What triggered the Cambrian Explosion? with Professor Rachel Wood

rachel slights there we go and without further ado rachel wood take it away thank you very much in indeed thank you jack our wonderful master of ceremonies so yes we are addressing the slight elephant in the room this evening uh what caused the cambrian explosion uh we've had a wonderful range of talks uh over the last uh weeks months uh probing all these various aspects of the cambrian explosion for example how are these fossils preserved how did they start to move uh the origin of skeletons biomineralization when did that appear in what groups and what did these animals actually look like and what they have been these are all very very profound problems and the the issue really with what caused the caribbean explosion is it is a very complex multifaceted issue that requires the input of many many different uh types of geoscience so this is what i would like to address tonight what triggered cambrian explosion now this has been something that has been very inspirational in science but also when i was putting this talk together i realized there's quite a musical theme here so i had no idea that the atomic meta pagans have actually put out an album uh called the cameron explosion and you can see what type of music it is you've got hallucigenia there playing a banjo it's a lovely sort of bluegrass uh um virginia music it's on youtube if you want to hear it so uh the camping explosion has been tremendously inspirational actually to art to a bit of music and also of course to scientists for many decades and as i said this is really a multi-dimensional multi-disciplinary problem so to tackle it we really have to look beyond paleontology alone to other uh forms other disciplines within geoscience particularly geochemistry but also sedimentology and stratigraphy to try and constrain the timing when everything happened so you can see here that the many of my my co-authors are my co-workers are listed and they are um from geochemistry isotope geochemistry aqueous geochemistry but also sedimentologists entomologists and many current and former phd students um i'm a firm believer in international collaboration as a one of our most profound aspects of being a scientist so there are many colleagues here who have had wonderful collaborations with for many decades from for example namibia germany and russia so what triggered the cambrian explosion just to recap uh where we are so from approximately three and a half billion years ago to 575 million years ago if you were snorkeling over the sea floor at this time you would have seen a lot the seafloor was dominated by microbial life so in other words things that are akin to modern stromatolites which you can see in the top right hand corner these are the famous stromatolites from shark bay and we see in the record many of these tomato lights the other images there are of uh precambrian general precambrian stromatolites and then at approximately three 575 million years ago bursting onto the scene uh many different areas we see the first signs of what we call complex life for example a and b there show clearly fossils at trace fossils things that have moved over the substrate moved over the sea floor so in other words we see evidence of mobility we see curious uh stemmed tubular forms there and sea and also a little bit later towards this 30 million year period we see forms that had bilateral symmetry and we've heard a lot about these possible bilaterians so here in here and here these forms with bilateral symmetry so uh and of course also this time we see the very very first forms with a skeleton so those little tubular fossils shown in h are the first forms actually gained a calcareous skeleton the problem is many of these forms we really have very little idea what they are in terms of affinity some may be animals and some maybe not so where we get more and more secure about these potentially being animals get as we get closer to the cambrian or the easier cambrian boundary and then of course from approximately 540 million years ago we see the the cambrian explosion itself where many of the fossils appear are are much more familiar and we can easily slot them into modern groups so such as the trilobites there but other other arthropods you've heard about those in wonderful detail from derrick briggs and various other groups um and these are almost certainly all animals so that sort of sets the stage in terms of the timing and if we put this timing now into a geological column you've been familiar with some of these uh time zones introduced in previous talks uh you can see we've got the pre-cambrian cambrian boundary there at about 540 million years ago here the time period before is the ediacaran and before that we have this period called the cryogenian it's when all the incredible snowball earths happened or when the two dominant snowboarders occurred and if we place our knowledge our fossil record now um within this time frame we can see first of all here's the evidence for the really well accepted fossil record of animals and the well accepted record really only goes back to very close to the ediacaran cambrian boundary but if we go a little bit further back in time we have these uh branching events of when we believe that showing the inter-relatedness of some of these groups and the these are not that controversial some of them are very secure in other words we know how these groups are related to each other as shown by these these branches of the family tree but what is incredibly uh unclear and poorly constrained is the actual timing so you can see there the timing with some of those pale blue lines i'll just highlight a few for example here between the mollusks and the annelids i think i've actually put it in the wrong place there sorry here yes here the time is actually very very uncertain it could be anywhere between those bits of time anywhere between the middle and the late edea current and you can see that all those branching events really are quite unclear in other words there's a huge uncertainty on when those events those branching events actually happened now if we go back even further to the very vexing problem of the origin of the animals the metazoa the uncertainty really is uh the uncertainty of timing really becomes huge you've heard a lot about these uh possible biomarkers shown by the star and the arrow indicating metazoa but there's a lot of controversy as to whether these really are the biomarkers of animals or not and then so if we go back to the the the branching event that we're particularly interested in the branching event of the metazoa themselves we can see there as i've highlighted in red although we have a a branching point between the um the the with the metazoa you can see that there's an enormous amount of certainty when the metazoa actually appeared did they appear at the beginning of the tionian any time during the toyonian or indeed any time during the cryogenia so this is the problem that we're grappling with there is really it's very um we have very little idea about when animals really did appear in the geological record or appear uh in geological time because the record may be mute so this is the problem really what one idea is that animals are actually very old they did appear in the titonian perhaps or even the cryogenian but they were simply very very small they didn't have a fossilizable hard skeleton and they were so tiny um that they um wouldn't leave any trace fossils either so i've just plonked to sort of a possible image of one of these very early possible ancestral bilaterian animals at messes earth so this this could explain this apparent gap between when we when the molecular phylogen is the dna tells us that animals must have appeared but when they actually come to dominance in the fossil record but there's another idea which is has gained a huge amount of traction in recent years it is still slightly controversial and it's also very complex but that is that the rise of animals to ecological dominance which is really what we see in the cambrian explosion is due to an external constraint and one of those external constraints could be levels of oxygen and this is because we know that oxygen is a prerequisite to the functioning of animals and this is an idea that was proposed really quite a long time ago by nursal in 1959 so the idea of the connection between sufficient oxygen being present in the atmospheres and therefore present in the seas and the rise of animals and this this is there's no doubt that all complex life with animals and many and plants require oxygen they require a certain amount of oxygen to undertake all their metabolic capabilities so there's no doubt there must be a connection in some shape or form between animals and sufficient oxygen being available to them now one issue is that part of this idea is that when you start to respire an energy that cells can produce via respiration uh once they've firing oxygen this increases by about 20 times so once you you go from anaerobic to aerobic respiration you can really create a huge amount of extra energy and the hypothesis suggests that this extra energy is is what is needed and that powers the generation extra complexity of life so for example it allows things to get very very large allows the evolution of more complex uh body structures and organs so for example a nervous system and muscles and also it allows the formation of skeletons we know that only you need a certain amount of oxygen dissolved in seawater that allows marine animals to start to produce a skeleton skeletons have all sorts of functions one of their primary functions is to defend and of course once your predator that what's going to eat you has developed teeth you need something that will withstand and defend you against those teeth so by hard parts i mean both defensive and predatory structures such as teeth and of course it also allows you to explore high energy lifestyles energy intensive movement in other words so for example a predatory lifestyle itself so this rather garish graphic is really just showing this idea that maybe life animal life went through a bottleneck of that allowed once oxygen levels have reached a certain threshold it allowed the manifestation of all this complexity in our geological deep past so let's consider what the earth was like um around the pre-cambrian uh around the uh during the cambrian or just before the cambrian explosion this is going back about 10 million years before the easier car and cambrian boundary so as far as we know our earth was very very different from today you can see this is where all the continents were our best guesses the continents were quite unlike uh the the continents would we recognize so for example siberia was a separate continent north china was a separate continent and south china was separate continents all sitting in very different places in the globe because of course they've had 550 million years of time to whizz around and form the configuration that we see today you can see that most the continents were micro continents they were very small and they straddled the equator and we know that the tropics are places where we often have the origin of new life new new species so already we have a feeling that because these were microcontinents they had a lot of shiny marine seas around them so the availability of lovely warm shallow marine seas to uh of innovate life was already very primed just by this paleogeographical configuration of our continents but we also know that uh the present is not the key to the past we're looking back in deep time here the world really was remarkably different for example there were no land plants no vascular plants living on land no trees also our best indications of the climate was that it was hotter than today and of course we're dealing with continents here that are all tropical and subtropical and also there's every indication that atmospheric oxygen levels and therefore oxygen levels in the seas were also lower how much lower is difficult to tell now where does where do we get that oxygen from today well most of the oxygen is actually formed by photosynthesis so about two-thirds of it are formed by tiny photosynthetic synthesizers uh in the seas and the other third or so are formed by land plants so and of course we have no land plants uh in the cambrian so some sources of oxygen simply or an abundant source of oxygen an important source of oxygen today was simply not present in the ediacaran and pre-cambrian times and cambrian of course now if we we think about where oxygen was sitting sits today in the modern oceans it's not equally distributed throughout our oceans here of course is uh the configuration of continents as we see it today and you can see here this is shown as dissolved oxygen in the oceans with very warm colours showing high amounts of oxygen going down to the very cold colours showing low amounts of oxygen and it's immediately apparent that you've got a lot of oxygen dissolved in the polar regions and you have these incredible blue cold uh oxygen depleted waters that form essentially um upwelling against the western sides of our continents and also we have a lot of um uh coal oxygen depleted water forming where you have these very very major rivers like the ganges and the brahmaputra this is because these uh upwelling of cold uh nutrient rich um waters from the arctic and also the rivers bringing down nutrient-rich waters they create a huge amount of production in the ocean of life and that life is did then depletes the ocean of water of oxygen so we have what's known as these oxygen minimum zones uh formed by continental runoff from rivers from major river systems and also from upwelling of cold nutrient-rich waters from the poles against our western sides of our continent now this has a major effect on structuring the ecosystems that live in these areas so for example in some of these areas not not all we where the auction levels grow get very very depleted in these areas you get a huge reduction of the diversity of life and there's very very little there that actually can it reaches any significant size the food webs are very simple and the food chains are very very short and also there's nothing there that produces a skeleton and this is in great contrast to the areas where we have a lot of oxygen i've just put an arrow here on the near the great barrier reef for convenience we have very complex food webs the water column is teeming with diverse life and we have the ability of animals to produce extensive areas of skeletal production by mineralization so in other words the amount of oxygen in the ocean really does have a huge impact on the diversity of life and also the type of life that you you find in today's oceans so how do we put this now into the context of what we're incident the cambrian explosion and the origin of the of the cambrian explosion so this is this idea that we have the history of life here shown in this graphic and it's shown here very much as a a linear narrative but we know that it's far more complex than this there are all sorts of feedbacks between life and the environment we have this this uh relentless change and increasing complexity is a very simple way of thinking about evolution and it's interrupted for example by mass extinctions so in other words we have this constant dialogue between the changing environment and the opportunity that gives for evolutionary innovation and the feedbacks between them so how can we think about oxygen as an environmental opportunity and how does changes in oxygen levels perhaps lead to evolutionary innovations so i want to just suggest two hypotheses of this talk and this is the first one which is that oxygen controlled the rise of animals so this is a very simple statement so how might we test this well of course part of the issue is how much oxygen did ancient animals actually need this is a very difficult problematic uh question to ask because if we look back um the time we have all these various possible animals but many of them are definitely not animals so for example we have here this is definitely not an animal this is a bit of a complex algae these forms here may almost almost certainly not be animals the jewelry rizzet really is out on whether this is an animal or not and as you get closer to the idiocar and cambrian boundary these forms are increasingly more certain of probably being animals and that's sort of all we can say at the moment but if we look at modern animals and their demands for uh oxygen the uh recently it was discovered that modern sponges which of course are right at the base of the food tree of animals modern sponges there can actually exist in very very low levels of oxygen so between 0.5 and 4 percent of present atmospheric levels are really very very low levels compared to humans for example however not all animals have the same oxygen requirements while sponges uh um have very or some of them at least have very low ocean requirements more complex life forms such as this have much higher requirements and these are worms polychaetes and in fact as polychaetes develop more and more predatory carnivorous lifestyles they demand more and more oxygen so we have here the first indication perhaps that all animals are not created equal that depending on your lifestyle you may actually need more oxygen with increasingly intense demanding lifestyles and particularly predatory lifestyles that demand very rapid movement to capture prey need high amounts of oxygen so if we go we think back about the oxygen in the oceans today uh we can start to put some terminology on it so everything with uh above two microliters and higher we term oxic and those uh colors shown in those cold colors in blue we can term dyssoxic and then very very low oxygen no oxygen at all vanishingly small amounts we call anoxic so these are terms i want to introduce you to just to show you how we try and uh interrogate this problem in deep time simply because we simply can't go back to the ancient record of animals and work out how much oxygen they needed so if we think about oxygen in the modern ocean so these are these terms i've just introduced to you oxic dysoxic and anoxic and you can see here below i've just put on uh the amount of oxygen this refers to so oxic is more than above two microliters between 0.1 and 2 we call dysoxide and then very very little here is anoxic now we have to use uh because we can't go back and actually measure the amount of oxygen in the oceans we have to use proxies so just before i mention the proxies just to show you that therefore in toxic conditions this has this huge effect on life so in uh opposite conditions we have a biodiverse life complex food webs and animals with skeletons in suboxic or dysoxy we see very small thin walled animals with no skeletons and then we get no life living on the sea floor at all in anoxic conditions so how can we probe this in deep time or we have to use what's called proxies so proxies are simply uh chemic things that we can measure in the rocks chemical signatures that we know change with oxygen so chemic in other words chemicals that react to different redox states so chemist chemicals that show either a different state in anoxic dysoxide or oxide waters and so we can take some particular elements and all the minerals that they form and use these as proxies to probe ancient oxygen levels so for example if we go to anoxic waters you can see here in this in this figure that we've got the sediment water interface and in oxic here's the sediment water interface here and you can see in oxygen conditions all the water column is oxygen and then a little bit of a seafloor is oxygenated osteonated as well but as we go into the dystic conditions you can see the seafloor is no longer oxygenated it becomes actually anoxic and as we move into these two anoxic states you can see that the water column away from where most animals are living on the sea floor has become anoxic so there are two different states of anoxia that we can look at one is where you have a lot of dominant iron and it's called pherogenous so we can look at the behavior of iron and say whether that water was anoxic or oxic also we can look at sea water that has a lot of free sulfur and this is called sulfidic or eugenic so we can look at this behavior as a proxy and then finally dystoxic waters very low oxygen waters often have some enrichment of particular species of manganese so they are manganese rich so we can use these proxies then to measure in rocks and build up a story of really how oxygenation has changed through geological time it's very labor-intensive work you have to take the rocks grind them into a powder and then put them through uh extraction by acids and i'm just going to show you to give you some nuts and bolts here of one method that we use and this is called iron speciation effie speciation and although it sounds complicated it's actually very simple all it is is it takes you measure the the amount of total iron in your rock and you just separate out these types of ions are iron carbonate iron oxide pyrite and so forth and you just create a ratio of the total amount of these to the total amount of iron so here's the ratio up here so a ratio of these reactive iron species if the ratio is below 0.22 we know that that rock was was deposited with free oxygen in oxide waters if it was deposited above 0.38 we know that it was spotted anoxic waters we can also tell by the ratio of pyrite to these reactive species whether it was produced in the presence of lots of iron or sulfur in other words whether it's feruginous or eugenic so in other words we have here a powerful technique to go into the rock record and work out what was the uh what was the um our sediment deposited in the presence of oxygen and if or in anoxia or whether it's in through free sulfur or in ferrogenous iron-rich conditions so we've been working for a very long time to try and put together a record and this is now the record the start of the record from a huge number of activity all over the world many many different research groups are working on this and you can see that in total we've we've reached a picture uh this is the sort of beginning of a picture summarized picture of really where oxygen was sitting on the planet for this key bit of bit of time so you can see here here's the eda car and cambrian boundary and you can see that right through the snowball earth and the cryogenian you can see the snowballers there these tropical glaciations the earth was really that the sea water was really anoxic it's dominated by these green ferrogenous waters and at mid depths you see this red coloration coming and going so this was almost certainly eugenic with free sulfur and then at some point after this final snowball earth here the gaskers in the middle of the in the ediacaran we think then this is when oxidation started you can see these blue arrows coming down and starting to oxygenate the ocean so uh this is a simplified view of how we think oxygen developed in our seas through this bit of geological time and review until just a few years ago now i want to take you now to the record that we have in just one area namibia the last 10 million years of the idi occurring so this this bit of time and this is very much the time the precursor to the base the cambrian and the cambrian explosion so here is um an outcrop uh just showing you from namibia it's a wonderful place to do field work um it the the outcrops are are extremely extensive there's no vegetation to cover them up and you can trace beds for hundreds of kilometers and the other reason we go there it's just this critical bit of time uh 10 million years just before the pre-cambrian cambrian boundary and also we have lots of trace fossils lots of soft bodied fossils and also lots of fossils with skeletons so we can we can really understand the relationship between oxygenation with in concert with the animals themselves so again looking at this map of the paleogeography this time you can see here shown in the red star this is where namibia was sitting at this time it was sitting just south of the equator probably 25 degrees or so to 30 degrees south of the equator so but remember uh we the the where the continents are has a huge impact on where oxygen is actually sitting in the earth so we can imagine that the continents had a very because they had a very different configuration at this time almost certainly where oxygen the distribution of oxygen the seas was not homogeneous through this area it could well have been very very different in different parts and these different micro continents so what we've been doing for the last 10 years or so is trying to build up a picture of what's happening to oxygen through time through these 30 million years or so of the record here in namibia but also in space and we've chosen namibia because the rocks are not only very well preserved but because we have fortuitously got these two basins or two c's and we can compare the behavior in these two c's through time so one is a base the north and the basin in the south so we can first of all ask the question do these basins have the same history or are they different and just to show you here here's a cross-section through the two basins so going from north to south here and these are just the names of all the places we've been uh gathering data and we've been gathering data to to gain uh to to look at the behavior so for example's vault murder here is in a very very shallow seas but we go right out to brac which is in very deep seas so in other words we can create a transect from very shallow waters out and deep waters and we can look at where exactly toxic waters were sitting and where anoxic waters were sitting so in other words we can we can in effect create a 4d framework so in space and in time and just to show you these dates here in red these are the dates we've got from ash beds these are what's called radiometric dates and just to show you that the the ages are pretty well constrained so we can actually put our oxygen framework our redox story into a very nice time framework so we've spent many many hours collecting samples it's very laborious taking samples maybe every meter and what we're looking for is evidence of anoxia so this is the second musical hint uh this is actually a heavy metal i just not it's a death metal band from nor norway so i decided i probably wouldn't play you a clip of this music so anyway we're looking for anoxia in these sediments and just to show you in a bit of detail this sort of data that we're getting this is data collected by a former phd student fred boyer and you can see an enormous amount of data goes into trying to understand these stories and here all you have to notice here is it's color-coded black is anoxic blue is oxic and then we've got this purple which is the manganese low oxygen waters and what's useful here is to then look at this cartoon showing the behavior of where anoxic waters were sitting in these two basins and what you can immediately see is the beginning of the record here there's a huge amount of anoxic water sitting in these basins only a very very thin veneer of oxygen oxygenated waters and you can see that our early possible animals our complex life is just sitting there clinging to these oxygenated waters here now as we pass up through you can see that the uh anoxic waters have retreated at this point then they're coming up again then they're coming up even further and then they retreat and then by this stage very close to pre-cambrian cambrian boundary we have fully oxygenated conditions so in other words we have a history of very very dynamic redox but the obvious thing is is when we put on on this record where all the actual oldest animals were sitting they are all clinging to these very very oxygenated waters in other words life was only living in these well oxygenated waters not even in low oxygen waters so we can start to say that this these these oldest animals at least in namibia needed well oscillated conditions now if we look at the story through time what you can see is this is these are just a bird's eye view of these two c's connected and if you were flying over it where these bodies of anoxic and oxic water were and you can see that in the oldest bit of time here there's a these huge amounts of the seas were covered a had a lot of anoxic water but as we progressed through time you can see them disappearing and the critical time around 545 million years ago they start to disappear all together from this northern basin the northern basement lovely and oxygenated a little bit still of anoxic water in the southern basin but then by the time we get to here um in the very latest bit of the record we have fully oxygenated conditions so we see a progressive change of oxygenation through time so how does this really affect the animals we've already shown that the animals are just clinging onto these oxygen very local areas of oxygenated waters but if we actually plot what's happening through time and where they're actually living we can see first of all let's look at the the burrow record the trace fossil record here at the beginning there's very few burrowers and there's very small percentage very low intensity of burrowing but you can see by the time we get to the top of the record pretty intensive burrowing on bedding planes and also if we compare the record of the soft-bodied form so here shown in green and the skeletal forms with hard parts shown in blue you can see also at the top of the record that they suddenly appear in deeper waters everything is clinging before that stage to shallow waters and medium depth waters and only at the very top of this record that they start to march down and get it start to inhabit deeper waters in other words what we're seeing is as anoxic waters fade away into the deeper waters and the whole base basins become very well occurred the animals are simply following it so there's a relationship a very close relationship between animals where animals lived and where the anoxic waters are so i think we can effectively prove our first hypothesis and say that it looks like the availability of oxygen can seem to have controlled where animals could live by expanding their habitats as oxygen became more and more available at least in this record in namibia close to the precambrian to the educar and cambrian boundary so let's think about this in a little bit more detail so this is the the figure i showed you before and i wasn't entirely straight because in fact this simple story turns out to be far more complicated rather than simply being one phase of oxygenation we may well have all these pulses of oxygen and these are the ones that have been recognized already and there may well be many more so in other words the earth it was as if the earth was oxygenating in a series of events whereas a pulse of oxygen then it went back to a more anoxic state pulse of oxygen more anoxic state and so forth and what's interesting is if you look at these pulses of oxygen what you can see is that they correspond to where our the record of our carbon cycle which is shown by these stable carbon isotopes shows a change you can see each of these red boxes is enclosing often a negative excursion really very very deep negative excursion here here here sometimes a twin positive and negative excursion now we don't fully understand the relationship between these but what is suggested is that there's a relationship between these pulses of oxygen and these perturbations to the carbon cycle so let's finally go to the cambrian excel itself the the record from beginning of the cambrian up to the end of the early campaign here's our second final hypothesis which is that actually this these dynamic redox events actually controlled the tempo of the cambrian explosion the idiocar and cambrian explosion in other words this very dynamic unstable oxygenation may itself have been a driver of evolution so i want to consider something that is easy to measure in the fossil record and that's just the size of something the body size so here you can see all these uh marine snails these gastropods that i've showed a photograph here they're very variable in size these are actually these are all one species these are one species and so on you can see they are very very variable in size now this idea that uh and there's a general idea that was produced or a general sort of hypothesis by this american paleontologist called edward cope and it's called cope's rule and he suggested uh back in victorian times that if you look at animals in one evolutionary lineage they tend to get bigger with time so this is called cope's rule and the reason for this was suggested was simple predator-prey relationships in other words big anim big fish eat little fish and little fish fish eat tiny fish and so on in other words it's good to be big the bigger you are the more you can eat you have you are a much more superior predator and competitor and you can produce more offspring so in other words all these very clear uh ecological and evolutionary reasons for being big and he suggested this was a trend in all images through their evolution now this has been gained a lot of attention more recently so for example first of all here you can see this amazing plot of many many different groups so all the arthropods in other words the things related to crabs and lobsters the lamp shells the brachiopods the sea urchins the crinoids all the mollusks in other words the eye vowels the keflapods and then vertebrates and you can see all of them when you plot up their body size through time it slowly gets bigger and this thick black line in the middle here shows you slowly increasing from the cambrian right up to the present day in other words cope's rule really seems to uh be true when you look at the whole um a huge data set uh through and look at it over a very very long large time scale long time scale however what about smaller changes so for example here we have uh just a plot showing you these um another type of fossils these are tiny microfossils and they show something opposite which is that they tend to get smaller when oxygen levels decrease in other words when you have a mass extinction which is often caused by anoxia uh they just go forward here a mass extinction caused by anoxia they tend to get very very small and we see this in many many mass extinctions where mass extinctions are caused also by this encroachment of anoxic waters into the area where things are living in the shallow marine areas or even deep water areas but an increased encroachment of anoxic um waters causes this quite dramatic change of body size and this was given the name the lilliput effect i think i've just i'm just my video here is covering up gulliver's image but lilliput after uh swift's fine novel so uh this is on yeah this is a shorter time scale so we have evidence for things getting smaller during mass extinctions but we it's all but also been suggested that we have evidence for things getting bigger through time as you saw before and this is been suggested to be related to oxygen levels so here you can see a plot just showing a where we how we think oxygen may have slowly increased in terms of um in terms of present day levels through geological time now this plot is a little bit dated now but you can see here is the edicar and cambrian boundary you can see the idea is that we didn't reach modern levels really until uh probably some time after the easier car and cambrian boundary but what this plot seems to suggest is that you can correlate the actual size of things through all of geological time here shown as they're complete they're the volume of animals you can see the general volume of all of life seems to have slowly increased through geological time whether you're a single cell a single primitive cell a prokaryote a single a more complex eukaryote or if you're a blue whale or a giant sequoia tree so this is prevalent idea that there's a relationship between oxygen and body sized increase of oxygen and decrease of oxygen but but what we really want to understand is how does this relate then to the cambrian explosion so here's our paleogeographic map again of the earth uh at this bit of geological time and now we're going to go to siberia here shown by the red star now siberia is a very different kettle of fish to do field work you're very much limited to outcrops along the huge major river systems this is the udema river of siberia all of the areas behind the river systems are dense forest and inaccessible with very few outcrops so but we have an incredible fossil record mostly started and developed and gained by the uh the paleontologists of the soviet union but also continued by many russians and international groups today and what i was into looking at with my colleague from moscow state university andrei gravloff is how do body sizes actually change during cambrian explosion so we went to the literature and we also made a lot of measurements in the field and we just took four groups which are relatively have got a relatively good record so first of all these sponges they're the first reef building animals of the cambrian some mollusks some very curious animals that are fairly primitive and possibly related to uh the um uh yeah the the distant relatives of mollusks and then these brachiopods the lamp shells and these numbers here just show you how many species we measured so we measured the size of all these uh fossils that we could get both from the literature and in the field and we plotted them up according to time so when they appeared in the geological record so here's our plot and what we've plotted here is just the longest dimension linear dimension and you can see it's color coded for these different groups don't worry about the the detailed names but you can see there are some very very dramatic time changes in body size so this is just the beginning of the cambrian here up to the end of the early early cambrian so this is a record only over about 30 or so million years or 25 million years first i want to point your attention to the first mass extinction in the geological record that's shown here by this red line is going to be shown as a red line in all the uh forthcoming lots just here this red lion and it's called the sims event and it's named after some rocks in the sinsk area and this is the first mass extinction and it's caused by anoxia so huge amounts of organic rich black shells are deposited in cyber at this time and you can see first of all from this plot that the body size of all these forms uh immediately after this synthetic event here get small yeah so a huge reduction in body size uh at this time it's the lilliput effect but more remarkable when you look at all these groups they showed increase here up to this point and then many of them show a decrease here and then perhaps a slight increase here and then an increase again sorry about my my lines a little bit not quite in the right place but increase again so in other words if we look at these body size changes through the cambrian explosion there are very very dynamic changes now if we then pull out these groups individually so first of all the sponges and you can show this is this is a plot that simply shows the minimum and the maximum and then where most of the data the measurements are sitting in the box in the gray box is here and you can see at the bottom of the x-axis this is the same time axis these 25 or so million years and i'll just pull out for you where the the the mean is sitting it's very very low in the early earliest early cambrian then it goes high and it goes low again high again and then at the sinsk event um it's there's a notable body size change in the archosius the mollusks show a very similar pattern and the high lifts as well show a very similar pattern in other words something remarkable is happening here all these groups which are not closely related are showing synchronous changes in body size they're all increasing at the same time and decreasing at the same time and they all show the lilliput effect after this mass extinction and i've just highlighted there in blue where the biggest body size is and you can see it's more or less at the same time now if we add the bracket pod data what's so interesting is these show a totally different pattern not the same at all they start big they get small and then they actually get bigger after the mass extinction so a totally different pattern and it's definitely very difficult to understand how to explain this but it's it's suggestive that whatever is causing these three groups to hold oh sorry wrong place um all these three groups here to show similar size changes the brachiopods are showing something very very different now let's also consider individual species well it turns out that individual species are also changing size you can see it's seen most closely here with the archaea sponges they start off small they get big then they decrease again then they get a bit bigger and here are a few data for the mollusks and the highlights but the brachiopods get small that those species that can change get small and then they get bigger again after syms so all these early cambrian animals are responding in some way to something going on in the environment that is triggering these changes in their body size both getting bigger and getting smaller and what's particularly remarkable is even individual species are showing really adaptive changes incredible changes in body size very very flexible changes so again i've just highlighted here this difference in where the biggest body sizes are in both the single species that change but between these three groups and then the bracket they showed very different records and the only explanation we have for this is that their physiological response to whatever is going on must be different and our best guess at the moment although this requires further testing is that these early earliest animals are responding to these pulses of oxygenation and these toxic pulses of oxygen oxygenation also may be bringing nutrients into these shallow marine lower cambrian seas and that also produces a increase of body size so for example here i've just highlighted where these changes are so in blue are these these three major groups and i've shown an arrow below this is where body size increases and in grey i've shown where the body size decreases so in other words gray the grey shading is body where the body size gets smaller and the blue shading is where body size gets bigger and if we look at the record on the siberian platform we look at the the geochemistry so some incredible very very dynamic changes in the carbon cycle in this platform exactly through this bit of time you can see here that the carbon cycle is going up and down and up and down and what this means geochemically has been modeled and if we model underneath in this brown line here it's been modeled to show these pulses of oxygenation so in other words uh where you um so i put this far in a slightly different wrong place in the sense but you can see here with a red arrow that's a pulse of oxygenation and a a dark arrow there the black arrow is a decrease in oxygenation in other words what we seem to be seeing is that the airing and flowing of this body size small body size large small body size is tracking these oxygenation pulses oxygenation and productivity impulses which is fascinating it means that the actual dynamics of the cambrian explosion may in part be driven by these pulses of oxygenation however what's so intriguing is not all these animals are responding the same as i said the brachiopods have a very very different pattern and also they are responding very very differently to this mass extinction event as i said this mass extinction event is the very first mass extinction event in the phanerozoic and it knocks out many of the really iconic groups of the cambrian never to be seen again like the these sponges and the highlights for example but the brachiopods of course we know march on and get more and more diverse through the paleozoic at least so in other words they have a very different response to this mass extinction which may be telling us something about their behavior their different behavior with changing oxygen conditions and that's significant because not only is that determining the actual dynamics of the cambrian explosion itself but it also determines the nature of its demise the end of the cambrian explosion shown here by this extinct event it may be determining what gets through and what doesn't in other words what gets us survive into the rest of the phanerozoic and in the case of the brachiopods become very very important uh animals in the biota so i think that we've gone some way to starting to prove the second hypothesis that it's this really dynamic oxygenation that may be controlling the tempo of the idiocar and cambrian explosion but it's far more complicated than that because different animals respond differently and this may be down to physiological differences with how they respond to this dynamic oxygenation and productivity so just to end uh i hope that i've convinced you with these just two little short tales from the cambrian that oxygen might well be a very important control in shaping the cambrian explosion but it is very very complex the oxygen story is not simple it's a very dynamic story we're only just starting to understand uh the real nature of the record and it certainly had a very very complex interaction with the record that we see of the cambrian explosion so thank you very much thank you so much to rachel for that wonderful talk that just sat so nicely in our series wrapping it all up and as and has been debated this week on we put out a twitter poll earlier in the week was the camera explosion caused by ocean chemistry or rising oxygen and 61 of our respondents agree with you that the twitter sphere is with you rachel and just before um we do some questions and if you have any questions do keep them coming in on the chat just a little talk about what's coming next because while first animals is over unfortunately um this this series of lectures will go on so in two weeks time we will have the first of our what we're calling our visions of nature talks so our first will be our very good friend dr ricardo perez de la fuente talking about fossil insect wonders ladies and gentlemen i think it will be a most interesting talk some of you might have seen uh ricardo's viral video on youtube earlier in the year how to tell the difference between real and fake amber but if that's anything to go by in two weeks time this will be quite the talk to see and then two weeks after that we have dr lauren sumner rooney who will be giving us the wonderfully titled talk all the better to see you with how do many eyed animals see the world so we're going to be looking into the many types of animals that have lots and lots of eyes so spiders come to mind but there are also many other things so do book on that and i will put the booking details up for ricardo's talk in a moment but for the time being we should go to our questions so i'm going to go to let's go to phil who i think is in bristol or yorkshire i can't remember that he asks about iron speciation so that measurement you were doing on the different types of iron and he says when using iron speciation as a proxy for sea water oxygenation how do you account for later diagenetic and metamorphic effects on mineral redox states so for everyone at home just to remind you diagenesis and metamorphism this is what happens when when rocks get squashed and heated and it can sometimes change the chemistry and i think what phil is saying is rachel can we believe your measurements or have they been well i'm i'm not a specialist on this but i think uh you can believe them um the so i just give you a bit of background iron speciation was a technique that was developed by simon paultnet leeds and don canfield in near copenhagen and it was calibrated on modern sediments so in other words they went all over the world to places where you have oxygenated and anoxic water so anoxic waters like the um the black sea and uh they found that this relationship was uh very very robust now in general when you're dealing with plastic sediments other words sediments that are uh reworked from pre-existing sediments shales silts um they found that you really cannot um use this technique on coarse-grained sediments like sandstones so sadly all of the rocks that have borne some of these amazing soft-bodied irikara fossils that we've heard about from uh um that that for example jack talked about from a mistaken point in um uh you could only really apply this this technique to fine-grain rocks now in the mistaken point and the avalon penis you do have very fine-grained layers that you can you can um tackle but uh the other classic sites like the edicara hills of australia these coarse-grained court sites you really can't use iron speciation you can't use it where there is uh very um and that's because of the dynamics of what's called the iron trap in very very shallow waters you also can't use it on any carbonates that have been undergone dynamitization late stage burial dynamitization because what we've discovered is in carbonates i mean the critical measurement here is the total amount of highly reactive iron and very few die genetic processes they they may move they may move around the highly reactive dietary iron within that pool but they don't uh um increase or decrease the size of that pool um the the settings that do is as i say deep late stage burial dynamization will mess that up shallow marine coarse grained material will mess that up and so will very very rapidly deposited sediments so those are the those are the main caveats but i would say i'm no specialist and you do have to always apply iron speciation with great care it will also change if you're looking at iron stones or um local burrowing activity you will get if you can get a full signature so you do have to be very very careful with how how it is applied but if it is applied properly with a good knowledge of the the the chemical constraints it is a very powerful technique there you go phil be careful with your iron speciation but it's very very useful and while we're on iron and oxygen we have a question from alex hans who asks is it true that the formation of banded iron formations held up the increase in oxygen levels of the oceans and therefore held up the explosion of life which eventually happened in the cambrian just a reminder those banded iron formations of much older generally much older than the rocks rachel's been talking about today and he's very iron rich of rocks and rusty red rocks i'm not sure the two things are connected and there's a huge amount of work going on of on the banded iron formations and what they really mean but the further back we go in time we realize that that image i showed you of a absolutely static anoxic c in the uh um deep free cameron if you like it's almost certainly not the case it turns out that there are probably other oxygenation pulses coming up through geological going piercing this bit of geological time we know of course there was a rise of oxygenation in what's called the great oxidation event um approximately 2.1 2.2 billion years ago uh and there were almost certainly other pulses as well um so the and i believe the bandit jack you're having a glass of wine um the banded iron the bat the banded iron formations are um they slightly peter out but then they slightly come back again after about 1.8 billion years so they disappear um i believe approximately 1.8 billion years ago but then they sort of come back they definitely do tell us something about oxygenation but i think the that story is still an area of that of active research to work out what it really means but of course we should it's one way to think about this is that all these events in geological time are potentially slightly predicated on uh events prior to that so if you think about these events as linear which i suppose we liked as human beings um you know you're just seeing the earth shift from a whole series of different states and sometimes these states are transient and sometimes they shift and they remain more permanent but you know even in the phanerozoic of course we see these incredible changes of oxygenation the very high levels for example that we saw in the carboniferous so um i i don't think there's any direct relationship between the banded eye informations and the cambrian explosion well thank you very much and i it this is specially selected rachel it pairs very well with discussions of erogenous uh certainly fermented yep south african pinotage um but we go to a question from matthew moving on to discussions of the some of the wonderful fossils and your brilliant data sets and matthew asks are the brachypods getting bigger after the extinction event because they are uh they are infilling an empty evolutionary niche left by the competitors who have got smaller well it's possible um the these these hypotheses of course are quite difficult to prove you've got to show that they really are living in the same niche which means living in the same place and eating the same things um the i wouldn't like to say yes at all the the demands the physiological brachiopods are actually rather interesting things they um they have very different energetic energetics compared to mollusks and my my suspicion is it's actually about metabolic demand but we have a huge problem uh we can measure metabolic demands in in other words which is one aspect of the physiology of modern brachiopods and modern mollusks but can we really say that these they were the same in the cambrian is the present the key to the past i i think it's a very it's very problematic uh it's very tempting because we have no no other way of guessing uh what these things needed um obviously if our geochemistry is sufficiently precise so for example let's say we could um we could potentially find fossils that lived in these very low oxygen conditions and then we could say something clearly about their demand but if they're all living in these uh if they're living in conditions that are indicated by oxygenation that i mean we're really at a limit of our geochemical techniques these these geochemical techniques are slightly crude so iron speciation is wonderful but it doesn't give you any idea of how much oxygen it just tells you there is free oxygen it doesn't tell you if there's a huge amount of it or really not very much it just tells you there is free oxygen and at the moment we have very few other really really good methods that allow us to quantify the amount of oxygen in the in the record that's what we would need in an ideal world so it your idea is is possible and i know it's this is something that is very um a very persuasive story for example the rise of the mammals after the um after the katy extinction of the you know the um dinosaurs and the cretaceous but for i i would i would like to say we have not yet tested that hypothesis for the bracket pulse after this first cambrian mass extinction just before we go to the next question um a reminder that if you have enjoyed um tonight's talk and the series as a whole and you are able to please do consider making your donation to the museum the museum is a charity and as well as putting on public engagement events like this one we are of course responsible for our wonderful building which you are more than welcome to visit now and of course over seven million objects in our collections and details of how to donate are currently in the chat um we go over to a question from huna deek who asks did other chemicals outside of oxygen in the water play a role in the growth of the sizes of animals such as salinity changes or more calcite being available for skeleton building hi huna um it's a good question i i could have given you an equally long talk on the role of seawater chemistry so i think there are some very interesting changes going on with for example uh how probably um uh magnesium calcium ratios in the sea and how they change and how they so for example when you when you look at these rocks on the siberian platform and you go to the late edea card and you look at the very first cements that precipitated in the poor spaces in grains and this tells you if these cements if you can show that these cements were precipitated from sea water and that's always slightly tricky but if you can show them precipitate from sea water what you see is some remarkable changes you see that in the late the last um 20 million years or so of the ediacaran those cements at least where i've looked at in in siberia they're made of dolomite which is calcium magnesium carbonate and you go to the early earliest cambrian the first few million years of the earliest cambrian and they're aragonite which is calcium carbonate and aragonite has a particular crystal form but then you get to the middle cambrian and all those cements switch to calcite which is still calcium carbonate but it actually has slightly less magnesium in its crystal lettuce and what we think is going on is that the magnesium calcium ratios are slowly declining through this time and we know from just normal aqueous geochemistry that if you if you have uh waters that are full of magnesium and actually other conditions uh if the other conditions are right you can form dolomite particularly if there's a temperature elevation but as you um dynamite's a bit of a tricky mineral to form but as you slowly decrease calcium magnesium ratios you go down to count you go aragonite and then calcite now what's so interesting is that dolomite nothing ever builds its skeleton out of dynamite because um uh we because it's actually a very very complex crystal to form it needs it's a very um complex crystal lattice but plenty of things form a skeleton out of aragonite and calcite and uh the um the uh what's so interesting is when we when the seas switch from dolomite to calcite that's when we see the very oldest animals forming a skeleton so forming a skeleton of aragonite and high magnesium calcite and not until we go to when the skeletons become calcite low magnesium calcite do we see all the animals that produce a skeleton of calcite and that includes the trilobites so it looks like and the trilobites of course their eyes are made of calcite so it looks like there may be a connection between changing seawater chemistry once the chemistry changes to allow the precipitation of what's called low magnesium calcite that seems to allow these triobites to produce a skeleton of low magnesium calcite but not before you can't produce a trilobite eye out of aragonite aragonite does not have the same optical properties so there's a very very interesting connection here between what animals are building their skeletons out of the minerals and also um you know changing seawater chemistry so of course the big question is what is driving sea water chemistry this time which leads us nicely into the net we'll take a final few questions and i'm going to combine a question from matthew with a question from sharon and we'll ask you talked about changes in oxygen but what is driving those changes in oxygen and matthew and sharon have said you know is it about overpopulation consuming all that oxygen or is it about climate change or glaciation or continental movements and tectonics so what do you think is driving these changes in oxygen that you've measured well this is the this is the million dollar question um we we probably don't have one driver for all the oxic events and there are many many of these and they possibly have some similarities but they may well have different drivers now if we think about just the the ones i i talked about on the um the siberian platform these event these are are um are very short-lived these are only two and a half or so two two to three million years long each of these these pulses so these carbon cycles i showed you each one is only about two and a half million years long now we don't have uh we don't think it's it's related to glaciations it may be related to changes in sea level but we don't know that for sure uh and it's far from proven it may be related to um productivity uh in other words it sometimes it's very difficult to know chicken and egg in these situations in other words what's what's driving what is the productivity driving the carbon record or is the carbon right record and manifestation of the productivity changes so uh the short answer is we really don't know there are there are ideas out there but they all remain to be tested so it is it is a critical question what is driving these and we may i think we're probably going to have to scrutinize each one of these events separately or at least in sort of clusters to really understand the drivers and the drivers might well change through time but you've got to evoke something that is changing over what is two and a half million years of course it's an immense amount of time but when we're talking about the idiocar and to cambridge these are really quite short-lived changes so we we can't we can't place this at the at the feet of plate tectonics that's that's a there's a very slow change it's got to be something that operates on a faster time scale there's a challenge to the young people in the audience come and join the geosciences and work out uh what the driver is behind the oxygen trigger of the cambrian explosion um and we'll go to a final question and um nobody's asked our usual question this week so i'm going to ask it and you talked you covered a great deal of geological time rachel and talked about all sorts of weird beasties and critters that are found in sites around the world um are they animals are they not who knows that's why they're so interesting um but the question i'm going to ask you is what's your favorite um i should have prepared for this question i i i don't really have a favorite fossil i what but i'm just going to ask a very different i'm not going to give you a very different answer maybe you maybe you sense that there is half of the cambrian and ediacaran subcommission colleagues are in the audience and you have to be very diplomatic i i what what i would dearly like to know is i would love to be at a time capsule and to be snorkeling over i'm afraid the easier car and seafloor not the cambrian sea floor had the cambrian forms i think we have a pretty good idea what most of these cambrian forms are we we we most of them fit into modern phyla not all but most of them are are um are recognizable but these easier current forms are truly truly baffling and we're making huge progress now and starting to understand what they might be but i would just love to be able to swim over a meadow of cloudiness tubes and normal clefus tubes these are these idiocorn cambrian it's like easier caron um skeletal animals and just pick one up and see what its soft tissue was and say ah it wasn't whatever blah blah blah after all that's what i would like so a a time machine to go and work out the as we said the taxonomic affinity of some of these beasties well we can we can dream maybe there's a young engineer watching sort that out for us in a few years at that point rachel thank you so much once again for a very brilliant and well-timed talk at the end of our series perfectly positioned with the question and answers you've been providing so thank you so much um lots of wonderful positive uh comments in the chat thank you uh once again and just a reminder to everyone at home that we although first animals is over and we very much hope we will be seeing you in two weeks time um when ricardo will be giving his talk fossil insect wonders so we'll be exploring a different bit of the fossil record in some much younger deposits um and ricardo knowing ricardo he will bring that alive and it will be a very very exciting talk so we very much hope to see you again in two weeks time fingers crossed will be coming live from the court of the museum as well so that's another exciting development on the way but at that point we say thank you once more to rachel hopefully see you in person soon um and thank you once again for joining us from all around the world and we hopefully see you again in two weeks time a wonderful bye thank you jack

Channel: Oxford University Museum of Natural History

Views: 36,571

Rating: 4.8157182 out of 5

Keywords: oxford university museum of natural history, geology, fossil, palaeontology, science, oxford, museum, natural history, lecture, rachel wood, oumnh, first animals, evolution, animal evolution

Id: ehkEltjGmpM

Length: 77min 21sec (4641 seconds)

Published: Wed Sep 30 2020