Alternative evolutionary series: If it didn’t happen by Neo-Darwinian means, how did evolution occur?
Biology’s Silent Big Bang
The Cambrian Explosion & Silent Eruption of Life before it
The Cambrian Explosion Reviewed in the Light of the Emerging Evolutionary Alternative
From an article in Discovery Magazine ‘Mysteries of the Orient’ the following description reveals the nature of the Cambrian explosion:
Thousands of exquisitely preserved fossils …offer a glimpse back to a pivotal event in the history of life. This moment, right at the start of Earth’s Cambrian Period, some 550 million years ago, marks the evolutionary explosion that filled the seas with the world’s first complex creatures. In a blink of geologic time a planet dominated by simple sponge-like animals gave way to one ruled by a vast variety of sophisticated beasts, animals whose relatives still inhabit the world today. This biological big bang reverberated through all facets of existence, altering not only the shape of animals but also the way they lived together.
– Monastersky (APRIL 1993 issue of Discover magazine)
Hopefully by now, (assuming that you have followed the previous articles in this alternative evolution series) you are beginning to see a distinctly different scenario of evolution emerging to our current version, that in combination, can begin to give us a very different view of how evolution may have unfolded, or leaped seemingly into rapid existence. For instance, when considering the Cambrian eruption of complexity, bear in mind how it follows the predictable pattern of growth of any complex and organised system, that of a lag phase (the pre-Cambrian period where the groundwork for what comes after has been laid), an exponential phase (the seeming spontaneous eruption of complexity to all the fundamental forms) and a stabilisation phase (I would say the period we have been in for millions of years – where the species have become adapted and fine-tuned to their environments and fulfilled all of their evolutionary potential). See the scaling laws article – the first in this series https://diggingupthefuture.com/2015/08/03/evolution-scaling-laws-something-universal-is-going-on/
Eugene Koonin’s summary highlights the particularly rapid and profound nature of biological evolution without seeming transitions or gaps in its complexity, and offers some solutions as to how this may have come about (via horizontal gene transfer, microbial mergers and genome remodelling etc) as seen in his paper on Biology’s Big Bang with particular reference to the Cambrian eruption of life. His summary is as follows:
Major transitions in biological evolution show the same pattern of sudden emergence of diverse forms at a new level of complexity… In each of these pivotal nexuses in life’s history, the principal “types” seem to appear rapidly and fully equipped with the signature features of the respective new level of biological organization. No intermediate “grades” or intermediate forms between different types are detectable..
The specific genetic mechanism is outlined in: ‘Hox Genes: Descent with Modification – Developmental’ by SF Gilbert – 2000 below and it is interesting that this mechanism itself evolved as seen below:
The number of Hox genes may play a role in permitting the evolution of complex structures. All invertebrates have a single Hox complex per haploid genome. In the most simple invertebrates—such as sponges—there appear to be only one or two Hox genes in this complex … In the more complex invertebrates, such as insects, there are numerous Hox genes in this complex. … By the time the earliest vertebrates (agnathan fishes) evolved, there were at least four Hox complexes…
The Hox gene complexes and their ability to build whole organisms rapidly and in a highly co-ordinated fashion, is only part of the evolutionary conundrum. Although the Hox genes (that work like master switches for all the other genes) presumably evolved as a short-cut means to activate the vast array of novel genetics exchanged amongst many different forms of earlier life – we need to understand how Nature knows what to build? What to switch on or off, or express genetically, when, where and how? This is where epigenetic modification processes via environmentally driven evolution comes into play. It is this epigenetic flexibility for differential expression, of otherwise conserved genes, along with genome remodelling that seemingly have played a major role in sculpting the species into perfectly adapted forms, via epigenetic mechanisms, which ultimately control the Hox genes or complexes.
For example, continuing from where we left off last week, recall the end reference to the Cambrian and the epigenetic explanation as seen in the following quote:
The concept ‘that epigenetic mechanisms are the generative agents of body plan and morphological character origination helps to explain findings that are difficult to reconcile with the standard neo-Darwinian model, e.g., the burst of body plans in the early Cambrian, the origins of morphological innovation, homology, and rapid change of form.
Abstract NEWMAN & MÜLLER
See previous articles on the great web of life and epigenetic evolution:
Epigenetic mechanisms are seemingly, the competent users of this genetic toolkit (the Hox gene complexes) as outlined in the book description from Cabej’s epigenetic theory of evolution titled:’Building the Most Complex Structure on Earth’ –
This is a novel theory that describes the epigenetic mechanisms of the development and evolution of animals and explains the colossal evolution and diversification of animals from a new post-genetic perspective. Modern biology has demonstrated the existence of a common genetic toolkit in the animal kingdom, but neither the number of genes nor the evolution of new genes is responsible for the development and evolution of animals. The failure to understand how the same genetic toolkit is used to produce millions of widely different animal forms remains a perplexing conundrum in modern biology. The novel theory shows that the development and evolution of the animal kingdom are functions of epigenetic mechanisms, which are the competent users of the genetic toolkit.
Epigenetics processes acting upon the genomes of various organisms, not only begins to account for the dramatic and rapid formation of fundamental ‘principal types’, epigenetic evolution also appears to be responsible for the great variation of forms of these fundamental themes, as it acts via differential genetic expression of these fundamental forms throughout the life span of a species, albeit in a less dramatic way as the species become more mature and reach their adult form. The continuation and fine-tuning (at a micro-evolutionary level of development) of all this epigenetic variation in gene expression is clearly seen as ongoing, as exemplified within to-day’s species. Recall the articles in last week’s post on rapid changes in size of some insect species according to the degree of methylation – an epigenetic process, or the change of sex of some fish larva depending upon the surrounding temperature as they were developing; a very clear example of environmentally driven epigenetic evolution in action. Therefore, this epigenetic mechanism, acting upon existing genes and driven by environmental cues, gives a real insight into the means by which the Cambrian eruption of all the fundamental body-forms and their variation thereafter, came to be.
Furthermore, as discussed previously, Nature appears to have given all the different forms of life a means to radically change and adapt rapidly if needs arise and rapid adaption is required, which is again environmentally stimulated/triggered and involves the remodelling of existing genes by so-called jumping genes. I referred to this in previous articles in this series as Natural Correction and also the fact that Nature has given all the species a means to not only survive, but to actually thrive. It is not survival of the fittest, but the thrival of the most adaptable and evolution via epigenetic means, is inheritently adaptable.
We will look at some possible environmental triggers for the rapid and radical eruption of life-forms – the means to adapt – and their subsequent diversity further on. But also bear in mind that seemingly, as the evidence strongly suggests: once an organism becomes more defined as a species, it seems to become much more genomically quiet and begins to refine its form (becoming a specialist species) as it has reached its fundamental level of metabolic complexity and efficiency (see scaling laws and predictable fractal networks of biological forms). Therefore, we can only assume that the younger and more primitive forms were much more susceptable to this type of genome remodelling and epigenetic expression resulting in dramatic and rapid transformations in the past than their adult species forms seen in the fossil record at a much later date.
All of these considerations begin to give us an insight into the means by which rapid speciation in response to environmental shifts erupted within the Cambrian period – which we could call a profound growth spurt corresponding to the exponential phase of the seemingly ‘universal’ growth and/or evolutionary development pattern represented by a Sigmoidal growth equation.
We are beginning to see mechanisms for rapid and profound evolution that are directed by natural laws of growth and form and actualised via epigenetic mechanisms that operate above the genetic level. This also gives us a means of the species developing from a common ancestral condition – going from the generalist to the specialist as proposed in Von-Baer’s laws which date back to the time before Darwin’s theory and supported by more recent molecular studies. See summary below:
Von Baer’s laws of embryology as applied to evolutionary development are summarised as follows:
General characteristics of the group to which an embryo belongs develop before special characteristics.
General structural relations are likewise formed before the most specific appear.
The form of any given embryo does not converge upon other definite forms but, on the contrary, separates itself from them.
Fundamentally, the embryo of a higher animal form never resembles the adult of another animal form, such as one less evolved, but only its embryo.
The atmosphere evolved & so did everything else –
As discussed above, environment is intrinsic to how species seem to have evolved. It is a type of co-evolutionary process judging by the research carried out on this topic. For instance, below is an article titled: Oxygen and Evolution by Berner et al (Science 27 April 2007: 557–558). [DOI:10.1126/science.1140273] which summarises the situation as follows:
The rise of atmospheric oxygen (O2) concentration during the Precambrian eon (~4500 to ~550 million years ago) was closely tied to biological evolution. Additional changes in atmospheric O2 concentrations over the past ~550 million years (the Phanerozoic eon) have probably also been intertwined with biological evolution. Here we examine the evidence for changes in O2 concentrations and their biological causes and effects during the Phanerozoic.
More detail is given in a similar article entitled: ‘Ancient Atmospheres and the Evolution of Oxygen Sensing Via the Hypoxia-Inducible Factor in Metazoans’ by Taylor and McElwain
(Physiology Published 1 October 2010 Vol. 25 no. 5, 272-279 DOI: 10.1152/physiol.00029.2010)
Although photosynthesis in the oceans likely began ∼3,000 million years ago …, it was not until ∼1,500 million years ago, when photosynthesis-derived oxygen began to accumulate to significant levels in the atmosphere. While atmospheric oxygen levels continued to increase over the following 1,000 million years, the existing life on Earth during this time remained as simple single-cell organisms … However, when atmospheric oxygen levels reached between 15 and 20% between 550 and 600 million years ago, the first animal body plans developed, and the evolution of metazoans started in earnest … During the following ∼550 million years of metazoan evolution, oxygen levels in the atmosphere have been estimated to have fluctuated between lows of ∼15% and highs of >30% … During this period, metazoan evolution progressed from simple aquatic species such as sea sponges and corals to a dramatic array of marine and terrestrial species including Homo sapiens (H. sapiens)…
… How the fluctuating levels of atmospheric oxygen over the last ∼550 million years impacted on evolution of metazoans is an area in need of further investigation … However, existing physiological studies indicate that fluctuations in atmospheric oxygen may have had a significant role to play in helping to shape evolution.
Now that we are beginning to understand the critical role that environmentally-driven epigenetic evolution plays in forming and shaping the species from the generalist to the specialist, continually adapting and making use of what is available in the environment, we can begin to see that bacteria was crucial in building the atmosphere for aquatic and eventually land-based life to form in the first place. The transition from soft-bodied embryo-like forms (I like to refer to these as species in the making), not quite sure what they are going to be when they grow up, is also explicable when we consider all these mechanisms together and within the context of the prevailing environmental conditions. Remember evolutionary complexity appears to be fundamentally linked to an organism’s innate metabolic potential. If a simpler organism or level of cellular complexity reaches full efficiency, (having a much simpler metabolic system), then, Nature appears to stabalise that system/organism at a macro-level (via genome silencing perhaps?) allowing for continued refinement of all variations on that theme. Therefore, organisms and systems that have much greater evolutionary potential will presumably specialise and become stabalised later than their more primitive counterparts. For instance, bacteria remain as bacteria once they became stabalised and and began specialising as free-living or symbiotic bacteria (as in the bacteria that keeps us alive and lives in our guts). These bacteria have actually co-evolved with other species and got pretty smart by hitching a ride. So in a sense, although they are still bacteria, (and they are not all bad), they have continued to evolve and adapt to many different conditions and environments and modes of existence. Sponge animals and jelly fish are very simple animals and remain as such to-day. However, some soft-bodied animals went on to become molluscs and grow protective shells of every shape, size, colour and hue and patterning, but still remain as molluscs. Obviously, embryonic soft-bodied animals also floating about in the primordial pond had much more hidden evolutionary potential and it could be that they simply hadn’t expressed all that they were going to be in the future – as yet. Therefore, we have set the scene for the type of conditions of life in the Pre-Cambrian leading to the Cambrian.
Now that we have an atmosphere and plants and fungi, the time is perhaps right for the silent eruption of complexity and cellular networking to co-evolve more complex animal forms. A new fundamental transfer system of exchange between the atmosphere and organisms begins in the Pre-Cambrian.
The Pre-Cambrian Big Damp Squid?: Which came first the chicken or the embryo?
Remember that just because the embryos or larval forms of species doesn’t look anything like its adult self, this species in the making, may not have activated or expressed genetically via environmentally-driven epigenetic processes, all that it had the potential to be when it grew up. Just as a caterpillar looks nothing like its adult form (a butterfly or moth or general flying insect), yet it has the same genes and Hox Gene tool-kit, in evolutionary developmental terms, the same could be implied only on a much longer timescale, but presumably no less dramatic than a caterpillar changing into an entirely different looking adult form, but only when the time is just right of course.
For instance, we could never explain the predecessors of this great explosion of complex life, and indeed, still cannot. However, back in the late 90s, Stephen Gould got very excited and wrote in a paper entitled: Fossils of tiny embryos 570 million years old may well be the greatest palaeontological discovery of our time (1998) http://www.neuroblast.net/team/ze/PDFs/gouldart.htm all about the fossil embryos found in this layer that might just begin to shed light on the next stage. He even went to great lengths to explain what had happened to their mothers in terms of a unique preservation process that happens only to tiny things. Subsequently, in a more recent article entitled: Precambrian fossils, once thought to be embryos, reinterpreted as… something else (2011), CAT scans now show that “they most certainly aren’t bacteria…but it’s not actually clear what they are.” They go on to say that most likely these are “a relative of the first metazoans” http://arstechnica.com/science/2011/12/precambrian-fossils-once-thought-to-be-embryos-reinterpreted-as-somethign-else/
The answer to the Cambrian explosion seemingly from no particular or obvious predecessors may be to go back to Gould’s original premise and see these ‘relatives of the first metazoans’ of animal life as embryos of what they were to eventually become but, without their mothers or their eggs. This brings us of course to the chicken and egg dilemma. Which came first? Well, as I have suggested in the title and other articles, it wasn’t really an egg at this early stage of evolution (this is a later developmental strategy – things are much more direct, radical and primitive in these evolutionary times), but an embryo or larval form of undifferentiated species prior to metamorphosis. These metazoan embryo-like or larval forms of life, therefore technically could not have had mothers as their adult forms were not around yet. It may have been a tiny damp-squishy- squid-like blob that got it all going or more to the point a squishy-squid-like shared ancestral CONDITION.
By taking this line of reasoning to its natural origins, or conclusion, we may not be looking at embryo-like or larval forms of organisms requiring an adult to make their existence possible. It doesn’t even matter at this stage of evolution or, at this point in the discussion, whether the embryo is packaged in an egg (spawn or otherwise), a pouch or within a womb. As the cells in any organism are at their most essential, organised colonies of well-coordinated networks and metabolic and molecular systems and we know that cells differentiate and become specialist cells epigenetically to form an organism, then once again, we don’t need to worry about the egg bit or the chicken for that matter – it is the collection of cells that make up an organism that we are interested in for the purposes of this discussion and it doesn’t have to originate with a specific ancestor or even a mother, although budding (a cell becoming a clone and growing up to be a large and epigenetically differentiated single-celled organisms is most certainly how simpler organisms reproduced themselves leading up to the Cambrian period.
The ancestral condition therefore may lie in the concept of embryo-like species being a highly coordinated colony of cellular life is exemplified in the cellular networks discussed previously where: cells that fire together: wire together. This is otherwise known as: Hebb’s law which, has been recently been verified via scientific studies. And to put this into perspective, the replay of the co-evolution of colonies of cellular-life such as slime-mould is a case in point. These slime-moulds can form slug-like pulsating cellular colonies working and co-operating together when certain circumstances prevail. In-deed, they approximate slug-like organisms in the making. They can take on all manner of plant-like, fungal-like and blobby simple-looking life-forms, doing hand-stands and twisting and turning according to environmental cues. They come together to co-ordinate their activities as this is a much better way to source fuel and therefore energy and further develop. Basically, they become greater than the sum of their individual parts by acting as one whole body. A great example of the co-ordinated intelligence of these creatures when they work together can be seen in the fact that tests have been done where, in a slime-mould without the use of a brain, eyes, or a nose or mouth and technically without a body, can complete a maze if there is some sugary substance to re-fuel and grow – to go where no slime-mould has gone before.
Then, if we bring all the genetic exchange across all domains of life into the equation, along with the microbial mergers or once independent organisms and horizontal gene transfer, jumping genes between even disparate forms of life and jumping genes operating within and between different species where they have the ability to remodel entire genomes as the record shows, you can perhaps begin to see the almost imperceptible foundations being laid for embryonic-like/larval forms identified in the late Pre-Cambrian and its subsequent leap of developmental speciation in the Cambrian. And as discussed earlier, we know that Hox gene complexes began to evolve (presumably in a quiet way within these blobby slug-like organisms of co-ordinated cellular networks) and provided in the end, a tool-kit – a short-cut means to rapidly express, and to actualise all this stored genetic novelity and epigenetic memory gleaned from the experiences and interactions in the primordial pond thus far, all that it could be – but only when the time was ripe.
By the time of the late Pre-Cambrian when a few sponge animals were floating about and the odd worm-like creature, whom we know existed from their borrowing activities etc, goodness knows what some of these that didn’t rapidly become simple worms, had the potential to morph into? And don’t forget the abundant and diverse microbial life-forms still with us today. There is no way of knowing what was stored within some of these embryos or larval miniature blobs, which palaeontogists aren’t even sure what to call them or what they are exactly. That is a clue in itself. I would suggest that they are species in the making, an embryonic/larval form of their potential adult selves.
Just like modern day larvae and embryonic forms (albeit originating from eggs with mothers and fathers for the most part), if we didn’t know any better from seeing this scenario repeated in predictable ways, we would not be able to say what the larvae, metamorphosing or embryonic form would be when it matured into its adult or even juvenile form. Remember, eggs and mothers or much of the species of animals have not actually evolved yet. We might be looking at how the first animals, plants, and their eggs, spawn and seed came into being in the first place, An egg or seed is a more complex cell and a more sophisticated and more efficient way of reproduce oneself via epigenetic modification, compared to spawning, budding or cloning your protege directly (like a literal chip of the old block).
These embryo-like or larval blobs (of the late Pre-Cambrian) may therefore be much more sophisticated free-living, diversified, multi-cellular life (species in the making), poised on the threshold of epigenetic expression, not yet actualised as the particularle environmental conditions to make this explosive leap in complexity may not have been present in the Pre-Cambrian. However, in the Cambrian period the environment changes dramatically, and seemingly, so does life. In other words, there is no point in slug-like algae-munching critters developing wings and flying around the air until there is an atmosphere to fly in and perhaps something to eat outside the primordial pond.
I call the Cambrian eruption of life, a controlled explosion and it was definitely had an environmental triggering mechanism as you will see below, because, it is perfectly timed and orchestrated according to changing environmental opportunities. I believe it is highly controlled and orchestrated because it seems that Nature had already in place, the means (via its Hox complex respective tool-kits) for these embryonic species to evolve later according to their innate (built-in) complexity, and to take full advantage of the environmental changes when they did arrive. As noted earlier, although the genes and epigenetic markers exchanged within a developing cellular collony now contained within a blobby organism can create vast variations of form, it is how these genes and their switches are expressed, when and where and to what degree that can make a huge difference in the end as to what evoltuionary journey a developing embryo-like organism might ultimately take.
For instance, if it only has two Hox gene switches to play with, it will problably only be able to do a basic blobby body-plan and quickly specialise as a jelly fish or something of that nature. On the other hand, if hidden in its genome and epigenome, it has a very complex Hox gene complex tool-kit, well, it will probably have the potential to build a very sophisticated body-plan. But, it might need special materials for the job. The opportunity seemingly did arise for this much more sophisticated body-form and without this opportunity, we would not be here.
This opportunity and the type of Hox gene mechanism an early developing organism was endowed with seemingly could make the difference between remaining as a simple invertebrate animal and forming a clone colony of coral (as soft-bodied animals of this type do to-day), or become a sluggy-type algae-nibbler – insect in the making awaiting long-term metamorphosis to bigger and better things. Or to become the first of the really fancy floating chordates- tadpole-like/worm-like or larval forms – take your pick (basically soft-bodied pooping, filter-feeders with rudimentary notch chord nerves and simple organs) to transform into the first vertebrate water-based animal (a fish and all variations on that theme) or becoming land-walking complex vertebrate tetrapod.
We have now set the scene for a possible scenario of how embryonic-like/ larval forms of pre-Cambrian life may have exploded into all the diversity of fundamental body-plans we see today, if we first explain the environmental trigger – the possible smoking gun for its cause. The clue lies in the following article entitled: ‘Evidence for a geologic trigger of the Cambrian explosion’ dated to April 18, 2012 by Jill Saka
The oceans teemed with life 600 million years ago, but the simple, soft-bodied creatures would have been hardly recognizable as the ancestors of nearly all animals on Earth today.,,
Then something happened. Over several tens of millions of years — a relative blink of an eye in geologic terms — a burst of evolution led to a flurry of diversification and increasing complexity, including the expansion of multicellular organisms and the appearance of the first shells and skeletons…
The results of this Cambrian explosion are well documented in the fossil record, but its cause — why and when it happened, and perhaps why nothing similar has happened since — has been a mystery… “The Great Unconformity is a very prominent geomorphic surface and there’s nothing else like it in the entire rock record,” says Shanan Peters, a geoscience professor at the University of Wisconsin-Madison who led the new work. Occurring worldwide, the Great Unconformity juxtaposes old rocks, formed billions of years ago deep within the Earth’s crust, with relatively young Cambrian sedimentary rock formed from deposits left by shallow ancient seas that covered the continents just a half billion years ago… “We’re proposing a triggering mechanism for the Cambrian explosion,” says Peters. “Our hypothesis is that biomineralization evolved as a biogeochemical response to an increased influx of continental weathering products during the last stages in the formation of the Great Unconformity”.
A Cambrian trilobite, with a shell made of calcium carbonate. (Photo: Shanan Peters)
So what if we do a thought experiment and imagine what a trilobite might look like naked and very tiny; an embryo-type blob perhaps? As it turns out trilobites did turn up naked. Perhaps it wasn’t quite its adult self yet. This evidence is presented below:
An un-named “soft-bodied trilobite” from the Flinders site in Australia … also might seem a reasonable Precambrian candidate antecedent to true trilobites …” http://www.trilobites.info/origins.htm
So guess where the more complex invertebrates got their exoskeletons from and all those molluscs built all their nice colourful shells or the fish got their cartilage to build rudimentary skeletons and the more complex vertebrates got their sturdier skeletons with limbs that could support their bodies? Fish are the most primitive vertebrates, so presumably (as the early Cambrian shows) these were the first of this kind. Furthermore, it seems, (again judging by the fossil record) that primitive (generalist) fish emerged first and later more specialist forms with jaws and teeth etc and so on until all variations on the theme of fish filled the seas and waters of our entire planet. But the other embryonic (tadpole-like) chordate still not sure what they were going to be when they grew up, may have taken a little longer deciding what they were going to specialise at. Some may have simply changed their program (genetic expression of all that saved up evolutionary potential) and long after the initial Cambrian explosion, where the basic body-form templates were established and stabilised, some embryo-like organisms or floating tad-pole types (chordate) may have opted to, as the amphibians do to-day, simply walk unto land and breathe air using a primitive mode of development via metamorphosis. It would beat having to gradually develop specialist breathing apparatus, a new form of skin, and whole new skeleton structure along with the limbs and digits required to walk on land. Presumably some major groups of generic tetrapods (four-legged vertebrates) developed and diverged and became highly proficient species of specialist kinds depending upon the level of their metabolic evolutionary potential. Ultimately some of these metabolically complex tetrapods grew up to become upright walkers – like ourselves. I’ll discuss the walking fish story in relation to the fossil record and some recent studies in next week’s article and hopefully, get to our own evolution in the following week.