing of old collections brings out new fossils and new species.
One other issue of human fallibility is something that can be referred to as collecting bias. Many of the early fossil sites were in quarry workings and mines, whereas most early palaeontologists were men of class and distinction – not the type to go around caves and quarries – and thus the people doing the collecting (quarrymen and miners) would only collect the major specimens that they knew would garner a rich payment from the palaeontologist This results in lots of individual skeletons of vertebrates and many large or novel or pretty invertebrates, but the more common and less beautiful fossils would be left or ignored, even if they were of high scientific importance. After all most miners and quarrymen would not know what was scientifically important at that time. Unfortunately for science, most of these pits and quarries no longer exist, have been filled in or collapsed, leaving no way to regain the knowledge potentially lost – not that it is anyone’s fault.
in a future world... Professor Ichthyosaurus opines on the existence of the great apes: cartoon by Henry De La Beche (1830)
So when you next hear someone decrying the lack of “transitional forms” or how useless the Fossil Record is for defining the history of life… remember that this lack of forms, this sparse nature of the record, the amazing variety of preservation… is all innate, is all integral to the record itself.
How likely do you think it is that in 200 million years time a future palaeontologist will dig up your bones? You might be scavenged, denuded out, metamorphosed or just were buried in the wrong environment… which is most likely, a church-yard is perhaps the most inappropriate place to hope to fossilise yourself. Try putting a clause in your last will and testament that you will be soaked in amber… or weighted and dropped into an anoxic shallow water environment… or thrown into a tar pit… these would all be better bets.
Then again, at the end of the day if Palaeontology saw a seamless transition from inorganic molecules to prokaryotes to eukaryotes to metazoans to you… it would be Soooooo boring… so easy, so lacking in challenge and intellectual thought… wouldn’t you agree?
Short-Link for this post: http://wp.me/pFUij-8r
Conway-Morris, S. (1997). The Crucible of Creation. Oxford: Oxford University Press.
Harding, I. C. (2009, March). SOES 2001 Palaeobiology Lecture 8. Southampton Uni. Original Source Unknown
Harding, I. C. (2009a, March). SOES 2001 Palaeobiology Lecture 9. Southampton Uni. Original Source: The Fossils of the Burgess Shales, by Briggs, Erwin and Collier, 1994
Parker, A. R. (1998). “Colour in Burgess Shale animals and the effect of light on evolution in the Cambrian”, Proceedings of the Royal Society of London, Series B: Biological Sciences, 265(1400), pp. 967-972. (ONLINE) Available: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1689164/ (Accessed 20 Nov. 2010)
With Thanks to Messrs Paddy Howe and Christopher Andrews of The Fossil Workshop, 55 Broad Street, Lyme Regis, DT7 3QF for reviewing, critiquing and commenting on this post and for all their kind help and assistance as both Palaeontologists and Personal Friends.