If Evolution didn’t happen by Neo-Darwinian means, how did it occur? Part Six: Why Fish can’t walk …

Part Six
If evolution didn’t happen by Neo-Darwinian means, how did it occur?
Why Fish can’t walk – they can’t grow Fishy Fingers – so what could have spawned it all instead?

This current article will focus on the alternative hypothesis to actual walking fishy ancestors, and propose instead: the alternative model used throughout this series – Descent from a common ANCESTRAL CONDITION by means of NATURAL CORECTION (adaptive programs underpinned by universal scaling laws of growth and development) via environmentally orchestrated epigenetic modification acting on novel genetic exchange across and between all domains of life.

how reptilian are you

Recall for example, last week’s article ‘How Reptilian are YOU?

https://diggingupthefuture.com/2014/01/31/how-reptilian-are-you/

This article essentially presents the evidence to why we are not descended from an actual reptilian-like anything and dispels the widely accepted myth, that we all have a reptilian part in our brain – the R-Complex – You don’t. Nor do reptiles. This is a misnomer and in fact as the evidence clearly shows: in vertebrates – which means any animal with a backbone and internal skeleton, the brain evolved independently at least four times, and brain studies in vertebrates point to evolution from a common ancestral condition to more specialised forms and essentially evolved in size relative to an organism and complexity according to environmental conditions showing metabolic/temperature dependent growth. Hence brain development has nothing to do with how reptilian your brain is or is not, but seemingly, it has more to do with the temperature of the primordial pond an organism developed/evolved in, which established its fundamental level of metabolic (brain building) complexity which ultimately led to specialising as a vertebrate fish, amphibian, lizard, mammal or man type species. This of course has implications for how Fish-like are You?

We can now begin to apply the concept of going from the generalist species to specialised form according to environmental conditions (from a common ancestral condition) as embedded in Von-Baer’s laws of embryological development with its extrapolation to evolutionary development, to FISH. Fish, as the fossil record and molecular studies show: are the first vertebrates of the Cambrian. But as the record also clearly shows: they quickly become stabilised as fundamental fish species and all variations on the theme of fish thereafter. Some are still jawless and primitive types, even today, while, many others went on to become highly diverse groups of real bony types. Fish would appear to be the simplest – (metabolically speaking) vertebrate form, therefore it is argued here that they evolved and became speciated sooner than other vertebrates with more evolutionary potential or inherent complexity as yet unexpressed (epigenetic) genetic forms.

But there wouldn’t be any fish or shelled animals or armour, or jaws or teeth or skeletons of the internal or external variety, if one important environmental change hadn’t happened. The changing chemistry of the waters (the carbonate, calcite for building bony parts etc) is strongly linked to all those new exoskeleton coats and rudimentary internal skeletons. Exoskeletons is the more typical garb of advanced invertebrates – you know, the ones that seemingly started out as soft-bodied blobs in the Pre-Cambrian as discussed in a previous article along with the evidence for the changing chemistry of the primordial waters? The first vertebrates were very primitive fish and would appear (as noted last week) to be inverted exoskeletons sporting their flipped skeletons inside.

I will argue that fish remained as fish due to their innate level of metabolic complexity and began specialising as a species rather rapidly from the Cambrian period onwards. Similarly, I don’t believe that complex invertebrates gave rise to fish, but instead, I will make the case that these fundamental forms of metabolic life shared a superficial ancestral (soft-bodied) condition, just as an embryo looks nothing like its adult or more developed form, it would be very difficult to peer into the little undefined blob and assess what it is going to be when it grows up. This is because it hasn’t grown up as yet and only history will reveal what these, as yet undefined species will become. Yes, developmental modes are seen over and over in our present era – but do bear in mind, that these developmental modes of becoming a species are themselves only evolving at this stage. They haven’t been as yet, or a better way to put it perhaps is that all the genetic novelty and epigenetic imprints picked up in the primordial pond haven’t been activated or fully expressed (epigenetically). Fundamentally, and based upon the principles of evolutionary development used here, it could be argued that these soft-bodied forms have completely different evolutionary potential already inherent in their unexpressed primordial forms.

This brings us back to the developmental modes in current species and why I believe that fishy ancestors never existed in that form. Furthermore, I strongly believe, based on sound supporting evidence which I will review further on, that fishy vertebrates could never have walked – they simply didn’t have the means to build strong limbs and develop digits, whereas other primitive pre-fish (soft-bodied floating basal chordate) did. I will make the case that these were more complex (metabolically speaking) vertebrates ‘in the making’ that hadn’t expressed their full potential as yet. Moreover, again based on sound evidence, I will argue that this miraculous transition to land is explicable via metamorphosis (a tadpole-like primitive ancestral condition), similar to how some modern-day primitive land-walking vertebrates use as a means to get out of the water and easily begin walking on land and breathing air. It seems a much simpler means of walking to land from water than a poor fishy-pod having to flop its fins and grow digits and a new set of lungs and breathing apparatus, not to mention a new skin.

This interpretation is founded on the alternative view of evolution using the principles discussed throughout this series can be seen in the direct application of the concept that current modes of species development, reflects the evolutionary species developmental modes of the past as proposed in Von-Baer’s laws. It is principally guided by the concept (again Von-Baer’s) that one type of species never passes through the adult form of another species during its development, then as discussed above, this begins to lead to a very different picture of evolution and how species emerged in the evolutionary past. It implies that walking vertebrates could never have passed through an actual adult species stage of fish even if it is proposed as an elusive fishy-pod primitive ancestor. Indeed, this is well supported by the fossil evidence as you will see further on.

Basically, as all other vertebrates – don’t show any development stage representing anything even resembling the most primitive ancestral form of a fish, it is assumed here that no non-fish vertebrate species had a fishy-pod stage in their development. We need to go back to a more primitive time when species were developing (more embryonic-like forms) when all vertebrates, including fish, shared a more common condition (pre-divergence) and assess their ancestral mode of development and therefore find clues to their evolutionary development. This is essentially the approach taken here where epigenetic (environmentally-triggered) metamorphosis is well supported in the scientific literature as an ancestral feature and means for primitive vertebrates to have emerged unto the world stage – but possibly not all at once. This alternative interpretation of the fossil record and reinterpretation in the light of more recent findings, essentially comes down to mechanisms that can alter (sometimes quite dramatically) how the genes are expressed by changing the programming as the quote below regarding epigenetic metamorphosis and the flexibility of these highly changeable ancestral programs of development and adaptation suggests.

Building the Most Complex Structure on Earth: An Epigenetic Narrative of …
By Nelson R. Cabej

Metamorphosis is an amazing example of the dexterity of animals to switch to different development programs. This certainly contradicts the prevailing opinion that an egg or a zygote is provided with a program that determines development up to the adult stage. This gains more significance when one remembers the ease with which some metamorphosizing amphibians can switch to a direct mode of development, or even skip metamorphosis altogether. …Metamorphosizing species, besides their own developmental program, have incorporated and executed ancestral developmental programs. Amazingly, like biological Houdinis, they shift the gears of development both forward (insects and amphibians) and backward (ascidians).
Cabej (2013, 179)

https://books.google.ie/books?id=THz_-R8BSs0C&pg=PA179&lpg=PA179&dq=Metamorphosis+is+an+amazing+example+of+the+dexterity+of+animals+to+switch+to+different+development+programs&source=bl&ots=EuYijkGWNu&sig=t-Z5Vua3HPuWx2_q7dJ8T7_Se5c&hl=en&sa=X&ved=0CCEQ6AEwAGoVChMIvI_Z17XixwIVTMAUCh20sAgH#v=onepage&q=Metamorphosis%20is%20an%20amazing%20example%20of%20the%20dexterity%20of%20animals%20to%20switch%20to%20different%20development%20programs&f=false

The key point being that vertebrate or invertebrate primitive and generalist organisms prior, to becoming mature adult species forms, is proposed to be timed according to environmental conditions. In other words, an insect is not going to morph into its final species form of a flying insect until the atmosphere is conducive to land and sky dwelling life, has been colonised by soils and plants and the entire eco-system is self-sustaining so that nibbling slug-like (previously algae eaters who could reproduce themselves via epigenetic differential of cloning), could co-evolve with self-seeding plants and eventually avail of the abundant flowering plants and help spread their seed while being fed and nourished much better if they were able to fly and flutter between them. Mimicry (an epigenetic environmentally-driven development program as discussed in the book – the Epigenetic Caterpillar) is a case in point. The flowers become colourful, so do insects in their adult species form if they have the genetic and epigenetic means to do so.

Similarly, therefore, like the insect species that commonly use metamorphosis, complex animals such as vertebrates with a much more sophisticated genetic toolkit (Hox gene master switches that are operated via environmental epigenetic controllers), can seemingly also transform themselves from a tadpole-like soft-bodied form that is confined to water until the time is right for it to get out of its pond (or primal pond in this analogy to earlier evolutionary development) and walk for the first time on land in its almost adult form (it has to mature and grow and develop more defined features in this form). If more primitive forms of vertebrate walking tetrapods can regenerate entire new limbs and body parts and lizards can lose their legs simply by not activating limb production during development, as discussed in previous articles on this site, well, it is not that strange I believe to propose that metamorphosis in vertebrates during evolutionary development could bring about equally dramatic changes to actually form a previously primitive generalist into what would seem to us an entirely new species. For example, Cabej continues to highlight the epigenetic basis for many of these processes of non-genetically driven evolutionary adaptation including metamorphosis. Again, this is taken from ‘Building the Most Complex Structure on Earth: An Epigenetic Narrative of …’

Genome sequencing of various species of unicellular and multicellular organisms, conservation of the genetic toolkit, biological phenomena such as developmental plasticity (intragenerational developmental plasticity and especially transgeneration plasticity), reversion of ancestral morphological characters, metamorphosis in invertebrates and vertebrates, cell differentiation, loss of morphological characters, etc., suggest that it is not changes in genes or DNA, but epigenetically determined change in patterns of gene expression in the process of individual development that may be responsible for evolution of structure and morphology. ..
Cabej (2013, 7)
https://books.google.ie/books?id=THz_-R8BSs0C&pg=PA7&lpg=PA7&dq=Genome+sequencing+of+various+species+of+unicellular+and+multicellular+organisms,+conservation+of+the+genetic+toolkit&source=bl&ots=EuYijkGQNw&sig=eATLIpUL8D6C30bWAuFEOnrs7fs&hl=en&sa=X&ved=0CCYQ6AEwAGoVChMIhKHturPixwIVBckUCh3_4QqJ#v=onepage&q=Genome%20sequencing%20of%20various%20species%20of%20unicellular%20and%20multicellular%20organisms%2C%20conservation%20of%20the%20genetic%20toolkit&f=false

Taking this metamorphic means of speciation at the fundamental boundary stage between invertebrate and vertebrate evolution, we could say: the tadpole-like ancestral condition of primitive basal chordates, for example, that hadn’t yet expressed epigenetically all that they had the potential to be, (unlike fish perhaps who had already began to specialise and stabilise as fundamental full-time aquatic vertebrates), would not morph into a tetrapod (limbed and air-breathing animal) until the time was right and not until there was very good reason to get out of the primordial pond in the first place (perhaps there were too many fish?) and a very good reason to want to expand its evolutionary horizons unto land in the first place. And of course, it would need to have the in-built complexity of perhaps metabolic potential not endowed in the simpler vertebrate fish, to achieve this quantum leap frog-type evolution in the first place, even if it wasn’t fully expressed as yet. Again, there would need to be plants to munch on and other creatures as well as their juicy eggs left lying around to be epigenetically reprogrammed into – goodness knows what? As said a number of times now: Mother Nature simply does not put all here eggs in one genetic basket.

Fish – the Vertebrates and then everything else…

Fish – the first vertebrates – gestation period is shorter for less complex metabolisms than for more complex biological systems. Nature cuts her cloth to her measure.
The fish of the Cambrian (the first vertebrates) were primitive forms. It could be said that they were not yet defined fully adult species forms in evolutionary developmental terms. They were dated to over half a billion years ago with no pre-fish or evidence of a non-fish becoming a fish. The simple organisms that existed before this layer show no resemblance to all the diversity of life that exploded into being in the lowest Cambrian, and scientists have looked really, really hard.
Lower Cambrian vertebrates from South China

D-G. Shu*, H-L. Luo†, S. Conway Morris§, X-L. Zhang*, S-X. Hu†,
L. Chen*, J. Han*, M. Zhu, Y. Li* & L-Z. Chen†

Fossil chordates first appear in the Cambrian (545-490 Myr BP), but their earliest record is exceptionally sporadic and is often controversial. Accordingly a coherent phylogenetic synthesis for the basal chordates remains problematic. To date the available soft-bodied remains have consisted almost entirely of cephalochordate-like animals from Burgess Shale-type faunas. Definite examples of agnathan fish do not occur until the Lower Ordovician, with a more questionable record extending into the Cambrian. The discovery of two distinct types of agnathan from the Lower Cambrian Chengjiang fossil-Lagerstätte is, therefore, a very significant extension of their range. One form is lamprey-like, whereas the other is somewhat closer to the more primitive hagfish. These finds imply that the first agnathans may have evolved in the earliest Cambrian, with the chordates arising from more primitive deuterostomes in Ediacaran times (latest Neoproterozoic, c. 555 Myr BP), if not earlier… These discoveries significantly predate previously published reports, but they also imply that yet more primitive vertebrates had evolved prior to the mid-Lower Cambrian.
http://www.nature.com/nature/journal/v402/n6757/extref/402042a0-s2.doc

The first vertebrates had a cartilaginous spine (not a fully hardened bone material) and everything about them is primitive – even if some remain in that primitive specialised form. In the living waters of the Lower Cambrian, these ‘two distinct types of agnathan were tiny primitive fish. They had no jaws, but they did have the all important notochord (a primitive backbone spine). Even today, lampreys (a possible living fossil of a epigenetically modified form?) do not have jaws. And as the article above noted: we still have a lot to learn about these primitive creatures. And interestingly, the article also noted a great many really primitive chordate (soft-bodied) forms and suggested that many more vertebrate types may have existed, even deep into the Cambrian period itself.

The following article relates to the Middle Cambrian period of approximately 505 million years ago and the following article is of interest, firstly as it proposes that these Middle Cambrian fossils are the earliest known vertebrate, which as you can tell from the above article – there are much earlier vertebrates, and because the article also proposes in the title that ‘Man’s earliest ancestor looked like an eel’. It is taken from NewsCore March 07, 2012
Sub-title: ‘HUMANS and other vertebrates evolved from a tiny sea creature resembling an eel,

British and Canadian experts have confirmed’.
Researchers from the University of Cambridge and the University of Toronto said today that the extinct 505 million-year-old Pikaia gracilens is the most primitive known vertebrate – making it the oldest ancestor of humans. Paleontologists analysed 114 specimens and discovered the presence of myomeres – blocks of skeletal tissue found in chordates, which is the group of animals that today includes fish, amphibians, birds, reptiles and mammals.

The study, published in the journal Biological Reviews, proves that the ancient creature is the earliest identified member of the chordate family.
Lead author, Professor Simon Conway Morris, from Cambridge University, said, “The discovery of myomeres is the smoking gun that we have long been seeking.”
He added, “Now with myomeres, a nerve cord, a notochord and a vascular system all identified, this study clearly places Pikaia as the planet’s most primitive chordate.”
Measuring about five centimeters long, with a sideways-flattened body, the Pikaia gracilens is found only in the Burgess Shale fossil beds in Canada’s Yoho National Park.
It was first discovered in 1911 by American paleontologist Charles Doolittle Walcott, who believed the Pakaia was related to leeches and earthworms.

However, scientists have long speculated that the creature belonged to the chordate family because it appeared to have a very primitive notochord – a flexible rod found in the embryos of all chordates, which advances to become part of the backbone in vertebrates.

The Pikaia’s body is divided into a series of segmented muscle blocks that lay on either side of the notochord and scientists believe that it swam above the sea floor by moving its body in a series of side-to-side curves.

Co-author, Jean-Bernard Caron, said, “It’s very humbling to know that swans, snakes, bears, zebras and, incredibly, humans all share a deep history with this tiny creature no longer than my thumb.”

http://www.dailytelegraph.com.au/mans-earliest-ancestor-looked-like-an-eel-uk-and-canadian-experts-say/story-e6freuyi-1226291259153?nk=2f8c01930522adb91b0d0fe930382bb1

Although I’m not specifically proposing that this is the actual ancestor of every living and seemingly extinct walking vertebrate, (as you know this alternative view of evolutionary development attempts to establish the ancestral primitive condition prior to speciation – possibly via metamorphosis, rather than literal ancestor), but I thought it was an interesting article to highlight the chordate ancestral form and the fact that other chordate primitive species of vertebrates had emerged significantly earlier.

Essentially, it seems that as fish are well-established even in the early Cambrian as essentially a species of fish, irrespective of their primitive form and variations on this theme. But it is interesting that some fish stabilise as primitive forms from the outset of the Cambrian period, while others seem to go on evolving as fish long after this period into the next major epoch of time. This is all before anything colonised land, not even the plants or soils or the insects or first tetrapods (walking four-limbed creatures). Therefore, it seems only logical to look for potential ancestral condition of all vertebrates as implied by more recent molecular studies and Von-Baer’s laws as applied to evolutionary development and discussed previously in this series of alternative evolutionary development.

For instance, as the earlier article ‘Lower Cambrian vertebrates from South China’ discussing the really primitive chordate indicates, there were many soft-bodied or basal/stem-chordata floating around in the Cambrian and contenders for basal position in the Pre-Cambrian. Many of these could have been the ancestral condition prior to and during the specialisation of some primitive vertebrate fish. Furthermore, others of this primitive ancestral condition may have not expressed all their evolutionary potential until later. Basal chordate may simply be more complex vertebrates in the making (possibly prior to metamorphosis into their more defined walking vertebrate form?). These are essentially floating, pooping, filter-feeding body parts with all their essential organs. For them to do this neat Houdini reprogramming, they certainly have the necessary tool-kit using the most sophisticated Hox gene complexes of them all. And so do fish, but by applying our evolutionary principles of once a species begins specialising as a species, it appears to remain fundamentally of that kind, then this is perhaps why fish didn’t leave their watery worlds and learn to walk – they had become speciated as FISH. Further support for this proposition is discussed below of why fish didn’t have the means to use this sophisticated tool-kit to produce more complex parts such as connected strong limbs and digits like other vertebrate species.

Recall the discussion of the Hox complexes in one of the earlier articles in this series (the genetic master switches and tool-kit of animal complexity ultimately orchestrated via epigenetic and environmentally-driven processes). And just to remind you about the Hox gene complexes shared by all vertebrates including fish, Gilbert states:

By the time the earliest vertebrates (agnathan fishes) evolved, there were at least four Hox complexes. The transition from amphioxus to early fish is believed to be one of the major leaps in complexity during evolution… This transition involved the evolution of the head, the neural crest, new cell types (such as osteoblasts and odontoblasts), the brain, and the spinal cord…
article link
http://ncbi.nlm.nih.gov/books/NBK9978/

The title of Gilbert’s study: ‘Hox Genes: Descent with Modification’, as it implies, Gilbert views evolution as being driven in part by a commonly shared process/mechanism, a tool-kit, if you like, by which basic body-plans of organisms such as animals can be laid out (built modularly) according to instructions. He states: “This means that the enormous variation of morphological form in the animal kingdom is underlain by a common set of instructions”
Gilbert (2000) http://www.ncbi.nlm.nih.gov/books/NBK9978/

However, this is not rigid preformed set of instructions, albeit fairly conserved as Nature doesn’t have to reinvent the wheel every time a species is fundamentally formed as a vertebrate, as discussed previously in these articles. These Hox gene complexes actually operate as master genetic switches but are ultimately orchestrated epigenetically according to environmental and chemical cues as adequately demonstrated by Cabej’s epigenetic theory of evolution: ‘Building the Most Complex Structure on Earth: An Epigenetic Narrative of Development and Evolution of Animals’ and other work entitled: Epigenetic Evolution (see his site epigenetics comes of age). When Hox gene complexes are viewed in this epigenetic light, we begin to see a clear mechanism, we begin to get a glimpse into how primitive chordate may have morphed into a land-walking tetrapod.

Furthermore, also bear in mind that body-plans of all invertebrates during the early stages of development, are not that fundamentally different to each other and this may be the point when all vertebrates in the making, shared a common ancestral condition. However, as they develop, more divergent forms emerge from these template body plan forms and that the timing of this divergence seems to correspond to the concept that not all organisms mature at the same rate and by applying our developmental model to evolutionary timescales, it seems that the more complex the metabolic system a species has: the longer its gestation period. It would seem (using Von-Baer’s principles and following other lines of evidence for evolutionary development being reflected in the main stages of embryological development on different scales works at every scale, that even the development of the brain evolving according to metabolic/temperature dependent factors and the model overall suggests that all complex evolutionary development proceeded from a shared ancestral condition – going from the generalist to the specialist form of speciation. Therefore, the more complex (metabolically speaking) vertebrates may have taken much longer to mature, specialise and fundamentally adapt to their particular niche, than their earlier and simpler vertebrate counterparts such as fish. In principle, Mother Nature takes more time to cook up the really sophisticated recipes, but once the main ingredients are brought together, she doesn’t fundamentally change the recipe.

This is supported by older evolutionary alternative models that are gaining more support by current molecular studies such as: Butler’s study on the ‘Evolution of Vertebrate Brains’:

… reptiles did not give rise to mammals any more than mammals gave rise to reptiles. In regard to embryological development, it likewise generally proceeds from the general (common ancestral features) to the specific (specializations of the taxon) … What is clearly established is that all taxa have their own specializations. Each taxon has a mix of primitive features.
pdf link
http://brain.phgy.queensu.ca/pare/assets/Evolution2.pdf

Indeed, increasing evidence suggests that all complex animals have evolved from their essential primitive ancestral shared condition and diversified as species by becoming increasingly specialised within their ecological niche. For example, a fish wouldn’t need feet because fins are more efficient for its watery niche. So this begs the question: if chordate was the ancestral condition for all vertebrates including fish, how did some of the inherently more complex chordates make it unto land, if the fish were already specialising (speciated) as fish? As outlined earlier, I am proposing metamorphosis for all vertebrate speciation including for fish as outlined below:

One small step for a salamander and one giant leap for mankind or Leap Frog-type Evolution?

For example the articles below demonstrate and confirm this idea of metamorphosis activation across all chordate. Furthermore, do bear in mind that all vertebrate animals, including ourselves, do go through the essential stages of being a chordate with a notch-chord and a primitive tube for a spine, but it isn’t a spine and full skeleton until later.

The history of a developmental stage: metamorphosis in chordates.
Abstract
Metamorphosis displays a striking diversity in chordates… This wide diversity has led to the proposal that metamorphosis evolved several times independently in the different chordate lineages during evolution. However, the molecular mechanisms involved in metamorphosis are largely unknown outside amphibians and teleost fishes, in which metamorphosis is regulated by the thyroid hormones… According to this definition, all chordates (if not, all deuterostomes) have a homologous metamorphosis stage during their postembryonic development. The intensity and the nature of the morphological remodeling varies extensively among taxa, from drastic remodeling like in some ascidians or amphibians to more subtle events, as in mammals.
Paris & Laudet (2008)
http://www.ncbi.nlm.nih.gov/pubmed/18932261
Volume 21, Issue 18, 27 September 2011, Pages R726–R737

The Origins and Evolution of Vertebrate Metamorphosis by Vincent Laudet
Introduction
Metamorphosis is classically defined as a spectacular and usually abrupt post-embryonic transformation of a larva into a juvenile… It is a very widespread life history transition: indeed, most animals undergo metamorphosis…Even in vertebrates, metamorphosis is frequent…: in addition to amphibians, most teleost fish, representing half of all vertebrate species, undergo metamorphosis … This post-embryonic transition also occurs in basal vertebrates, such as lampreys. In the various classes of vertebrates, these metamorphoses are morphologically very diverse; however, there are some common principles in all these transformations ….
…metamorphosis is triggered in close connection to the environment: in most species, environmental cues play an important role in pushing the larvae to transform to an adult… Both in insects and in amphibians, hormonal systems play a very important role in these processes, and in both cases nuclear hormone receptors and neuroendocrine signalling are used to translate environmental cues into a coordinated program that remodels the organism
…. The first data on the origins of vertebrate metamorphosis came from lampreys… In these species, the larva, known as an ammocoete, is a filter-feeding organism that, after several years of larval life, can be transformed into an adult. This metamorphosis involves a significant reorganization of the body, including of the thyroid gland itself. ..
The data on lampreys nevertheless suggest an ancient association between thyroid hormones and metamorphosis, and this has prompted a study of invertebrate chordates. …. This allows us to propose a new, molecular definition of ‘metamorphosis’, according to which it is viewed as a post-embryonic remodelling period that is controlled by thyroid hormones and their receptors, and that is shared by most, if not all, chordates and that was present in the ancestral chordate … The alternative would be that the spectacular metamorphosis observed in many amphibians and teleost fishes evolved secondarily and convergently; however, that basal chordates undergo a spectacular metamorphosis, and the existence of shared features between the molecular cascades controlling metamorphosis in amphioxus, teleost fishes and amphibians, argue for metamorphosis being an ancestral feature of chordates.
http://www.sciencedirect.com/science/article/pii/S0960982211008311

The Origins and Evolution of Vertebrate Metamorphosis
http://www.sciencedirect.com/science/…/S0960982211008311
ScienceDirect by V Laudet – ‎2011
Metamorphosis, classically defined as a spectacular post-embryonic transition, is well exemplified by the transformation of a tadpole into a frog. It implies the appearance of new body parts (such as the limbs), the resorption of larval features (such as the tail) and the remodelling of many organs (such as the skin or the intestine)…Metamorphosis is actually widespread in the vertebrates, though quite diverse in the way it manifests in a particular species. Furthermore, evolutionary and ecological variations of this key event, from paedomorphosis to direct development, provide an excellent illustration of how tinkering with a control pathway can lead to divergent life histories. The study of invertebrate chordates has also shed light on the origin of metamorphosis. The available data suggest that post-embryonic remodelling governed by thyroid hormones is an ancestral feature of chordates. According to this view, metamorphosis of the anurans is an extreme example of a widespread life history transition.

And finally, another except about vertebrate metamorphosis:

Chordate Metamorphosis: Ancient Control by Iodothyronines by Robert J. Denver doi:10.1016/j.cub.2008.05.024
A complex life cycle, where an animal begins life as a larva then undergoes a metamorphosis to the juvenile adult form, is a widespread and ancient life history strategy … Larvae generally exploit different ecological niches from adults, thus avoiding competition for resources. There is considerable morphological diversity among larvae and the transformations that they undergo during metamorphosis … which raises the question whether the complex life cycles of extant species reflect an ancestral or a derived state.
http://www.sciencedirect.com/science/article/pii/S0960982208006611

These articles strongly point to the universality of metamorphosis within vertebrate forms. It is more common amongst both invertebrates and vertebrates, beyond amphibians, than previously realised. Recall that in vertebrates that even fish ancestrally may have arisen – not out of egg spawn, but via metamorphosis. This supports the idea that all vertebrate animals evolved from a general ancestral condition (a metamorphic chordate stage) an ancient mode of development perhaps? After this point, it seems that metamorphosis begins to become less dramatic in animals as they become more complex. Metamorphosis is just a rather rapid and profound remodelling of organisms and is reflected in all animals from reptiles to mammals, only in a more controlled and stable form.

As also highlighted above, simple environmental factors can trigger the secretion of a hormone, influencing metabolism and induce metamorphosis in young larvae forms. Ancestrally, something similar may have triggered chordate to metamorphosis at different times and according to environmental cues into a much more mature species form. Prior to this metamorphosis however, as indicated in research with modern species, chordate or any primitive condition prior to metamorphosis shows that larval or immature forms are actually free-living and can feed in different environments to their matured (metamorphosed) form. Furthermore, it can be inferred from many studies that developmental stages in the form of polyps, medusa and cloning and segmentation as used by present-day primitive type species, may have been a means of simple reproduction prior to a metamorphic leap to specialist speciation. In other words, primitive or larval type ancestral forms may have sustained and reproduced themselves indefinitely until the time was right for making the evolutionary leap and expressing all their accumulated diversity.

In other words, the independent larval stage in different environments may point to evolutionary timescales of primitive reproduction via epigenetic cloning/budding to reproduce these primitive and differentiated life-forms that could have remained in this stage of development indefinitely (a few million years even after the first diversification of primitive fish) or until new ecological niches opened up such as the possibility of a changing the chemical composition of the shallow water perhaps, triggered by a significant increase of iodine (sea weed) and causing a cascade effect of many varied free-living organisms that munched on primitive algae and seaweeds of the shallow seas in the Cambrian. This iodine component is highly pertinent as you will see below:

… the main triggers of metamorphosis are the thyroid hormones, iodinated derivatives of the amino acid tyrosine, produced by the thyroid gland. http://www.sciencedirect.com/science/article/pii/S0960982211008311

For instance on a fossil site in the Uk, the following description sets the scene as well as revealing where that extra iodine might have come from and why metamorphosis for land-dwelling vertebrates and indeed, invertebrates, may not have happened until after the Cambrian period:
http://www.ukfossils.co.uk/timeline/Cambrian.html
________________________________________

The Cambrian period lasted from about 545 up to about 495 million years ago. The start of the Cambrian period was more active than at the end. Mountains were still being created but only in a few parts of the world. By the end everything had quietened down somewhat and was much calmer. Land was being gently eroded by shallow seas, leaving most of what is now the British Isles, under water. Certain types of sponges that are now found in the rocks all around the world, seem to suggest that the climate was very likely quite warm.

Animals and plants were still confined to the water so no life existed on dry land. At the beginning of the Cambrian period, many types of invertebrates were existing in the waters and seas. Brachiopods and trilobites evolved. The trilobites took over the seas and developed into many forms and soon began to outnumber the over invertebrates. Sponges, worms and jellyfish evolved along with molluscs and early echinoderms. Plant life was probably a lot slower to evolve, but this is difficult to know with any certainty as the only recorded plant remains found in rocks from this period today are of calcareous algae (a type of seaweed).
GEOGRAPHY AND CLIMATE: Sea level rise, sandstone and limestone deposited, Atmospheric levels of oxygen rise, North America and Eurasia separate.

TERRESTRIAL ANIMAL LIFE: No life on land.

PLANT LIFE: No plant life.

SEA LIFE: Primitive algae and seaweeds, Jellyfish, Sponges, Starfish
Worms, Velvet worms, Shelled animals appear, Trilobites and
Brachiopods dominant, Reefs built archaeocyathids.

Who knows, it may all be down to a thyroid hormone and according to environmental cues that triggered chordates to morph into fish and in time for a whole new eruption of complex vertebrate tetrapods (land walking vertebrates) which ultimately led to ourselves and every non-fish vertebrate on the planet? Well you were, in your current life, once not unlike an elongated tadpole and then you grew into a recognisable embryo of vertebrate form with a notch-chord and basic filtering system. Metamorphosis is simply direct development speeded up a little and the remodelling is just a tad more radical. It shouldn’t really matter that we might have started out as a swimming tadpole-like slender worm in the primitive Cambrian seas; do you not start out as a speck and grow to a embryo in fluids of another sort? As you become more definable, first as a limbed vertebrate, you are not that different to all the other limbed and backboned forms, (except fish perhaps as they don’t have limbs with digits) it is only later after you leave your watery environment do you crawl about on all fours before, finally standing on your own two feet.

These studies begin to really open up the whole issue of how common was this metamorphic developmental – speciation process back in the primordial pond and provide a sound basis for proposing that the shared ancestral condition of all vertebrates (including fish) was embodied in a chordate primitive form which used metamorphosis to make a quantum leap to its next level of complexity – whether it be to specialise as a fish, or to simply grow limbs and digits and develop a whole new breathing apparatus to walk on land as a vertebrate tetrapods of all varieties of primitive wan’a-be – amphibians, lizards, and mammalian forms in the making. Remember, like the fish, there would appear to be many variations on the theme of walking tetrapod until further specialisation and therefore speciation becomes established. Metabolic complexity (inherent) in these tetrapods would appear to be the main driving force that ultimately determines their actual species specialism. Don’t forget the importance (as the record clearly shows) of continued genetic exchange and particularly hybridisation as a means of genetic novelty directed by environmental epigenetic factors of expression, as a means of continuing to shape and mould these species in the making into many varied and diversified forms.

Could Fishy-pod ancestors walk?

An article on a popular science website discusses an interesting experiment relating to the Hox family of genes (that act like master switches during development and turn other gene sequences on or not). It is entitled: How the genetic blueprints for limbs came from fish…

… the transitional path between fin structural elements in fish and limbs in tetrapods remains elusive. Both fish and land animals possess clusters of Hoxa and Hoxd genes, which are necessary for both fin and limb formation during embryonic development. Scientists compared the structure and behavior of these gene clusters in embryos from mice and zebrafish. The researchers discovered similar 3-dimensional DNA organization of the fish and mouse clusters, which indicates that the main mechanism used to pattern tetrapod limbs was already present in fish…Does this imply that digits are homologous to distal fin structures in fish? To answer this question, the geneticists inserted into mice embryos the genomic regions that regulate Hox gene expression in fish fins. ‘As another surprise, regulatory regions from fish triggered Hox gene expression predominantly in the arm and not in the digits… Fin radials are not homologous to tetrapod digits.The researchers conclude that, although fish possess the Hox regulatory toolkit to produce digits, this potential is not utilized as it is in tetrapods. Therefore, they propose that fin radials, the bony elements of fins, are not homologous to tetrapod digits, although they rely in part on a shared regulatory strategy.
http://sciencedaily.com/releases/2014/01/140121183412.htm

The key phrase in this article in my opinion is …fin radials, the bony elements of fins, are not homologous to tetrapod digits, “although they rely in part on a shared regulatory strategy”. The other important part of the above article is that the fact that the fish Hox complex was not able to activate digits in the mouse,and only activated a slight developmet in the arm region.

Furthermore, there is complimentary evidence for fish remaining fish as the fossil record does not present any convincing evidence for the transition between certain types of lobbed-finned fish and walking tetrapods either. I will discuss this evidence below. All in all, it would appear that reason why fish cannot make fishy fingers, but other vertebrates with the same tool-kit can, is because fish began to specialise earlier that their more complexly endowed counterparts. For example, feet are no good to fish, but fins are very useful to vertebrates that live in water and Mother Nature would appear to be very efficient at stabalising a species once it starts to become metabolically efficient and used up all its inherent complexity picked up in its primordial environment.

Returning to the fossil record, I will now review some of the conventional interpretations in the light of some recent studies addressing the so-called transition of fin to foot – Walking Fishy-pod hypothesis. According to conventional thinking it took almost 200 million years for this fishy-pod to evolve limbs that could walk and a whole breathing apparatus and many, other things that allow a fish to live in water rather than land. Yet we have no evidence of this great feat of biological engineering, at least not using our conventional understanding of this scenario. For instance, below is a typical outline of the conventional scenario, followed by the usual puzzlement when trying to reconcile the evidence with the Neo-Darwinian version of events

“Our first four-legged land ancestor came out of the sea some 350 million years ago. Watching a lungfish, our closest living fish relative, crawl on its four pointed fins gives us an idea of what the first evolutionary steps on land probably looked like. However, the transitional path between fin structural elements in fish and limbs in tetrapods remains elusive…
article link http://pnas.org/content/97/9/4430.full.pdf

Millions of dollars later and with much concerted effort in 2004, they found it: a real contender for the walking tetrapod was an early lobe-finned fish, which propped itself up in shallow fresh water known as tiktaalik. The only problem was the more recent discovery of fossilized tetrapod tracks, which are dated to some 18 million years before the tiktaalik fossil, and actually several million years before the time that lobe-finned fish were believed to be paddling about in muddy water trying to grow some walking limbs. The following article explains:

The fossil record of the earliest tetrapods (vertebrates with limbs rather than paired fins) consists of body fossils and trackways. The earliest body fossils of tetrapods date to the Late Devonian period … and are preceded by transitional elpistostegids such as Panderichthys and Tiktaalik that still have paired fins. Claims of tetrapod trackways predating these body fossils have remained controversial with regard to both age and the identity of the track makers. Here we present well-preserved and securely dated tetrapod tracks from Polish marine tidal flat sediments of early Middle Devonian (Eifelian stage) age that are approximately 18 million years older than the earliest tetrapod body fossils and 10 million years earlier than the oldest elpistostegids. They force a radical reassessment of the timing, ecology and environmental setting of the fish–tetrapod transition, as well as the completeness of the body fossil record. (In Nature by Niedźwiedzki et al, 2010 Abstract)
http://nature.com/nature/journal/v463/n7277/abs/nature08623.html

Maybe we really do need another way of non-vertebrate tetrapods to become land-walkers. How about Leap-frog evolution as discussed above via metamorphosing primitive basal chordate?
I also think that it is worth taking a look at the rest of the fossil record for fish and ask ourselves: why would some fishy-pods learn to walk and all the rest of that complex stuff, while all the rest of them stayed essentially as fish? We will now look at one famous so-called living-fossil and let you decide and do recall the earlier article in this series that discusses genome silencing when a certain level of evolutionary complexity has been reached: In an article in Science Now, the title reads: “Living Fossil” Gets its genome sequenced (2013) and states the following:

coelecanth fisha live coelacanth

“The coelacanth isn’t called a [“living fossil”] for nothing. The 2-meter-long, 90 kg fish was thought to have gone extinct 70 million years ago—until a fisherman caught one in 1938—and the animal looks a lot like its fossil ancestors dating back 300 million years. Now, the first analysis of the coelacanth’s genome reveals why the fish may have changed so little over the ages. It also may help explain how fish like it moved onto land long ago..

.
Then, the same article goes on to explain what they found after sequencing its genome.  They calculated the number of estimated changes that occurred in the genes over the time since the coelacanth branched off from other vertebrates on the animal family tree. Finally, they compared those data with the corresponding rates of genetic change in various mammals, lizards, birds, and fish.

“The coelacanth genes changed at a [“markedly”] slower rate than those from other animals,… The genes of lizards and mammals evolved at least twice as quickly as those of the coelacanth, the team reports online today in Nature. That could explain,… why the fish has changed so little in 300 million years. ”
article link
http://news.sciencemag.org/plants-animals/2013/04/living-fossil-gets-its-genome-sequenced

Are you a little confused? I know I am and I think they are too.
Next week I will review the entire model for this overarching principle of evolutionary development from the Pre-Cambrian to the Cambrian and beyond. Then, hopefully I will get to moving unto land with our primitive and rather experimental tetrapods (species in the making?) Of course this sets the scene for our own evolution which is a massive story in itself and I will maybe leave this to the final article in the series

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s