Ron Abel attempted to show that the fossil record did not support what he called ‘vertical evolution’ by claiming that it needed to show a ‘finely graded sequence’ from simple to complex over evolutionary time. He asserted that the fossil record was discontinuous, showed at best succession rather than descent and concluded by stating that fossils appeared suddenly at the Cambrian. Abel’s argument was a gross misrepresentation of the palaeontological evidence for evolution, and ignored the fact that:
- The insistence on a ‘finely graded sequence’ is a misrepresentation of what we would expect to see in the fossil record based on known mechanisms of speciation
- Evidence for large-scale evolutionary change abounds in the fossil record
- The fossil record is fragmentary because (i) fossilisation occurs only under certain conditions, with some animals fossilising more readily than others (ii) many fossils are destroyed by the movement of the crust over time and (iii) the short amount of time between when fossils are accessible to palaeontologists and when natural processes destroy them
- Fossils do not appear ‘suddenly’ at the Cambrian. Not only is there evidence of complex multicellular life prior to the Cambrian, the appearance took place over several million years.
Its reliance on dated special creationist arguments is painfully obvious.
Evolution? The Fossils say yes.
Abel offered the following strategy for Christadelphians seeking to rebut the evidence for evolution from palaeontology:
- Evidence required - A reasonable argument in support of vertical evolution must show that there is a finely graded sequence from simply to complex over a long period of evolutionary history.
- Evidence lacking
i. At best paleontological enquiry can only show that one form of life came after another, (granting the generous assumption that all inverted orders of fossil deposits can be explained). Present paleontological enquiry does not prove descent.
ii. The fossil record does not show a finely graded sequence from simple to complex. Its record is one of discontinuity. Types spring suddenly in the Cambrian deposits.
He was wrong. The fossil evidence for large-scale evolutionary change is overwhelming. Palaeontologist Steven Gould, in response to special creationist claims that the fossil record showed no evidence of evolutionary transition stated:
Since we proposed punctuated equilibria to explain trends, it is infuriating to be quoted again and again by creationists—whether through design or stupidity, I do not know—as admitting that the fossil record includes no transitional forms. Transitional forms are generally lacking at the species level, but they are abundant between larger groups.[1]
Such transitional forms, as Gould said, abound in the fossil record.[2] One of the better-attested transitions is the evolution of tetrapods from lobe-finned fish, which took place around 380 to 365 million years ago:
Eusthenopteron was a lobe finned fish that lived around 385 million years ago. Acanthostega was a tetrapod, which lives around 365 million years ago. Tiktaalik, a lobe-finned fish that lived around 375 million years ago is a perfect intermediate between tetrapod and fish, with features of both fish and tetrapod. Its front fins have a wrist, while it also has a neck – something fish don’t have. Its mix of fish and tetrapod features have led to the unofficial term ‘fishapod’ to describe this transitional form.
Source: Coyne "Why Evolution is True" (OUP 2009)
Eusthenopteron was a lobe finned fish that lived around 385 million years ago. Acanthostega was a tetrapod, which lives around 365 million years ago. Tiktaalik, a lobe-finned fish that lived around 375 million years ago is a perfect intermediate between tetrapod and fish, with features of both fish and tetrapod. Its front fins have a wrist, while it also has a neck – something fish don’t have. Its mix of fish and tetrapod features have led to the unofficial term ‘fishapod’ to describe this transitional form.
Another well-documented example of large-scale evolutionary change is the evolution of birds from dinosaurs. Birds share many anatomical similarities with the bipedal carnivorous dinosaurs known as theropods. Since the late 20th century, we’ve discovered a considerable number of fossils transitional between birds and theropods, some of which have feathers.
Source: Coyne "Why Evolution is True" (OUP 2009)
The comparison between modern chickens, the transitional fossil Archaeopteryx and the theropod Compsognathus show these similarities. Archaeopteryx had feathers and an opposable great toe, but also had a tail, teeth and reptilian pelvis.
Source: Coyne "Why Evolution is True" (OUP 2009)
One of the more remarkable dinosaur discoveries of recent times is the feathered theropod Sinornithosaurus millenii, which lived around 125 million years ago. Its fossilised remains show clear evidence of filamentous feathers – structures that today are only seen on birds – but whose dinosaur nature is clearly shown by its tail and teeth. Other feathered dinosaurs include Microraptor gui which lived around five million years later than Sinornithosaurus, and had well-formed feathers on its forelimbs and hindlimbs:
Source: Coyne "Why Evolution is True" (OUP 2009)
As with the evolution of tetrapods, where large-scale evolutionary change from fish to tetrapod is clearly seen, the evolution of birds from theropod dinosaurs is fairly clear. We are not looking at succession, but large-scale evolutionary change. This is well seen in the evolution of whales from artiodactyls, hooved mammals. Around fifty million years ago, we see in the fossil record a remarkable transition from hooved mammals to aquatic mammals over a scant ten million year period:
Source: Coyne "Why Evolution is True" (OUP 2009)
This is a cursory overview of a small part of the evidence for large-scale evolutionary change in the fossil record, but it serves to show that contrary to Abel’s assertion, the fossil record shows unarguable evidence of evolution. Geneticist Jerry Coyne notes:
The fossil record teaches us three things. First, it speaks loudly and eloquently of evolution. The record in the rocks confirms several predictions of evolutionary theory: gradual change within lineages, splitting of lineages, and the existence of transitional forms between very different kinds of organisms. There is no getting around this evidence, no waving it away. Evolution happened, and in many cases we see how. [3]
Cambrian Explosion - Slow fuse, not big explosion.
One of the classic creationist claims is that all the major animal groups appeared suddenly at the start of the Cambrian around 541 million years ago, with no evidence of ancestral forms prior to then. Needless to say, Abel’s assertion is wrong. Not only do we have evidence of complex multicellular life before the Cambrian, including ancestors of some of the animal groups that appear in the Cambrian, the appearance did not take place in a geological instant, but over several million years.
Apart from the unique Ediacaran biota, complex metazoan life has been found in the Ediacaran period, and these are usually interpreted as related to modern phyla:
- Vernanimalcula guizhouena: small (< 180 micrometres) organisms believed to be the earliest bilaterians, that is, animals with bilateral symmetry. They have been found [4] in the Doushantuo Formation in southwest China, and are dated to 580-600 million years ago, at least 40 million years before the Ediacaran-Cambrian transition. This interpretation has been challenged by some [5], but if they represent genuine bilaterian fossils [6], they would provide evidence to support the idea that all the modern phyla did not emerge at the start of the Cambrian.
- Kimberella: this is generally accepted as a bilaterian, and is dated at around 558 million years ago, which is about 15 million years before the beginning of the Cambrian. Palaeontologists generally regard it as being related to the molluscs. [7]
- Spriggina: currently considered to be an arthropod, Spriggina [8] bears a passing resemblance to trilobites, and may well be related to them. It has been dated to around 550 million years ago.
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| Spriggina floudensi - Source Wikipedia |
Model of Spriggina floundersi.
Image: Benjamin Healley
Source: Museum Victoria
The length of the Cambrian explosion is anything but short. Keith Miller, a palaeontologist at Kansas State University who is also an evangelical Christian notes that if one defines the Cambrian explosion as the length of time from the start of the Cambrian period to the earliest appearance of diverse invertebrate communities in the fossil record, the length of time is around 20 million years.[9] That is a lot of time in which this apparently instantaneous appearance of major phyla appeared. On that point, Miller notes:
However, not all living animal phyla with a fossil record first appear within this time window. The colonial skeleton-bearing bryozoans, for example, are not known from the fossil record until the end of the Cambrian around 491 million years ago. More significantly, several living invertebrate phyla have a fossil record that extends into the late Neoproterozoic before the Cambrian. Sponges have been recognized as early as 580 million years, cnidarians (the group includes jellyfish and anemones) are present among the Ediacaran animals at around 555 million years, and the stem groups...for some other phyla were also likely part of the Ediacaran communities.
Defining the Cambrian “explosion” is not as straightforward as it might seem. Although there was clearly a major burst of evolutionary innovation and diversification in the first 20 million years or so of the Cambrian, this was preceded by an extended period of about 40 million years during which metazoans (multicellular animals) arose and attained critical levels of anatomical complexity. The Ediacaran saw the appearance of organisms with the fundamental features that would characterize the later Cambrian organisms (such as three tissue layers, and bilaterally symmetric bodies with a mouth and anus), as well as the first representatives of modern phyla. The base of the Cambrian is not marked by a sharp dramatic appearance of living phyla without Precambrian roots. It is a subjectively defined point in a continuum. The Cambrian “explosion” appears to have had a “long fuse.” - Emphasis mine
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| Source: http://biologos.org/questions/cambrian-explosion |
The ‘Cambrian Explosion’ much touted by creationists, namely the sudden appearance of all major phyla instantaneously at the base of the Cambrian without any evolutionary relationship between each other, is not a problem for science because it never happened, not at least in the way creationists assert.
A finely graded sequence is not what we would expect to see
We’ve seen that contrary to Abel’s assertion, the fossil record shows large scale evolutionary change, not just ‘succession.’ Furthermore, his claim that major classes of animal life appeared suddenly at the start of the Cambrian is false. As Keith Miller puts it, the ‘explosion’ had a long fuse.
What about Abel’s claim that “a reasonable argument in support of vertical evolution must show that there is a finely graded sequence from simply to complex over a long period of evolutionary history”? While there are technical reasons why the fossil record is fragmentary, [10] this is not the reason why we don’t have a ‘finely graded sequence. The reason we don’t is that we have no reason to expect one due to the nature of speciation.
Special creationists often assert that Darwin believed the fossil record should show such a sequence. He did not. In the Origin of Species, Darwin argued against the idea that the transformation of an ancestral population was even and slow (emphasis mine):
But I must here remark that I do not suppose that the process ever goes on so regularly as is represented in the diagram, though in itself made somewhat irregular, nor that it goes on continuously; it is far more probable that each form remains for long periods unaltered, and then again undergoes modification. [11]
"It is a more important consideration ... that the period during which each species underwent modification, though long as measured by years, was probably short in comparison with that during which it remained without undergoing any change." [12]
Darwin also argued against the idea that such a transformation of the ancestral population would involve the entire population or occur over the geographical distribution of this population:
"... natural selection will generally act very slowly, only at long intervals of time, and only on a few of the inhabitants of the same region. I further believe that these slow, intermittent results accord well with what geology tells us of the rate and manner at which the inhabitants of the world have changed." [13]
If Darwin was right, then a fossil record of this process is hardly going to show a ‘finely graded sequence.” Rather, he referred to the process of speciation as being intermittent and irregular, and flatly stated that:
... we have no right to expect to find, in our geological formations, an infinite number of those fine transitional forms, which, on our theory, have connected all the past and present species of the same group into one long and branching chain of life. We ought only to look for a few links, and such assuredly we do find--some more distantly, some more closely, related to each other; and these links, let them be ever so close, if found in different stages of the same formation, would, by many palaeontologists, be ranked as distinct species. [14]
Special creationists are fond of quoting Darwin out of context by touting this quote:
But, as by this theory innumerable transitional forms must have existed, why do we not find them embedded in countless numbers in the crust of the earth?
but failing to continue this quote to provide his answer:
It will be more convenient to discuss this question in the chapter on the Imperfection of the Geological Record; and I will here only state that I believe the answer mainly lies in the record being incomparably less perfect than is generally supposed. The crust of the earth is a vast museum; but the natural collections have been imperfectly made, and only at long intervals of time. [15]
To this one can add Darwin’s comments, which show that he did not expect to see an innumerable series of ‘finely graded transitions’. Ultimately, in order to understand why we don’t see such ‘finely graded transitions’, we need to look at speciation.
Allopatric Speciation and the Fossil Record
Large populations tend to be resistant to evolutionary change as novel genes that arise in this population tend not to be fixed. However, new genes that arise in small populations stand a much better chance of becoming fixed and dramatically changing the gene pool. Examples can be found in small isolated communities where genetic disorders often predominate.
This is the idea behind allopatric speciation, advanced primarily by German evolutionary biologist Ernst Mayr. He argued that the most likely source of new species would be small reproductively isolated populations on the boundaries of larger populations. With gene flow from the larger population to the smaller population eliminated or greatly minimised, the two populations eventually became distinct species which would not interbreed even if the barriers separating them were removed.
The nature of allopatric speciation – something which takes place in small areas over a geological instant – means that we would not expect finely-graded transitions to be seen in the fossil record. Vertebrate palaeontologist Donald Prothero points out that:
If the allopatric speciation model applied to the fossil record, then we should not expect to see speciation in the fossils from the main population. Instead, speciation should occur in small peripherally isolated populations that have little chance of being fossilized. In addition, all of the data from biology showed that this process of speciation typically takes place in tens to hundreds to thousands of years, which is a geological instant as far as paleontologists are concerned. The age difference between two bedding planes is often many thousands of years. Thus we would not expect to see the gradual transitions between species preserved very often; instead, we expect to see new species when they immigrate back in to the main population after their isolation and speciation event. In other words, they would appear suddenly in the fossil record. And once they were established, speciation theory would predict that the main population would remain stable and not change gradually through time but that new species would continually arise on the periphery and migrate back to the homeland. [16]
Source: Prothero "Evolution: What the Fossils Say and Why it Matters"
The diagram above compares what one would expect to see from phyletic gradualism, the ‘finely graded series’ model, and what we generally see in the fossil record.
Sharp eyes readers would have noted the reference to punctuated equilibrium in the diagram above, and concluded that PE is therefore what one would expect from allopatric speciation. Douglas Theobald, in a paper on PE concludes by noting:
Punctuated equilibrium is a valid scientific hypothesis, and when geological strata are complete with good temporal resolution and the fossil record is well-represented, the hypothesis is testable. PE, as construed by Eldredge and Gould, is founded upon the modern allopatric speciation model which lies well within mainstream population genetics. However, PE is not novel, and in large part PE originated with Charles Darwin in The Origin of Species (Darwin credits British paleontologist Hugh Falconer with first proposing that stasis is more predominant in the fossil record than periods of morphological change). Thus, in any meaningful sense of the word, the theory of Punctuated Equilibrium is resolutely "Darwinian." 17
Conclusion
Ron Abel's claim that the fossil record does not support evolution is false. We do not expect to see a 'finely graded sequence' of transitions for two reasons:
- What we know of speciation via the allopatric model states that we would not expect to see those changes, which occur in small, isolated populations over a geological instant, preserved in the fossil record
- The fossil record is fragmentary due to the fact that fossilisation is rare, many animals fossilise poorly and the geologically active nature of the earth means that many fossils are never found.
Contrary to what special creationists assert, Darwin recognised that speciation was intermittent and irregular.
The fossil record does not preserve many transitions at the species level, but at higher levels, they are abundant, and show not succession, but large-scale evolutionary change.
Finally, Abel’s assertions about the Cambrian Explosion are flat-out wrong. It took place over several million years, with evidence of complex multicellular life, including some ancestors of Cambrian organisms present in pre-Cambrian data.
The fossil record shows unarguable evidence of evolution. Believers who use Abel’s suggested strategy in order to rebut evolution will be embarrassed.
References
1. Stephen Jay Gould, Evolution as Fact and Theory, Hen's Teeth and Horse's Toes: Further Reflections in Natural History, New York: W. W. Norton & Company, 1994, p. 260
2. The examples cited appear in Jerry Coyne’s “Why Evolution is True”
3. Jerry Coyne, Why Evolution Is True (2009: Oxford University Press) p 57
4. Chen J et al “Small Bilaterian Fossils from 40-55 million years before the Cambrian” (2004) Science 305:218-22.
5. Bengtson, S.; Budd, G. (2004). “Comment on “small bilaterian fossils from 40 to 55 million years before the Cambrian”. Science 306 (5700): 1291a.
6. Chen, Jun Yuan, Paola Oliveri, Eric Davidson and David J. Bottjer. “Response to Comment on "Small Bilaterian Fossils from 40 to 55 Million Years Before the Cambrian"” (2004) Science 306: 1291
7. Fedonkin, M. A.; Waggoner, B. M. (1997). "The Late Precambrian fossil Kimberella is a mollusc-like bilaterian organism". Nature 388 (6645): 868–871
8. Glaessner, Martin F. (1958). “New Fossils from the Base of the Cambrian in South Australia" Transactions of the Royal Society of South Australia 81: 185–188
9. Miller K “The Cambrian ‘Explosion,’ Transitional Forms, and the Tree of Life BioLogos Blog Dec 3 2010
10. Most animals are never fossilised as their bodies are eaten or scavenged before they have a chance of being fossilised. Furthermore, fossilisation is more likely in certain environments (marine) than others (forests) for obvious reasons. Finally, there is the sheer unlikelihood of a fossil being found in the first place before it is eroded or destroyed by natural processes. The fact we have a fossil record in the first place is nothing short of remarkable.
11. Darwin C On the Origin of Species Chapter 4, "Natural Selection," p. 152
12. Darwin, Chapter 10, “On the imperfections of the geological record,” p 428
13. Darwin, Chapter 4, p 140-141
14. Darwin, Chapter 10, p 428
15. Darwin, p 172
16. Donald Prothero, Evolution: What the Fossils Say and Why It Matters (2007: Columbia University Press) p 80
17. Theobald D “All You Need to Know about Punctuated Equilibrium (almost)"
