The fossil record, as mentioned above, is one of the most compelling bodies of evidence for evolution – that there are fossils of extinct species at all is interesting, that we can see where modern most genera first appear in the fossil records in the last few million years is amazing.  We’ve recovered billions of fossils, from single cells to complete mammoths, from impressions left by ferns to the impressions of feathers surrounding an intact dinosaur skeleton.

And yet, the fossil record remains frustratingly sparse.  We may have billions of fossils from hundreds of thousands of species, but there are likely as many as 25 million species of insects alive today.  When you include other invertebrate and vertebrate animals, the number can only grow; when you include plants and single-celled organisms, the number skyrockets.  Does this undermine the fossil record?

In a word, no.  We expect the fossil record to be sparse relative to both the number of creatures and number of species that have ever existed.  Fossilization is an extremely rare event – not only must the creature remain undisturbed after its death (already this is a severely limiting factor, if we expect ancient carnivores and carrion eaters to be as effective as their modern cousins), but they must also be almost immediately removed from the oxygen and bacteria that cause decomposition. (For example, they might be covered in a sandy landslide, or sink to the bottom of a peat bog).  Once the creature has been removed from the immediate threat of decomposition, they still must encounter the unlikely conditions which allow the the skeleton (or other features) to mineralize; then, they must still remain undisturbed through thousands or millions of years of geological activity.  Finally, and perhaps most unlikely, we have to be lucky enough to stumble across them.  Aside from really exceptional fossil beds (http://en.wikipedia.org/wiki/Lagerst%C3%A4tte), fossils are generally rare, very difficult to access (thanks, solid rock!), and extremely expensive to extract and clean, without any of the lucrative payoff you get from other digging activities like diamond mining.  We should perhaps be amazed that we have anything approaching a fossil record at all.

If we had only a single complete fossilized skeleton of an extinct creature, it wouldn’t do much to prove evolution.  (A serious argument could be made for its effect on a creationist theory, however, and what an extinct species would have to say about the foreknowledge and power of a creator god, but disproving creationism is not the same as proving evolution.)  Similarly, even thousands or millions of fossils of extinct species wouldn’t lend themselves to support for evolution if they did not first lend themselves to patterns – to patterns showing that a) the fossils actually represented stable species and not one-off mutations, and b) the species, genera, and families they represent show progression over time, as we’d expect in the “family tree” predicted by phylogenetics.

As to the first point, while there are certainly controversially taxonomied fossils (especially in the hominid line) clearly there are ample examples beyond controversy.  We have more than thirty examples of Tyrannosaurus Rex (some skeletons complete or nearly so), more than 1200 specimens of just one species of smilodon, and Meganeura specimens (giant dragonflies) have been found in the UK, France, and the US, just to name a few examples.  The second point, though – that we can see examples of “progression” (not in the sense of intentionally building toward a specific form, but in terms of a transition across forms from one morphology to something substantially different) – requires a bit more evidence.

I could hardly begin to discuss anything like a broad overview of the family tree of all life and where the multitude of fossils fit in – that’s a topic that deserves at least a book (like http://www.amazon.com/Written-Stone-Evolution-Fossil-ebook/dp/B004HD4UEI/ref=sr_1_3?ie=UTF8&qid=1299381783&sr=8-3 or http://www.amazon.com/Evolution-What-Fossils-Say-Matters/dp/0231139624/ref=sr_1_1?ie=UTF8&qid=1299381783&sr=8-1).  I will touch briefly on five different fossil series showing intermediate or transitional forms, each of which are interesting for their own reasons: the horse, the whale, camelids, the bird, and humans.

Evolution of the Horse

Our modern horse, genus Equus, is a particularly interesting creature – even if for no other reason than its single-toed limbs. While the horse seems to have the same-shaped limb as most other mammals, it’s missing the eponymous feature of  the “pentadactyl” limb – the five toes.  We might suspect, though, that somewhere in its history, there is a horse ancestor – a proto-horse – with five toes.  Interestingly, we have fossils of just such a species, along with many others that show a gradual transition from five toes to our modern single toe – as well as a gradual transition to grazing teeth and the skull structure of the modern horse. (You may wish to read through http://en.wikipedia.org/wiki/Evolution_of_the_horse for a more thorough discussion than I can manage here.)

We start about 52 million years ago (hereafter “mya”) with a creature called Hyracotherium (http://en.wikipedia.org/wiki/Hyracotherium), which looked something like a cross between a rabbit and a deer, about the size of a dog.  (Note that Hyracotherium is definitely NOT a rabbit, as some have claimed: http://members.cox.net/ardipithecus/evol/lies/lie015.html.  The original confusion that caused the species to be named “hyrax-like” occurred because the the initial naming occurred when only a few teeth – not even an entire skull – had been found)  Hyracotherium had the five toes we’d expect in a mammal – four padded fingers and a fifth “thumb” off the ground, like a dog’s dewpad.  Fast forward a few million years through the fossil record by means of Orohippus (50mya http://en.wikipedia.org/wiki/Orohippus), Epihippus (47mya http://en.wikipedia.org/wiki/Epihippus), Mesohippus (40mya http://en.wikipedia.org/wiki/Mesohippus), and Miohippus (36mya http://en.wikipedia.org/wiki/Miohippus), and we can watch several changes occurring gradually: the creatures grow larger, their teeth begin to adapt to grinding and chewing (i.e. they become grazing teeth), and our proto-horses begin to walk on only three toes – meanwhile, the first and fifth toes have become vestigial nubs.

Note three important points – first, each of the examples listed above are not single species but entire genera.  There was no orderly escalator from Hyracotherium to modern Equus, but a series of branches constantly sub-branching.  Our Equus is simply the highest point in a brambles of Equids.  Secondly, each genus did not “replace” the one before it.  Orohippus may have appeared only a couple of million years after Hyracotherium, but species of Hyracotherium thrived for up to 20 million years – in other words some species of Hyracotherium existed even after Miohippus had appeared.  Lastly, the sample fossils that are used to define the genera are not proposed to be the specific ancestors of our modern horse – they are almost infinitely more likely to have been offshoot branches near the ancestors of the modern horse on the phylogenetic family tree.  They show a general trend in progression, but they may also possess features that don’t continue through to future genera, like a changing number of vertebrae.

Continuing back to our Equus progression, Parahippus (http://en.wikipedia.org/wiki/Parahippus) is perhaps the first true equine – at least among the fossil record we have.  Parahippus still had three toes on each limb, but only the center one was substantially load-bearing. By this time, the ancestral horse had also reached the size of a large pony.  Merychippus (http://en.wikipedia.org/wiki/Merychippus) followed, and from it we see three branches of equines develop, Hipparion (http://en.wikipedia.org/wiki/Hipparion), Pliohippus (http://en.wikipedia.org/wiki/Pliohippus), and Protohippus (http://en.wikipedia.org/wiki/Protohippus), the latter of which continued through Dinohippus (http://en.wikipedia.org/wiki/Dinohippus) and Plesippus (http://en.wikipedia.org/wiki/Plesippus) to our modern genus Equus about 4.5mya.  By the time of Plesippus, the protohorse had had abandoned the two vestigial nubs of the second and fourth fingers and stood on a single toe; and our proto-horse had reached the size of an Arabian horse.  Of course, even after we reach the genus Equus, the fossil record shows a wide variety of species before we get to the zebras, donkeys, and horses we see today.

If you haven’t already, I highly recommend following the link to the wikipedia page on horse evolution (http://en.wikipedia.org/wiki/Evolution_of_the_horse), at least to see the pictures illustrating the gradual changes in toes and teeth from Hyracotherium through Equus.

The horse lineage is important evidence for evolution because while the difference between Hyracotherium and Equus is dramatic, an uncommonly populated fossil record shows gradual changes across many genera and millions of years accumulating to that dramatic difference.  This evidence is particularly consistent with the theory of evolution, but does not fit well into a creationist theory that does not include evolution.  And, while the horse lineage is probably the best documented, it’s certainly not the lineage in the fossil record.

Evolution of the Whale

The whale (and cetaceans in general) are a peculiar bunch – mammals who live entirely within the water, though they have to breathe air to live.  If we didn’t already know they existed, we’d think they were unlikely even as science fiction, yet until the intervention of human over-hunting, cetaceans thrived.  It’s clear that whales had terrestrial ancestors from the appearance of internalized, vestigialized features (like the “fingers” in their fins, the frequent floating bones of their now-missing hind legs, and the internal ear that now picks up sound from their jawbone), and while their lineage is not as well-documented as Equus, it is possible to trace back through proto-cetaceans to likely precursors of the whale (judged by unique features like ear construction and the shape of their teeth).

You can follow the link to Wikipedia (http://en.wikipedia.org/wiki/Evolution_of_cetaceans) for more depth, but in short, the lineage runs back through the first fully marine cetaceans (like Basilosaurus (http://en.wikipedia.org/wiki/Basilosaurus) and Dorudon (http://en.wikipedia.org/wiki/Dorudon), who still had hind legs, though their pelvic bones had completely separated from the spinal column), through the protocetids (like Protocetus (http://en.wikipedia.org/wiki/Protocetus) and Rodhocetus (http://en.wikipedia.org/wiki/Rodhocetus), whose sacrum had already defused into separate vertebrae, and whose nostrils had already begun to move from the front of their snout toward the top of their head, and what would eventually become blowholes), and through the mammalian-amphibious Ambulocetus (http://en.wikipedia.org/wiki/Ambulocetus) and remingtoncetidae (http://en.wikipedia.org/wiki/Remingtonocetidae) (who were like alligator/otter hybrids, equally at home on water or on land with their load-bearing legs), and all the way back to pakicetids (http://en.wikipedia.org/wiki/Pakicetid) and Indohyus (http://en.wikipedia.org/wiki/Indohyus), both proto-whale possibilities whose ears and teeth show the first indications of the unusual structures in whales.

Again, the intent is not to show that these fossils were directly in the line of descent between a  dog-like creature and our today’s Cetaceans, but rather that the creatures they represent show the transitional forms we’d expect between an Indohyus and a modern whale – that transitional forms exist and thrived during the times we’d expect.  Whether a specific creature is or is not directly in the ancestral line is a detail for paleontologists to argue about, but doesn’t bear directly on the fact that transitional forms do exist.

The tone at the beginning of this link (http://www.talkorigins.org/features/whales/) is a bit shrill, but the details are terrific.

Evolution of Camelids

Camelids make another interesting case study, not so much because they shows the transition from one form to another, but because their history explains the origins of three different “families”: the llamas and alpacas of South America, the dromedaries of Africa and the bactrians of Asia.  You can find a bit more detail here (http://en.wikipedia.org/wiki/Camelid#Evolution), but the key is in this image: (http://en.wikipedia.org/wiki/File:Camelid_migration_%26_evolution_DymaxionMap_01.png).  North America is where camelids first appeared in the fossil record, and in that same fossil record we can see (http://en.wikipedia.org/wiki/Camelid#Extinct_genera_of_camelids) that until very recently (geologically speaking at least, probably until the last ice age and the proliferation of man in North America) that camelids were still varied and common here. The three “families” mentioned above were the migrants from the central location.

Evolution of the Bird

The evolution of the bird (http://en.wikipedia.org/wiki/Origin_of_birds) – and specifically their descendance from dinosaur reptiles – is probably the second most contentious case study in evolution, specifically because of Archaeopteryx (http://en.wikipedia.org/wiki/Archaeopteryx, http://www.talkorigins.org/faqs/archaeopteryx/info.html), the first-discovered “early bird”.  Though plenty of other early birds are now known (http://en.wikipedia.org/wiki/Jeholornis, http://en.wikipedia.org/wiki/Rahonavis, http://en.wikipedia.org/wiki/Unenlagia, http://en.wikipedia.org/wiki/Buitreraptor, http://en.wikipedia.org/wiki/Hesperornithiformes, http://en.wikipedia.org/wiki/Ichthyornithes, etc.) and Archaeopteryx isn’t even considered a direct ancestor of modern birds, it is still an excellent example of a “transitional” form between a raptor-like dinosaur and something like today’s raven – it has feathers and the opposable big toe of birds, but the unfused thoracic vertebrae and rear-skull entry of dinosaurs.

There remains some considerable contention as to what branch of dinosaurs produced birds, or even if another upstream branch of reptiles called archosaurs were their ancestors, but there is no remaining doubt amongh those who study evolution that birds came from reptiles (or, as some would say, that they are still reptiles).

Humans

The evolution of humans is – at least between evolutionary theorists and creation theorists – the most contentious case study. However, because we’re a topic of great interest to ourselves, we’ve received a great deal of study from ourselves; more that can be usefully summarized here in a way that’s more convincing than the examples above.  The fossil list (http://en.wikipedia.org/wiki/List_of_human_evolution_fossils) and the writeup of human evolution on Wikipedia (http://en.wikipedia.org/wiki/Human_evolution) are extensive, and worth at least a cursory glance if not careful study.  The matter is complicated by the concept of human exceptionalism (http://en.wikipedia.org/wiki/Human_exceptionalism) – the idea that humans aren’t just part of the fauna – but once the human fossil records and history are compared to those of other species, it becomes more difficult to deny that humans have a history significantly different from other animals, and that exceptionalism is a matter of philosophy and psychology, and not genetics or biology.

Arguments against the Fossil Records as Evidence for Evolution

Several specific arguments were raised in comments on these posts, and I’ll address them directly here.  (I note that there are in fact -countless- arguments against the fossil record in general and fossils in specific – many of them completely groundless, and many of them very relevant and worthy of engagement.  Answering every argument is far outside the scope of this essay, but if there are any particular arguments you feel are persuasive and applicable, please direct me toward them.)

Argument One: (Source: http://www.strengthsandweaknesses.org/Weaknesses/Fossil.Record.Gaps.htm)The gaps in the fossil record are a significant weakness.

I can’t argue against the the fact that there are substantial gaps between the species we see in the fossil record when we fit them into the tree predicted by phylogenetics.  The hypothetical “bough” in the link above shows a 3% fill rate, but I’d be surprised to see it even as high as 1%.  But gaps in the fossil record are not evidence against evolution, nor do they undermine the theory in any way.  Due to how uncommon fossilization is, we will always have gaps in the fossil record – even if we were able to excavate the entire Earth.

Still, if we have even one transitional form between two species – if we can draw even a very dotted line between two genera – we have evidence for evolution.  But we have countless dotted lines, and some of them (like the horse), are less dotted and more solid.

The question is, how much accumulated evidence do you require before it becomes overwhelming?  Once set, you cannot move your goalposts without sacrificing your dignity.  A singled dotted line might be interesting but an outlier – a work of fancy rather than reason – but two or three are a pattern, and two or three hundred are substantial by any measure.  The more fossils appear, the more gaps we will have between them since transitions are never as abrupt as a single generation.  And the more fossils we find to fill the gaps, the more we may realign our theory about the descent about a particular line; but continued refining of the details of the theory does not affect the validity of the theory as a whole.  We may not have even a dotted line for some genera, but if evolution is indicated for some, would a lack of intermediate fossils suggest two different forces in play – evolution for some, and divine creation for others?

Argument Two: (Source: http://www.truthinscience.org.uk/site/content/view/48/65/) –  A) There is significant debate between evolutionists as to the mechanism by which it occurs (particularly between smooth transition and punctuated equilibrium), thus creationism. B) The Cambrian Explosion (http://en.wikipedia.org/wiki/Cambrian_explosion) is a major stumbling block for evolutionary theory.

The first argument does not take much to refute – there is evidence for both smooth transition and punctuated equilibrium (and, in fact, evidence that each occur in in difference scenarios; smooth transition as creatures refine competition to maximize a particular niche and punctuated equilibrium as creatures encounter new niches that have not been filled, as in the case of adaptive radiation), just as there is evidence on both sides of a number of current academic debates among those who study evolution.  None of those debates undermine our understanding of evolution as a general theory, both sides of the debate are only refining aspects of evolution and require the basic tenets of evolution to make sense.

To say that ongoing academic debate about aspects of the theory becomes an argument against the theory in general is along the lines of encountering a debate between artists arguing if a particular shade is blue-green or aquamarine, and therefore proposing that colors do not exist and the shade is gray.  The argument mentioned in particular is not “Evolution can only exist if in the form of punctuated equilibrium” or “Evolution can only exist if in the form of smooth transition” but “We have evidence for a greater frequency of smooth transition” and “We have evidence for a greater frequency of punctuated equilibrium” and then a debate following as to whose evidence could in fact be reinterpreted in the light of the other’s understanding.

As mentioned in the first chapter of this essay, scientific theories are always open to be challenged, both in part and in whole, when new evidence presents itself.  The evidence in both cases supports evolution; the difference is that different models of evolution are better supported by one kind of evidence than another.

The Cambrian Explosion in the second part of the argument (http://en.wikipedia.org/wiki/Cambrian_explosion) is a challenge, and much more worthy of discussion.  The particular quote much-bandied by creationist websites that comes from Dawkins’ The Blind Watchmaker is:

“…the Cambrian strata of rocks, vintage about 600 million years, are the oldest ones in which we find most of the major invertebrate groups. And we find many of them already in an advanced state of evolution, the very first time they appear. It is as thought they were just planted there, without any evolutionary history. Needless to say, this appearance of sudden planting has delighted creationists.”

Note that the actual full quote is:

“Before we come to the sort of sudden bursts that they had in mind, there are some conceivable meanings of ‘sudden bursts’ that they most definitely did not have in.mind. These must be cleared out of the way because they have been the subject of serious misunderstandings. Eldredge and Gould certainly would agree that some very important gaps really are due to imperfections in the fossil record. Very big gaps, too. For example the Cambrian strata of rocks, vintage about 600 million years, are the oldest ones in which we find most of the major invertebrate groups. And we find many of them already in an advanced state of evolution, the very first time they appear. It is as though they were just planted there, without any evolutionary history. Needless to say, this appearance of sudden planting has delighted creationists. Evolutionists of all stripes believe, however, that this really does represent a very large gap in the fossil record, a gap that is simply due to the fact that, for some reason, very few fossils have lasted from periods before about 600 million years ago. One good reason might be that many of these animals had only soft parts to their bodies: no shells or bones to fossilize. If you are a creationist you may think that this is special pleading. My point here is that, when we are talking about gaps of this magnitude, there is no difference whatever in the interpretations of  ‘punctuationists’ and ‘gradualists’. Both schools of thought despise so-called scientific creationists equally, and both agree that the major gaps are real, that they are true imperfections in the fossil record. Both schools of thought agree that the only alternative explanation of the sudden appearance of so many complex animal types in the Cambrian era is divine creation, and both would reject this alternative.”

Again, the tone is a bit more shrill than constructive, but its point is clear.  The problem is one of a fossil gap, not one of an unexplainable feature of the fossil record.  As we see with human fossil lineage, once attention has been drawn to a subject, studies and funding follow – now pre-Cambrian multi-cellular creature fossils have begun to accumulate.  See the latter sections in the wikipedia link: http://en.wikipedia.org/wiki/Cambrian_explosion#How_real_was_the_explosion.3F.  Still, the Cambrian Explosion was a particularly aggressive adaptive radiation, which leads to the question of what caused or allowed for it – continue on through the Wikipedia link for possible explanations of the new “niche” the radiation exploited.

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