Category Archives: Evolution

The Tree of Life needs to be chopped down?

In January 2009, the popular science magazine New Scientist ran a controversial cover story declaring that “Darwin was wrong,” about the Tree of Life. Darwin famously presented the concept of life branching like a tree in his seminal work and it has been a mainstay, almost an icon, of evolutionary theory ever since. Unsurprisingly, anti-evolutionists aim a lot of their criticisms at this understanding of evolution. It is not uncommon for them to reference the New Scientist article and the evidence it presents. Such an article was doing the rounds recently on Facebook, so I felt the urge to have my say.

The Tree of Life is a model for understanding evolution, but it is not applicable in all cases. Organisms often swap genes through a process called Horizontal Gene Transfer (HGT) which can render the use of a tree ineffective – when genes are hopping about from branch to branch they can no longer be traced in the linear fashion necessary for a tree with dichotomous branching. Whole organisms can also combine, through a process known as endosymbiosis, wherein an organism becomes part of a larger host cell as they become mutually dependent – it’s how eukaryotes have mitochondria. When whole organisms combine, you suddenly have two branches growing into each other. The same problem arises near the tips of branches, where hybridisation amongst closely related species messes with the tree metaphor. All of these things combined make evolution more like a tangled web than a tree of life, so do we need to chop down the tree and find a new metaphor?

There are two ways a model can be used in science which are relevant here. Firstly, the model can be used to literally describe the important features which fit every example of the phenomenon in question. Secondly, the model can be used to give a detailed description of one example, which is used as a basis for understanding more complex examples. The Tree of Life does not fit the first approach very well, so any evolutionary biologist (or critic) looking for it to function this way are not going to find it useful. It does, however, fit the second approach, as multicellular organisms generally do pass on their genes in the linear fashion required for the Tree of Life model to work, so it can be used as a basis for understanding the more complex additions of HGT, endosymbiosis and hybridisation. In this latter sense, it is also useful pedagogically – students learn the basic branching concept of the ToL before moving on to more complex models; that’s how many concepts are taught in science, for example, students learn about electron shells before they learn about how we understand the positions of electrons in light of quantum mechanics.

I like to think of the theory of evolution as a bit like a bungalow. Darwin and Wallace laid the foundations and built the frame of the building, but it needed more to be a home. The Modern Synthesis gave it walls, windows and doors, but it’s not quite the same building we can walk around today. Since then, some walls have been knocked down, some new ones added, rooms redecorated, even a conservatory and a porch built. Darwin and Wallace would possibly not recognise this home at first – they would need a good look around, but they would still recognise it as a bungalow; no extra floors have been added – it certainly isn’t a tower building masquerading as a bungalow. Theories are often relatively simple and that allows them to cover a broad range of phenomena. That’s a strength, not a weakness.

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Intelligent Design’s problem with the Cambrian explosion

Last night I made a grave error. I made the mistake of attempting to peruse the Intelligent Design movement’s main website, Evolution News and Viewsto see what they have said about recent developments on the Cambrian explosion. I personally don’t think that researchers should give them much thought, but as I am currently just a blogger I will occasionally address them. It is my intention to get my hands on Stephen Meyer’s book Darwin’s Doubt and review it though that may be a long time coming, especially considering that they have made a follow-up to address critics. Until I have read their books, I don’t plan on getting into the details of their arguments. Right now, however, I am more concerned with their approach to the supposed debate.

Firstly, they sound desperate to appear original and as though scientists researching the Cambrian explosion are slowly coming round to their way of thinking. They often state things in a way which suggests that Meyer got there first, for example, Graham Budd was apparently recently “confirming Meyer’s denial” about Precambrian organisms, when the reality is that the interpretations Meyer favours were first offered by evolution-accepting palaeontologists. Cambrian and Ediacaran experts are constantly described as admitting to something which the ID crowd believes; their loaded language is meant to give the impression that they are way ahead of the experts.

The ID presentation of the Cambrian explosion appears to be that is that it was relatively short (10 million years appears to be a figure they will accept), that bilaterian phyla appeared very suddenly in the Cambrian, that there were no precursors found amongst Ediacaran organisms, and that there is no satisfactory evolutionary explanation for it. You will find all of these views, even the last one, preceding Meyer’s publications and from experts in relative fields. The problem for them here is that they are not trying to solve any problems – they already think that they have the answer, yet it really offers no explanation.

Secondly, they consistently complain that they are being ignored. Nobody name-drops Meyer, nobody cites his book, nobody addresses his main thesis. They even do this when discussing papers which focus on specific phenomena, as though every single paper relating to the Cambrian explosion must address their pet theory (I use that term loosely). They are like the guy in a bar who seems to want everyone to fight him (perhaps better left ignored). Scientists are quite happily dissecting every aspect of the Cambrian diversification, looking at the genetic changes involved, the divergence times, the environmental changes, taphonomic changes, identifying fossils and working out how they fit in, looking at the timings of the events and so on. The debate is ongoing, there are many, many voices clamouring to be heard, trying desperately to tie together an overwhelmingly large, yet incomplete, dataset which befuddles even the most astute mind. Teasing out cause and effect in deep time is difficult and frustrating, people come at it from different angles, new evidence and new ideas can cause major shifts in thought. Meyer and his crew are desperate to be the most heard voice, they want their issue addressed and until someone addresses it they will assume that they are being ignored (which is tantamount to admitting defeat, by the looks of it).

Their third issue is that their main focus isn’t actually at the heart of the Cambrian explosion, despite their best wishes. The diversification can be perceived in many ways, with current thought often favouring its interpretation as an ecological explosion. It has often been perceived as an explosion in disparate body plans, which is not exactly the wrong way to look at it, but can be seen as the result of the ecological driving forces. The ID proponents take this a step further; it isn’t simply about body plans – it’s about the new information behind those body plans. With their rapid appearance narrative of the Cambrian explosion, this perspective on the diversification seems like a major issue, a sudden, unprecedented influx of biological information. Understandably, when addressing some of the ecological forces at play we don’t necessarily need to address the genetic changes, but they can’t always accept that. The genetic changes are important, but it does seem to be the case that the genetic toolkit necessary was already largely in place well before the Cambrian explosion (sponges, for example, appear to have some functioning genes which are used in more complex organisms in the development of the nervous system). So some Precambrian organisms may have had the capacity for evolving some of the body plans we see in the Cambrian, but nothing to cause them to do so – having a football pitch, a ball and 22 people does not ensure that a football match will take place.

This pushes the issue back, which ID theorists would like to present as a retreat, as hiding from the problem. The reality is that when you are concentrating on the Cambrian explosion you look at the stage which has been set and then analyse the changes. The environment is part of the stage, the organisms which preceded the radiation are part of it too, and the genetic toolkit is part as well. This is not to say that no new genes were necessary for the Cambrian explosion, but that it is not a major issue. The evolution of regulatory networks and of new genes is a separate question, which I get the impression that they know as it allows them to paint this picture of retreating evolutionary biologists. They can keep pushing back and back because ultimately they know that the origin of information goes back to the origin of life and that is where they truly set up camp, not the Cambrian explosion.

In summation, one tactic of the ID proponents is to try to sound original, when the reality is that the majority of their views on the Cambrian explosion are taken from actual researchers who accept evolution. They also complain repeatedly that they are being ignored, often because their personal favoured views are not being addressed. Finally, their issue is not really with the Cambrian explosion, but with the origins of information at life’s beginnings. The Cambrian explosion isn’t what they think it is, but as long as they continue to present it their way they will always feel ignored and as though experts are conceding to them. They will just persist in offering only criticisms and complaints.

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Ediacaran ecosystem engineers – the Savannah hypothesis and our Skynet-type origins

Image from Wikipedia.

Out on the savannah, it is easy to find certain resources as they tend to be concentrated in limited areas. Trees, termite mounds, changes in terrain, all contribute to this concentration of resources. It is this sort of environment which has been hypothesised as resulting in our own bipedality which enabled human ancestors to move efficiently between resource hotspots. In a recent review, Budd and Jensen have proposed a ‘savannah’ hypothesis as an explanation for the evolution of bilaterian animals and consequently the Cambrian explosion.

Bilaterian animals are widely thought to have driven the Cambrian explosion, particularly as they function as ecosystem engineers altering the environment around them. The burrowing activities of bilateral organisms altered ocean chemistry and the nature of the sediment, opening up more resources to be exploited and resulting in a cascade of diversification. But why did bilaterians evolve in the first place? Ecological causes for the Cambrian explosion tend to presuppose features they should be explaining, such as the ability to burrow or the presence of predation (both likely contributed enormously to the diversification, but were also caused by it). Environmental causes tend to suggest limiting factors such as a lack of oxygen, which may actually be mistaking cause for effect.

The savannah hypothesis suggests that the Ediacaran biota also functioned as ecosystem engineers, causing carbon hotspots in the sediment and water around the organisms which were exploited by bilaterian animals which went on to diversify, eventually displacing their Ediacaran providers. Dissolved organic carbon in Ediacaran seas would not likely have clumped together, instead being spread out through the water column – not an economical resource for active organisms. Burrowing is highly energetic and would require dissolved carbon to be concentrated; without the Ediacaran organisms it would have been too diluted and sequestered away by the abundant microbial mats. Just like trees on the savannah, the Ediacaran biota concentrated dissolved organic carbon, providing sufficient resources for active burrowing and the need for motility.

In their thorough review, Budd and Jensen challenge the view that Ediacaran organisms went extinct by the start of the Cambrian period having been outcompeted and devoured by bilateral organisms. Instead, they survey putative evidence that shows that bilaterians first appeared towards the end of the Ediacaran period and that Ediacaran-type organisms persisted well into the Cambrian (and perhaps longer). At first, bilaterians would have been dependent on the Ediacaran ecosystem engineering, but went on to evolve their own sessile forms, such as crown-group sponges, and predatory habits which made them a threat to the Ediacaran biota – comparable perhaps to humans in the Terminator franchise creating Skynet and setting up their own demise.

They also reviewed the phylogeny of basal animals and take the view that sponges form a single clade which is the sister group to all other animals. They coined the term “Apoikozoa” which encompasses all animals and their sister group the choanoflagellates. And they made a case for Ediacaran organisms being early animals, albeit hugely problematic, whilst being highly critical of some of the optimistic interpretations. It is a paper which has provided a lot to mull over.

References 

Budd, GE. and Jensen, S. 2015. The origin of the animals and a ‘Savannah’ hypothesis for early bilaterian evolution. Biological Reviews. doi: 10.1111/brv.12239

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An optimistic approach to the Ediacaran biota?

There’s a bit of a problem with the Ediacaran fossil record – it’s not what was originally expected and the organisms we do find are problematic. Based on the complex and recognisable fossils of the Cambrian, it was anticipated that more primitive forms would be found in the Precambrian, and, in a sense, they were. When they were first recognised, Precambrian organisms appeared to fit what was predicted; amongst them, palaeontologists recognised possible sponges, jellyfish, assorted worm-like creatures, putative arthropods and echinoderms. The more they were studied, however, the more problems in classifying them arose.

At some point along the line, near enough every Ediacaran fossil which had been linked to modern phyla have been reassessed and their connections found wanting. There is a handful which can still tenuously be linked to modern groups, but there is an apparent dearth of expected animal fossils, especially when the molecular clock data is taken into account. There appears to be an evolutionary gulf between the Ediacaran biota and the Cambrian explosion fauna.

Part of the problem is preservation – comparing the fossils of the Ediacaran and the Cambrian is difficult considering that they are mostly preserved in very different ways; the fossils of the Ediacaran are soft-bodied organisms preserved mostly as moulds, the fossils of the early Cambrian are mostly tiny bits of shell and other hard parts, and then there are the exceptionally preserved organisms from deposits such as Chengjiang.

One approach we can take to link the Ediacaran and the Cambrian is to avoid trying to fit them into recognisable taxonomic groups, and instead focus on the attributes they share with modern animals, particularly their ecology. This was the process adopted by Mary Droser and Jim Gehling in a paper earlier this year, titled The advent of animals: The view from the Ediacaran. We can look at the Ediacaran period and see things which are usually associated with animals, even if we cannot properly classify the fossils in question.

Mobility

One thing which clearly sets animals apart is movement – worms wriggle through sediment, fish swim about, and, of course, us humans find as many different ways to move as possible. Many animals don’t move about for most of their lives, not least sponges and corals, both of which we might expect in the Ediacaran in some form, but movement on or in the sediment would potentially be evidence for bilateral animals milling around. Most of what we see from the Ediacaran are stationary organisms, attached to the sediment by a holdfast or resting on the surface. The earliest animal traces are from 565 Ma and are most similar to traces by the polyps of anemones, providing evidence of muscular contraction, evidence of which also comes in the form of the body fossil Haootia quadriformis which possessed bundles of muscle fibres and is a possible cnidarian. The most common trace fossils in later Ediacaran rocks are in the form of grooves and levees, called Helminthoidichnites, and are interpreted as being caused by an animal too small to be preserved and limited in size by the chemical conditions of the sediment. They appear to have been mining the microbial mats, also showing evidence of avoidance behaviour, and are likely to have been created by bilaterian animals.

A few Ediacaran body fossils are associated with traces as well, lending to their interpretation as bilaterian in nature. Kimberella is a box-shaped body fossil which is often associated with scratch marks (Kimberichnus) that has been commonly seen as bilateral and has even been considered to be a possible mollusc. The associated traces have been interpreted as evidence of mat grazing though there are differences between the grazing habits of Kimberella and those of molluscs. The likely related Dickinsonia and Yorgia have been found associated with faint casts of their bodies, which appear to be resting or feeding traces where they sat ingesting the microbial mat before moving on to another patch. They often also have possible muscular contraction marks, though this interpretation depends somewhat on their phylogenetic affinity. The advent of mobility is therefore not confined to the Cambrian period though it does see an increase in the number of modes of mobility, as it is a behavioural trait of bilateral animals found in the Ediacaran.

Reproduction

Sexual reproduction is another trait associated with modern animals found in the Ediacaran period. The puzzling organism Funisia is a collection of tube-like structures which were previously not even recognised as body fossils. They demonstrate branching patterns which are potential evidence of asexual budding, whilst their distribution appears to be due to the production of spats, a form of reproduction mostly found in sexual organisms. Though their phylogenetic affinity is puzzling, the likely sexual reproduction of Funisia highlights another metazoan trait found in the Ediacaran period.

Skeletonisation 

The Cambrian explosion was first recognised in the fossil record due to the geologically sudden appearance of skeletal parts. The evolution of hard parts appears to have been a key stage in the evolution of Metazoa though it is not restricted to the Cambrian. Droser and Gehling discussed the example of Coronacollina, a cone-shaped organism with long, straight spicules radiating outwards, interpreted as a sponge-grade organism which is important for being the oldest known multi-element organism. Other Ediacaran shelled organisms include Cloudina and Namapoikia which are possibly cnidarian-grade organisms but had their study been released more recently they would likely have included the latest interpretation of Namacalathus as a lophophore. Even if we cannot place them phylogenetically, the appearance of skeletal parts, particularly multi-element organisms, is a key step in metazoan evolution found in the Ediacaran.

Ecosystems 

Ediacaran fossils tend to have been preserved in the places they lived, as opposed to having been transported and dumped elsewhere. This allows them to be studied as communities and permits insight into their ecological nature. The Flinders Ranges of Australia contain a succession of beds which are characterised by a range of organisms in shallow marine settings. The same organisms tend to appear on each bed but with different abundances, suggesting a level of sophistication in communities similar to that in the Phanerozoic despite there being a lack of predation, organisms living in the sediment, and widespread skeletonisation.

Conclusions

Setting aside phylogenetic affinities, traits of modern animals are found in the Ediacaran period. An optimistic approach to the Ediacarans allows us to see signs of mobility and the presence of muscles, skeletonisation, sexual reproduction, and the beginning of complex ecosystems – all possible links to the animals found in the Cambrian, suggesting that poriferans, cnidarians and bilaterians were all found in the late Precambrian.

References

Droser, M.L. and Gehling, J.G. 2015. The advent of animals: The view from the Ediacaran. Proceedings of the National Academy of Sciences 112: 16. [Link]

 

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The Cambrian Explosion – The old fossils vs animals debate

In a world without fossils, we might never have known about the event commonly known as the Cambrian explosion. This perplexing episode in evolutionary history has been known since early geologists and palaeontologists first started piecing together the history of life on our planet; what they found was that the oldest rocks contained no fossils, overlain by rocks containing complex fossils. A lack of precursors for complex fossils, such as trilobites, did not fit the evolutionary narrative gaining traction at the time. Charles Darwin’s explanation for this puzzle was that the fossil record was incomplete; these ancestral forms had existed, but had either not been preserved or not been found. Since then, more fossils have been found, including a large variety in Precambrian rocks, and the timescale has been constantly refined, allowing for a better understanding of the timing of key events. The questions raised by Darwin’s ideas have not quite gone away: is the Cambrian fossil record a true representation of the emergence of major animal groups? or is it an illusion caused by the paucity of the fossil record?

Many palaeontologists would like to read the fossil record literally and support the idea that the Cambrian explosion was a real event, however, there is a story in the genes which is somewhat different. Geneticists can estimate the divergence times between lineages using molecular clocks, a technique which uses mutation rates to count back to the time when two species or higher groups split. When applied to the origins of major animal groups, these estimates have varied widely and have been constantly refined. One consistent theme is that these dates are always substantially before the Cambrian, including the origins of phylum-level groups. It is always to be expected that a group or species would have evolved earlier than they appear in the fossil record, but not to the degree shown by molecular clocks for the Cambrian. If the story of the genes is read literally, then there is a lot of cryptic evolution in the fossil record, making the Cambrian explosion either an illusion caused by an absence of fossils, or by an increase in preservation potential due to the origin of hard parts and/or increased size across multiple groups. The conclusions of a recent molecular clock study appear to support the idea that the Cambrian explosion is a preservation event, not the rapid appearance of major animal groups.

In a paper titled Uncertainty in the Timing of Origin of Animals and the Limits of Precision in Molecular Timescales, dos Reis et al. present some robust new data and provide an excellent overview of the limits of molecular clock studies. Their estimates vary widely depending on certain assumptions, which are addressed in detail, but all of them do support an early divergence of major animal clades. They conclude:

This is the long-standing conundrum of the Cambrian—whether the first animal fossils faithfully reflect an explosion in animal biodiversity or merely an explosion of fossils. The results of our study—which integrates fossil and molecular evidence to establish an evolutionary timescale—suggest that the Cambrian explosion is a phenomenon of fossilization, while biological diversity was established in the Neoproterozoic. Integrating all of the sources of uncertainty that we explore (Figure 6, Table 1) allows us to conclude that crown Metazoa originated 833–650 Ma, fully within the Cryogenian, while the component clades of crown Eumetazoa (746–626 Ma), crown Bilateria (688–596) Ma, crown Deuterostomia (662–587 Ma), and crown Protostomia (653–578 Ma) all diverged within a Cryogenian to early- or mid-Ediacaran interval.

The results of our analyses leads us to reject the hypothesis that metazoans, eumetazoans, bilaterians, protostomes, deuterostomes, ecdysozoans, lophotrochozoans, or, for that matter, any of the component phylum-level total groups, originated in the Cambrian.

Figure 6 from dos Reis et. al (2015)

The conclusion that the fossil record is not accurate for the Cambrian may seem dismissive, just as many palaeontologists dismiss molecular clocks as inaccurate when they ostensibly contradict fossil data. This incongruence between the two datasets might itself be an illusion. Might there be reasons to think that both are accurate, but that we are mistakenly expecting them to answer the same questions?

It is relatively easy to picture a number of scenarios which could create the illusion of a surprising increase in complexity when really we just do not have the fossils. The Cambrian fossil record clearly does show that there was a dramatic increase in fossilisable hard parts, as we see a handful of shells at the end of the Ediacaran, through the increasing diversity and complexity in the Small Shelly Fossils (SSFs); soft tissues do not preserve well, so it might be a case that hard parts evolved in animals which had been milling around for millions of years prior. Or could it be the case that they simply were not big enough to be preserved? An impression of a soft-bodied animal in sand cannot be smaller than the individual sand grains. The meiofauna live in those sorts of surroundings, in between the grains of loose sediment, where oxygen levels are often low; animals too small to see without a microscope, complex yet only found fossilised in exceptional circumstances. And maybe it is the case that we have fossils of them but are looking with the wrong expectations? In these scenarios, the genes are right and it is the fossils which do not give enough of the true picture.

The Ediacaran period is famous for its menageries of soft-bodied organisms, preserved as impressions in sand, or by volcanic ash smothering their soft forms; any organism large enough would surely have been preserved. Within this period of time, we also find the Doushantuo fossils, visible only through the use of high-powered microscopes. Preserved in phosphate, they are largely an assortment of little ball-shaped clusters, thought by some to be embryos, yet by others to be single-celled organisms in the process of division. During the Cambrian and later, phosphate deposits such as these contain the elusive meiofauna, yet they are absent from the Ediacaran. Instead, we see a finer degree of fossilisation. Preservation in the Precambrian is arguably better than the Cambrian and later – preservation has evolved, but not towards higher fidelity.

Geneticists can also shed light on this question of preservation potential. Animals have a number of microRNAs, which are regulatory RNA genes that can be used to track the evolution of complexity, put simply because more complex animals have more microRNAs than simpler organisms. When some of the simpler animals are analysed, such as acoel flatworms and rotifers, it is found that they have lost microRNAs and become less complex, so they cannot be used to work out what a simple bilaterian ancestor might have looked like. Instead, they seem to indicate that the common ancestor of bilaterians was relatively large and complex, and at the very least a persistent puzzle.

It is possible that we already have fossils of bilaterians from the Precambrian, but that we are not identifying them correctly. The Ediacaran form Kimberella is often considered to be a possible mollusc, whilst its contemporaries Spriggina and Parvancorina are sometimes linked to arthropods, yet all of these are questionable. Recently, the shelly Ediacaran organism Namacalathus has been identified as a lophophorate, placing it with animals like brachiopods and bryozoans, but this is undeniably controversial.

Bilaterians don’t often sit still. They wriggle around on the sea floor, they churn up the sediment in search of food and shelter, they swim around looking for food. Burrows and trails are abundant in the fossil record, often found alongside body fossils, but also in sediments where body fossils could not be preserved. They give us insight into the nature of the sediment, the life habits of animals, and the evolution of complexity. Trace fossils tell a similar story to the body fossils. The sediments in the Precambrian were quite hard and largely devoid of oxygen – not conducive to the existence of meiofauna –  and lack any clear evidence of bilaterian activity, then animals began to wriggle about in the sediment in the Cambrian, allowing oxygen to reach greater depths. The first stage of the Cambrian shows a large increase in the ways animals interacted with the sediment, but it was not until the second stage of the Cambrian when they started to burrow up to a metre deep and ecosystems took on a new level of complexity. The trace fossil record suggests that the Cambrian explosion was a very real event, but that body-plan diversification happened before changes in ecological structure.

The data from trace fossils hints at how the record of body fossils and the information in the genes could both be telling the same story. It is possible that molecular clocks are right about major animal groups diversifying much earlier than the Cambrian, but the conclusion that the Cambrian explosion is an illusion caused by a poor fossil record is misguided. The fossil record does indeed appear to record an actual evolutionary event, but it might not be the origins of these major animal groups – it may instead be their ecological expansion, not least the widespread evolution of hard parts. Under this scenario, we are still lacking in key fossils, but that lack is not the explanation for the incredible diversification through the start of the Cambrian period. The Cambrian explosion would therefore not be a case of “animals or fossils” but an ecological restructuring which could not have been known without the fossil record.

References

dos Reis, M., Thawornwattana, Y., Angelis, K., Telford, M.J., Donoghue, P.C.J, and Yang, Z. 2015. Uncertainty in the timing of origin of animals and the limits of precision in molecular timescales. Current Biology. [Link]

Erwin, D.H., Laflamme, M., Tweedt, S.M., Sperling, E.A., Pisani, D., Peterson, K. 2011. The Cambrian conundrum: Early divergence and later ecological success in the early history of animals. Science 334: 1091-97. [Link]

Mangano M.G., Buatois L.A. 2014. Decoupling of body-plan diversification and ecological structuring during the Ediacaran – Cambrian transition: evolutionary and geobiological feedbacks. Proc. R. Soc. B 281: 2014003. [Link]

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The Cambrian Explosion – An Enigma of Deep Time

Over half a billion years ago the biological world was a more peaceful place, or so the story goes. Simple multicellular organisms abounded, doing little more than filtering out food particles from the ocean waters. Then, in a geological blink of an eye, animals began to diversify, irreversibly changing the prevailing ecosystems, paving the way for modern life. They began eating each other, causing some to burrow into the sediment deeper and deeper, disturbing the slimy microbial matgrounds which had been key in the fading Ediacaran environs. Some developed armour, sporting hard parts which protected their soft flesh, leaving behind more distinct traces in the fossil record. Yet others became more mobile, learning to swim around to evade predators, or to seek out their own prey.

This event is often referred to as the Cambrian Explosion and has earned the nickname “biology’s Big Bang”. It is one of the most important events in the history of life on Earth and, due to its singular nature, is one of the most misunderstood. Misinformation proliferates on the internet, not least due to anti-evolution websites considering it to be a weapon in their arsenal, but also because it is a very mysterious time, where datasets often support multiple interpretations and cause and effect are difficult to untangle.

Image credit: Brest van Kempen

When Charles Darwin first pondered the Cambrian mystery, he was facing a fossil record where no Precambrian fossils were known – the lowest rocks contained the surprisingly complicated trilobites, not at all what he had hoped to see. Darwin invoked the incompleteness and imperfection of the fossil record as his explanation, in the hope that someday new fossils would elucidate this remarkable absence. It wasn’t until 1958 when Precambrian fossils were first recognised, but even they did little to diminish the seeming leap in complexity in the Cambrian. The Burgess Shale fossils, found by Charles Walcott in 1909, revealed a menagerie of bizarre creatures, many of which clearly had modern body plans, but many others seemed to have unique body plans, as though this Cambrian diversification was even bigger than expected.

The Burgess Shale was properly studied from the 1970s onwards and by the 1990s the diversification had been established as one of the key mysteries of palaeontology. By this point, a picture of the Cambrian Explosion had emerged which provided quite the conundrum. Those Precambrian fossils had originally been interpreted as belonging to modern animal groups but had since been questioned and even proposed as belonging to their own kingdom. Then, in perhaps as little as four million years, every modern phylum arrived on the scene, along with potentially three times as many more. It was an unprecedented diversification, expounded beautifully by Stephen Jay Gould in his book Wonderful Life. This is a picture which crops up repeatedly, despite twenty years of advances which show it to be wrong. In future posts, I will explain what has changed in our understanding of the Cambrian Explosion, which questions have been answered, and the mysteries that lie ahead.

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What’s That Swimming Towards Me? – Reblog

I recently took the decision to share some of my older blog posts in order to pad out this iteration of the blog a bit and get me back into the swing of things. It beats ranting about Jurassic World and provides some hopefully readable content. Here is a piece I wrote in August 2010 as a bit of pop. science writing. 

With the vast size of the Earth’s oceans, it is not unlikely that many of us will swim in them from time to time. Whether you are going for an innocent paddle, catching waves on your surfboard, or sailing the seas on a fishing trawler or pirate ship, you will be sharing the waters with myriad different animals, some of which are dangerous. If you were to find yourself swimming under water with a big, moving animal coming ever closer, how do you know whether you have a friendly dolphin swimming towards you or whether it might be a hungry shark? Well, the easiest way can be done at a distance and is a simple observation with an interesting explanation – evolution.

So, with that large, looming shadow swimming towards you, what do you look out for? If you can see its tail, simply look at the orientation of the tail fluke, is it horizontal or vertical? Failing that, look at how it moves, does it undulate its body up and down, or does it move its body from side to side? Dolphins and whales have a horizontal tail fluke, which means they have to undulate their body up and down in order to propel themselves through water, so if you see either the horizontal tail fluke or the undulating movement, you have a dolphin or whale coming towards you.

Sharks, on the other hand, have a vertical tail fluke and so must flex their body from side to side for propulsion. If you see the vertical tail fluke or side to side motion, then what is coming towards you is a fish and so might be a shark.

As our fishy ancestors used a side by side motion to propel themselves through water, so did our earliest terrestrial ancestors and so do reptiles today. Snakes are an extreme example of this sort of movement, but the side to side motion is still there. During the Mesozoic era things began to change, as our ancestors (and convergently in dinosaurs too) developed a more upright posture, instead of the sprawling gait of reptiles. With an erect posture the more effective way to rapidly move is to flex the spine up and down whilst running, rather than side by side.

Many vertebrates have some of their vertebrae fused to facilitate particular movements, so future evolution can often be restricted to working within the confines of that movement. As dolphins evolved from terrestrial mammals, their semi-aquatic ancestors also used this up and down movement and so adapted this to movement in the water. Side to side motion, like that of a shark, would require a larger number of changes when there was the simpler solution of up and down movement (though note that evolution does not have the foresight, it simply uses what is available – quick fix solutions often work in evolution). The motion of whales and dolphins is testament to their ancestry, having descended from active land mammals.

During the Mesozoic, another group secondarily took to the waters and adopted the torpedo shape of dolphins and sharks. These were the ichthyosaurs, descending directly from terrestrial reptiles. As their ancestors used the side to side motion, so did the ichthyosaurs when they swam, also possessing a vertical tail fluke. They had some unusual traits for reptiles, giving live birth and being warm blooded, but their swimming motion gives away their reptilian status. This also quite ably demonstrates that they are not dinosaurs as many laymen mistakenly think, for dinosaurs did not have a sprawling posture which uses the side to side motion. So, if you are somehow in Mesozoic waters with a shape swimming towards you, it may be too late before you can discern whether or not it is an ichthyosaur or a shark.
Ichthyosaurus anningae

Artist: James McKay

 

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