What Was The First Complex Life on Earth?

reverend robert plott is an educated man a professor of chemistry the prestigious university of oxford but he is as excited as a child at christmas when new parcels arrive in 1677 he hurries around the grass squad of university college and bursts through the heavy oak door of the ground floor lodgings it is just as well there on the ground floor because the parcel that greets him fills the entranceway it is a huge crate battered and splintered it must have taken three or four porters to bring it inside not to mention the poor horses that pulled it all the way from cornwall plot wastes no time in prying the wooden slats apart and revealing the bounty within nestled among straw and wooden shavings there are rocks curious shapes and patterns each accompanied by notes written in loose flowing cursive he pulls each one out briefly admiring it turning it in his hands before setting it down and reaching in for the next when the crate is almost empty his searching hands encounter a smooth rounded surface it takes two hands to pull it from its nest the reverent marvels at this unique specimen two rounded bulbs join where they touch and morph into a smooth cylindrical stem with a seemingly hollow center to plot it looks like a bone but it is made from stone and it is unimaginably huge at this time in the late 17th century debate runs rife among natural historians as to the true nature and origin of fossils some believe them to be the remains of ancient creatures long since dead while others prefer to dismiss them as tricks of the eye shapes that are formed naturally by the growth of salt crystals beneath the ground still suggest that fossils are the so-called seeds of plants and animals that have descended deep into the earth and begun to germinate there but robert plot is a skeptical man of science and with no way that he can see for living things to be preserved in solid rock he tends to favor the salt crystal interpretation he has spent the last few years studying the shell-like fossils found in oxfordshire quarries likening them to the crystal patterns produced by of all things crystallized urine [Music] but not all structures to be found in the rocks can fit this simplistic crystalline explanation plot eventually concludes that this magnificent specimen is in fact a petrified leg bone of some unimaginably large creature perhaps it comes from something like the impressive roman war elephants he saw in the flesh just a year earlier or perhaps it belongs to a long lost race of giants and so 100 years later the bone now part of oxford's natural history collections is studied again by another man of science richard brooks [Music] the understanding of fossils had advanced by this point and many more accept the possibility that they are remnants of long dead creatures and brooks is keen to use his knowledge of animal and human anatomy to interpret this curious specimen for the first time he gives the rock a name [Music] he calls it scrotum humanum making clear the fossil's kinship with a certain specific part of a giant human's anatomy of course brook's interpretation was a little off the mark we have no evidence of a race of giant humans either by the bones or the stones they left behind plot's cornish bone has now been identified as the thigh bone of a megalosaurus the first dinosaur ever to be described since the dawn of paleontology scientists have been forced to challenge the assumptions about what is and isn't possible about what earth's rocks really tell us plot and his contemporaries were occupied with whether fossils were even real at all but now we use them as a tool to probe our own multi-billion year history how much can the fossil record show us how far does it go back can we use it to find the first complex life and how can we be sure our interpretations are correct [Music] this video is sponsored by blinkist an excellent tool for absorbing knowledge wherever you are ever wanted to read a classic book but simply couldn't find the time well blinkist is here to help you out breaking down epics ranging from sapiens to the origin of species into 15-minute bite-sized chunks you can easily absorb on your commute or other times that would otherwise be wasted it's a truly excellent tool for lifelong learners and a great accompaniment to our channel should you wish to learn more [Music] like other educational apps all you need to do is download the app sign up for an account and you can listen wherever you are and when you do make sure to use our link we've teamed up with blinkist to offer you an exclusive 7-day free trial and 25 offer premium membership more than 5 000 titles are already available with more being released all the time so click on the link in the description below to activate your free trial now thanks now back to earth's distant past [Music] the chances of anything becoming a fossil beard a towering t-rex or diminutive dragonfly are vanishingly small the circumstances and conditions that allow traces of life to persist through geological time are so few in number as to be almost impossible to engineer or predict normally when an organism dies it is simply eaten by another creature recycled back into the great circle of life even if not consumed by a predator or scavenger microscopic detritivores and saprophytes colonisers of forest floors riverbeds and ocean bottoms make short work of any organic matter that falls into their domain [Music] bacteria break down and water washes the rest away leaving only the hardest bones and shells in time these two will oxidize and crumble away to dust that becomes soil and nutrients for the next generation of life this is the fate of almost anything that has ever lived to be so thoroughly broken down to our component molecules and reincarnated in the next cycle of life but in some exceedingly rare cases a dead organism against all probability is able to break the wheel to escape the endless cycle [Music] there are a few places on earth where the relentless recycling of organic matter is sufficiently slowed to allow some remnant of a creature to persist long enough to leave a trace in the rocks perhaps an animal is washed away to the ocean depths never stopping anywhere long enough to become food perhaps a fallen tree is so quickly covered by mud and sand as a river breaks its banks that it's swamped before the rod sets in or perhaps a freak change in water chemistry is enough to kill everything stuck within its toxic extent excluding potential scavengers for many years to come [Music] of course life on earth is famed for its tenacity and there are a few physical or chemical phenomena that can keep voracious bacteria at bay even in the deep dark ocean depths meters below the surface of atari swamp or in a seemingly toxic pool of acid there will be bacteria able to survive and feed on the organic bounty that comes their way soft flesh is easy pickings for these microscopic scavengers and they leave only those hard parts bones teeth and shells that are too tough to consume so in these dark buried chemical exclusion zones the hard parts persist and become one with the rock [Music] over time the soft sediments are compressed by the weight of more deposited on top water is squeezed out new minerals are precipitated and old minerals are lost fossilization is now a race between chemical and geological processes as sediments turn into rock can new minerals move in and preserve the structure of the bones teeth and shells before the original minerals are lost [Music] in the rare cases that they do a fossil is born if not then even the hardest of shells is dissolved and the void it left crushed out of all existence [Music] this unforgiving gauntlet which every dead organism must run if it hopes to be granted a fossil afterlife sees only one bone in a billion preserved within the rocks for creatures with no natural hard parts or which live far from the kind of environments that favor preservation the odds are stacked against them paleontologists estimate that less than 0.1 percent of all species that have ever lived have become fossils and far fewer of those fossils have actually been found nevertheless and perhaps testament to the sheer number of organisms that have walked swam flew or slithered across the earth there is an astounding wealth of fossils to be found in our rocks the very existence of a fossil record is what defines an entire eon of earth history the phanerozoic from 541 million years ago to today is so named in greek after the visible life within rock layers dating to this time there are bones teeth and shells but also the impressions of trees and leaves and in especially rare cases footprints fur and feathers reverend robert plot was among the first to tap into this phanerozoic treasure and in the centuries since paleontology has grown to interpret life evolution and environment on the basis of these fossil remains [Music] and tracing the fossils of the phanerozoic back through time a pattern emerges of increasing simplicity with increasing age [Music] large and complex skeletons give way to more rudimentary carapaces and modest shells and then to first appearances at least in the lowermost cambrian rocks that mark the beginning of the phanerozoic the fossils just stop [Music] charles darwin himself remarked in 1859 upon the strange and sudden termination of our paleontological lifeline into the [Music] past the problem of earth's missing pre-cambrian life has come to be known as darwin's dilemma how could complex hard-shelled life have seemingly sprung from nothing 541 million years ago [Music] where did all that complexity come from and why can't we see it the answer we know now is that we simply didn't know where to look against all possibility there are impossible fossils lurking in rocks from long before the eon of so-called visible life [Music] in the northwestern corner of arizona the colorado river cuts deep through flat layer cake strata of the phalarozoic carving out one of the most magnificent geological features on earth the grand canyon [Music] and on a cool autumn day at the end of the 19th century the chief paleontologist of the u.s geological survey picks his way down the narrow switchbacking path descending deeper into the canyon and deeper into time [Music] charles doolittle walcott is a self-taught geologist he began his paleontological career as no more than a professional fossil collector and yet his eye for something new and special among the ordinary proves the key to his academic success never having been told what should or should not be geologically possible he looks at the world with fresh unbiased eyes seeing things that his contemporaries want to overlook [Music] descending into the cool shadowy depths of the grand canyon woolcott spans thousands of years with each downward step until near the chasm's floor a single stride suddenly transports him 200 million years deeper into the past he has crossed an unconformity a sharp boundary in the rocks marked not by gradual deposition but by ages of erosion in this case 200 million years of time have been lost walcott was first walking upon familiar cambrian rocks but is now adrift in the pre-cambrian among rocks that are some 750 million years old these are the layers he has come to see and with his collector's eye he is quick to assess the geology before him his attention is soon caught by a strange pattern something he has never seen before in rocks of any age seen from the side perpendicular to the rock's bedded strata there are domed cabbage like layers sometimes broad sometimes tight and compact even pointed when surfaces parallel to the bedding are exposed the tight layers are successively cross cut to form fine concentric rings without any comparison walcott considers what could have formed these curious structures their layering is much finer than the grains of the surrounding rocks would seem to allow and their shapes are not those easily formed by any physical processes he knows in his notebook alongside their sketches he posits that they could have been shaped by some kind of ancient life although precisely what kind of life he isn't sure he gives them a new name cryptozuon meaning hidden life and reflecting the many questions still surrounding these strange formations [Music] many more examples of cryptozuon were subsequently discovered in pre-cambrian rocks but it wouldn't be until the 1950s long after wolcott's death that this hidden life would be revealed for what it really was in several salty bays of western australia rocky blobs cluster in the tidal shallows and ecological analysis revealed these smooth domes to be composed of a layer upon a layer of fine sediment and organic matter and in the uppermost layers photosynthesizing cyanobacteria reach for the sun these are stromatolites multi-layered microbial tower blocks in which only the uppermost levels are occupied they grow with geological slowness driven by the photosynthetic need for the sun tidal waters wash fine sediment over their tops and the cyanobacteria must form a new layer on top if they are to survive shaped by tidal ebbs and flows and the gentle action of waves in the bay the layers are molded into individual domes cauliflower-like in cross-section and concentrically ringed when eroded flat [Music] these living stromatolites are a perfect match for the cryptozoons walcott found in the grand canyon and just as the modern domes are formed by life so too are there petrified precambrian twins charles doolittle walcott had unwittingly found the first evidence for life before the phalerozoic proving darwin's assertion that life did indeed exist in the precambrian and offering a hint as to why it was still hidden to us quite simply because it was too strange to be believed [Music] since the mid-20th century realization that the pre-cambrian fossil record does indeed exist if only we know where to look proterozoic paleontologists have had to challenge their assumptions and put aside their biases about where and how some of the earliest forms of life might be preserved now the scales have fallen from our eyes and the jewels of the precambrian are revealed we may not find bones or teeth or even simple shells but we do find hints and clues of the life that existed before this mineralized protection when the stage was set for this complex life to find its figurative feat [Music] it may seem an impossible task to preserve a soft squishy organic body in the face of biological recycling but as stromatolites have shown us there are certain quirks of rock formation that allow even microscopic bacteria to be preserved throughout eons in this case sufficiently large communities of tiny organic organisms are able to exert enough influence on their local environment that the normal processes of rock formation are subtly altered with a thorough understanding of the kinds of structures life can build today we can search the rocks for such traces as well for instance tidal estuaries are ideal habitats for microorganisms that don't mind the brackish parts salty waters that wash over them nutrients delivered by the river are deposited along with fine sand and mud in shallow tidal waters that are protected from the worst of the battering waves here vast layered communities of microbes can grow on and in the sediment spreading out to make the most of the sun's rays even if the conditions don't contrive to build stromatolite tower blocks these biomats can still leave a trace in the resulting rocks their surfaces become tufted and wrinkled by the action of water on their organically woven fabric as the delta streams shift or storm surges dump new sediment on top these tufts and wrinkles are locked in place and the unique chemistry of these aqueous environments tips the balance of petrification allowing the patterns to be preserved as molds and casts before the organic matter of the biofilm rots away today pre-cambrian paleontologists scour the top and lower surfaces of rocks for these shadowy hints of wrinkled biomats searching at sunset when the sun is low reveals intricate patterns formed by sun hungry biological fabric many hundreds of millions of years ago for many paleontologists today turning over a slab to find its bottom surface decorated with fine tufts and wrinkles formed by ephemeral microscopic organisms is a thrill akin to discovering a dinosaur footprint it is a brief moment of time miraculously captured for the ages [Music] but there is an even more miraculous process occurring in some of these ancient rocks if pre-cambrian paleontologists were encouraged to set aside their biases before now they must throw away the rule book entirely in the 1950s around the same time that australian ecologists were discovering living stromatolites geologists on the opposite side of the world were uncovering something even stranger [Music] elso baghorn and stanley tyler investigated curiously carbon rich rocks from the gunflint range of western ontario layers of coal-like rock contain no fossils but they hint that life was extremely abundant here abundant enough to accumulate carbon within the sediments themselves elsewhere among the gunflint rocks they find a dark glassy chert adding it to their pack for analysis when they return to the lab [Music] standard geological analysis often involves slicing paper-thin sections of a rock making it thin enough so light can pass through it and experts can identify the minerals by their microscopic characteristics but when tyler and barkhorn subject their chert samples to this process they find something they don't expect speckled all throughout the fine crystals of silicon dioxide there are tiny grains of carbon it's this that is responsible for the chert's black color but in places these specs are gathered together in three-dimensional spheres filaments and star-shaped groupings each no more than around 10 microns or a hundredth of a millimeter across in both shape and size they resemble living microbes but they are nearly 2 000 million years old could they really be biological in origin such delicate forms preserved for so long over the last 70 years since this incredible gunflint discovery scientists have found chemical and petrological justification for how such impossible fossils could come to exist [Music] remembering that fossilization is a race between degradation and preservation for the most delicate of organic structures to be preserved then that preservation needs to happen at an unprecedented rate indeed the petrification process must begin while these cells are still alive certain quirks of chemistry more common in the exotic and undisturbed seas and lakes of the precambrian can see certain mineral fluids become supersaturated in natural waters [Music] silicon dioxide or phosphate ions build up and are further concentrated by the microbes that inhabit the sea floor as they reach unsustainable levels and the kind of gel begins to precipitate the luckless microbes are encased in a mineral tomb from which there is no escape deprived of the nutrients they need to survive the cells die [Music] but they are already beyond the reach of any that might reuse their carbon as the mineral gel crystallizes and hardens the cells are frozen in their death mask protected even from the lifeless chemistry that would reduce them to formless blobs if crystallization happens fast enough then the cells are preserved in the three-dimensional positions they inhabited at the moment of their death [Music] if not and the gel is flattened by sediments deposited on top then the detail is retained but not their positions three dimensions are reduced to two as the delicate fossils are flattened like pressed flowers between layers of mud and sand this mineral sarcophagus is enough to allow even the most delicate cells to stand the test of time as long as the rocks themselves survive so does their microfossil treasure when hundreds of millions of years later paleontologists slice those rocks and put them under the microscope or dissolve away the crystalline tombs with acid they are greeted with delicate organic walled forms that look remarkably similar to modern cyanobacteria algae and other single-celled microbes just like insects preserved in amber these appear to be cells that have been frozen in time only they are seven times as old and many times smaller and more flimsy it seems almost too perfect to be true and indeed the true nature and antiquity of such exceptionally preserved microfossils has been called into question many times naturally the older the rock the more questionable it is that the microfossil bounty has survived intact and these are not intricate insects in amber but forms that are much more simple are they really biological cells that are billions of years old or is it simply wishful thinking it has been said by pre-cambrian paleontologists from both camps that extraordinary claims require extraordinary evidence impossible fossils that seem to buck the trend of all fossilization before them must be subjected to more rigorous validation than any other we must prove that they are formed by life and we must prove that they were formed at the same time as the rocks themselves not introduced later luckily the science of exceptional preservation has matured to the point where such verification is now routine along with documenting the shapes sizes and associations of microfossils within an ancient rock experts also look for chemical signals that suggest a biological rather than chemical origin they look for certain complexity and structure which can't be achieved by physical processes alone and the surrounding rocks are analyzed in minute and microscopic detail to calm any fears that the microbes could have been introduced after the minerals had formed [Music] now organic walled microfossils from the precambrian are no longer impossible but an important part of our paleontological inventory stretching back at least two and a half billion years [Music] it may not be a full picture but some two centuries after his birth it is a better picture than charles darwin ever hoped to have and we now begin the more challenging task of interpreting exactly what it is that picture shows lynne margulis was not your traditional scientist once married to the famous cosmologist carl sagan she looked at the world of evolutionary biology in her own unique holistic way she wasn't interested in how one organism functioned but pioneered considering the links between even the most disparate creatures she didn't consider individual habitats as their own isolated environments but rather looked at the entire earth system as a vast interconnected and self-regulating super organism her theories were wild and unconstrained by popular opinion earning her the title science's unruly earth mother [Music] one such theory took shape in her mind in 1966 as she took up her post teaching biology at boston university she posited that the more complex eukaryotic cells from which all higher multicellular organisms are built came about not from competition but by cooperation normally evolution by natural selection brings about changes in living things by favoring certain traits within a species naturally occurring variation under competitive conditions where resources are scarce but margulis suggested that in the case of eukaryotes things happened a little differently [Music] when nutrients ran short in the ancestral prokaryotic world different bacterial cells contrived to work together to increase both of their survival chances this pact or symbiosis was secured by a physical association one type of prokaryote would physically come to live inside another and reproduce alongside it and thus margulis's theory of endosymbiosis took shape it seems like a strange assertion and indeed it was too strange too unsupported by actual evidence for many to accept it straight away margulis submitted her paper to about 15 scientific journals before one the journal of theoretical biology finally agreed to publish it she spoke about the theory everywhere she went at scientific seminars conferences workshops and even among friends but for a long time the idea of bacterial cooperation was just too much for many to countenance for about 15 years lynn's ideas were considered fringe science and perhaps never taken completely seriously but by the 1980s a new analytical technique gave the endosymbiotic theory the spark of credibility it needed it was discovered that mitochondria the famed powerhouses of the eukaryotic cell have their own unique dna that is unrelated to the dna of the host later the same was shown to be true for chloroplasts margulis had claimed that these structures were once independent organisms and now their genetic signatures were indicating that was true it looked like she had been right all along having ridden the endosymbiotic storm from rejection through suspicion and eventually to orthodoxy helene margulis's unruly approach to science only deepened and she developed a taste for the controversial she worked with james lovelock to give substance to his gaia theory that the earth is a single self-regulating super organism and fought hard to make her case for five kingdoms of life animals plants fungi single celled eukaryotic protists and prokaryotic monera she even went so far as to reject the three domains of life that emerged from genetic analysis of prokaryotes despite the fact that it was this very genetic analysis that had given substance to endosymbiosis [Music] nevertheless as unorthodox as her opinions and approach may have been there can be no denying lynne margulis's uncompromising passion for microscopic life and she was one of its most vocal advocates right up until her death in 2011. [Music] and of course there is a surprising diversity of primitive life are most commonly depicted as simple pill-shaped rods but they can also be spheres filaments loose spirals as well as various bunched and tangled associations of all these in size they typically range from less than a tenth of a micron to five microns in length modern eukaryotes by contrast are larger and more complex inside their cells they contain the organelles obtained by endosymbiosis including the mitochondria chloroplasts and tail-like flagella but they are also packed to bursting with other cellular machinery and internal membranes thought to have evolved by more traditional natural selective means genetic material doesn't float freely throughout the cell but is instead contained in a membrane-bound nucleus and there is an ephemeral but highly organized structure to the gel-like cytoplasm which helps coordinate the complex processes of cell splitting and sexual reproduction it is called a cytoskeleton it is this internal organization that allows eukaryotes to become the basis of all higher forms of life the macroscopic multicellular organisms that evolved much later [Music] if we are to search for eukaryotes in the pre-cambrian fossil record we must take a lesson from lynn margulis and think laterally if the internal features of this more complex life aren't preserved then instead we must search for features from which we can infer a more complex organization [Music] these are things that something only a cell with organelles like a cytoskeleton can accomplish one such feature is the microfossils size in general prokaryotes tend to be smaller than 5 microns in diameter and eukaryotes are at least 10 microns across but even today we see exotic super-sized bacteria or miniature eukaryotes encapsulated colonies of bacteria can easily reach a larger size and once self-digestion occurs there is no way to tell who the outer case belongs to so size is a good first indication but is by no means reliable a much more reliable indication can be found in the organic walls of the microfossils themselves a simple cell wall is easy enough to build but any patterns or embellishments require more cellular organization than a prokaryote can muster so paleontologists agree that any organic microfossil that is adorned with spines or other external structures and ornamentation must in fact be derived from a more complex eukaryote [Music] more simple outer walls and sheaths are more difficult to pin down but again some clues can point towards a level of organization only possible with complex internal machinery many modern single-celled eukaryotes are able to survive through periods of environmental stress by encasing themselves in more resistant cysts from which they emerge when conditions are improved and nothing is more environmentally stressful than suddenly being encased in mineral gel so we can expect to find these cysts among the microfossil assemblages [Music] [Music] the simple structureless innards of the average prokaryotic cell could never build a tree or a jellyfish or a human it takes the complexity of a eukaryotic cell to make the visible life on earth as we know it of course such complex machinery takes time to evolve even with endosymbiosis providing a leap off the starting line and it's thought that the boring billion the period of unprecedented geological stability between 1.8 and 0.8 billion years ago provided the perfect conditions to gestate these changes now that exceptional preservation throughout the pre-cambrian is offering us a window on the microscopic life that thrived at this time can we find evidence of these monumental evolutionary changes in the fossils themselves [Music] and even possibly answer the question what and when was the first complex eukaryotic life [Music] a proterozoic fossil hunter is unlike any other kind of paleontologist walking among ancient rocks she doesn't hope to find a promising curl or lump producing from a rock's surface and she would never expect to find her prize washed up among the scree at the base of a cliff [Music] instead she is meticulous about documenting every geological detail of the freshest exposure of rock she can find collecting evidence from far and wide to interpret the environment in which these layers formed and what geological torture they have endured until now in strata that is sufficiently unaltered she scours the rock for minute differences in the minerals that make them up extracting seemingly uninteresting slivers from within these layers she labels them carefully noting their position and orientation before unceremoniously tossing them into our backpack among tens of others just like them packing them securely back at the hostel for the flight home [Music] back at the lab it's time to process her fines some she will cut exposing fresh surfaces that haven't seen the sun for a billion years and then mount those surfaces onto glass slides and grind them until they are just a few tens of microns thick other samples she will dissolve in hydrofluoric acid which will melt away all rocks and minerals leaving only the organic matter behind this too will be mounted onto glass slides and fixed in place finally in a darkened room thousands of miles from the journey start and thousands of hours later she slips the glass slide beneath her microscope and peers with some trepidation through the eyepiece only then will she know whether she has found something or not whether her training and geological eye has revealed a treasure of the proterozoic or if all that work has been for nothing [Music] it is an exhilarating feeling looking through the eyepiece and finding the bounty of the proterozoic laid out and it's a feeling that many paleontologists have experienced over the years there have now been countless discoveries of microfossils from proterozoic rocks all over the world spanning huge swathes of time [Music] [Applause] the gunflint biota including the curiously star-shaped usatrian and umbrella-like kakabikia is found consistently in the gunflint church of western ontario dating to 1.88 billion years other microfossils are found from rocks all over the world indicating their global dominance within the mesoproterozoic finely reticulated spheres of dictiosphera sit alongside tapania and shui use spheridium with their branching and interlinking spines all are found in rocks of china and australia dating between 1.75 and 1.45 billion years old slightly younger rocks of the proterozoic dating to 1.2 billion years are found in the hunting formation in canada and these are host to a curious association of seemingly differentiated cells different shapes are confined to different parts of the organized structure and paleontologists believe this is one of the earliest instances of complex and consistent multicellularity and sexual reproduction by 1 billion years ago eukaryotic cells have even made it out of the oceans and onto land or at least into the non-marine waters of the turidan lakes of modern day northwest scotland one species native to these ancient lakes is by selem brassieri also featuring differentiated cells even before the end of the boring billion it seems that life is already well on its way to complex multicellularity but what of the deeper history of eukaryotes [Music] our modern protozoa paleontologist no longer needs to worry about a paucity of fossils to study to compare her finds with ones that have already been documented she visits the smithsonian museum of natural history she wastes no time with the exhibits impressive as they may be walking past t-rex skeletons and gigantic fossil stromatolites to a small unmarked door in the corner of one of the halls [Music] she steps through the door and into a different world gone are the information board's creative lighting and screaming children and she's instead faced with passageways lined with cabinets gathering dust that dulls their already monochrome colour scheme navigating this backstage labyrinth she eventually finds herself in the same room as the fossils she is searching for but she still has to work to find them the good thing about microfossils is that they don't take up much room but 70 years of proteozoic paleontology has nevertheless accumulated an impressive number of specimens cabinets stretch floor to ceiling and are crammed in back to back with scarcely room to slide a piece of paper between them but these stores are mounted on wheels driven by geared winding handles and as the young paleontologist winds the cabinet slowly rolls to the side allowing her to walk along the newly created space and commence her search in earnest the store is filled with brown shoeboxes each full of proterozoic treasures mounted on their glass slides and labelled with a careful hand [Music] small red dots in the corner of some labels indicate sections that are priceless beyond measure they are the designated holotypes the first and presumably finest example of a given species to which all subsequent specimens can be compared one box alone may contain a dozen slides and in the entire room there are thousands of such boxes indeed there is no shortage of fossils from which to draw our conclusions ornamented microfossils of the type that can only be formed by eukaryotic internal complexity have been found in rocks as old as 1.6 billion years [Music] dictiosphera germanosphera pterosperm obsimopha simea and valeria are all morphologically complex forms recovered from the paleo-proterozoic chang cheng group of northern china [Music] more ambiguous eukaryotic evidence is to be found in even older instances of exceptional preservation fossils found in the 1.8 billion-year-old changu rocks in northern china seem to show some of the outer ornamentation expected from eukaryotes but without any of the other features to suggest complexity they are not the most compelling of examples remembering the tenet that exceptional claims require exceptional evidence the claim of the earliest eukaryotes must be backed up by more than just a faint pattern seen in the right light [Music] and yet remarkably even this example is surpassed in age by another claimant for the title of the earliest eukaryote long thin spirals scrolled in dark carbon were discovered in 1899 in 1.3 billion-year-old rocks of montana at first they're interpreted as trace fossils figurative footprints in the sand left behind by a simple burrowing worm but in 1976 the fossil was reanalyzed and set against the emerging picture of late animal evolution to provide a new interpretation these curious doodles are extremely thin flattened films of biological carbon up to nine centimeters long if straightened out paleontologists believe them to be the flattened remains of a large seaweed-like organism that they have named grypania [Music] in order to grow so large we assume eukaryotic organization is at play this is fine for the grapenia discovered in montana it sits alongside other examples of crude multicellularity but when grapenia was discovered in rocks more than 2 billion years old we must challenge our assumptions once again is the eukaryotic interpretation right if so could these giant eukaryotic specimens be among the first on earth even older carbon-rich fossils have been discovered in fine-grained shales dating to 3.2 billion years old some have argued for their eukaryotic nature based on their large size at 300 microns or three tenths of a millimeter they are much smaller than grapania but still larger than most known bacterium for this extraordinary claim however there is no more evidence of course other methods have been devised to try and trace eukaryotes back to their roots through earth history just as the cellular machinery of these more advanced cells can create complicated structures on the outer wall so too can they produce complicated chemistry that no physical or prokaryotic process can replicate and so there is still no consensus on how to verify the antiquity of a molecule other techniques can involve the analysis of genetic change and differences among living eukaryotes reconstructing family trees of the eukaryotic kingdoms and tracing those trees back to their roots using an assumed rate of evolutionary change so-called molecular clocks can put a date on the origin of eukaryotes with no fossils necessary some of these analyses also pin the beginning of complex cells to a date of around 2.7 billion years but there is by no means a consensus on the approach used to arrive at that date there are so many assumptions in the shape of the evolutionary tree and the rate of change over evolutionary time that any proposed dating becomes all but meaningless [Music] today the pre-cambrian fossil record has been revealed to us in all its glory we are pretty certain we know how complex eukaryotic life came about and we have a good and growing history of how it developed in its earliest billion years or so but finding the very first eukaryote the basis for all complex life to come is a much harder task it is likely that eukaryotes as we know them today actually came about gradually beginning with the sealing of the endosymbiotic pact and continuing with the iterative development of internal architecture throughout the relative stasis of the boring billion [Music] approaching evolution from the more traditional direction the invention of membrane-bound organelles and nucleus as well as an organizational cytoskeleton and the internal chemistry needed to support it would take an extremely long time thus instead of modern eukaryotes appearing fully formed at some point in the proterozoic we can expect a sliding scale of complexity from cells to the barely improved prokaryotes to cells that are capable of combining in their multitudes differentiating and building the world as we see it today [Music] this increasing simplicity as we trace the fossil record deeper into the precambrian is matched by the increasing uncertainty we have when interpreting these most ancient fossils we may never know what was the first true eukaryote if there even is such a thing upon this sliding scale and yet with the wealth of fossil evidence now available to us for the bulk of the proterozoic we do stand a better chance of understanding the transitions that came about after eukaryotes have taken the stage [Music] the fog has been well and truly lifted on one of the most cryptic periods of earth's history and as organisms grow ever larger and more complex the late pre-cambrian world begins to look ever more like the world we inhabit today [Music] the next 800 million years promise great things for complex eukaryotic life [Music] you've been watching the entire history of the earth don't forget to like and subscribe and leave a comment to tell us what you think next up we are no longer billions of years in the past but hundreds of millions we're diving into one of the strangest times in earth's history when the entire planet was covered by ice [Music] you

Channel: History of the Earth

Views: 1,254,336

Keywords:

Id: HDim45WdlLs

Length: 59min 33sec (3573 seconds)

Published: Tue Nov 30 2021