The origin and rise of complex life: integrating models, geochemical and palaeontological data

09:05-09:30 Quantitative reconstruction of Earth’s surface conditions during the rise of complex life

Benjamin Mills, University of Leeds, UK

Abstract

The link between global environmental change and the rise of complex life during the late Precambrian remains uncertain. The time period in question contains two massive 'Snowball Earth' glacial events, and also contains abundant but sporadic evidence for atmospheric and marine oxygenation, but forming a more precise picture of Earth's surface environment through the Neoproteorzoic Era is essential in order to understand whether these conditions might have spurred evolutionary advances, or alternatively may have been a direct result of 'bioengineering' of the environment by new organisms. The evidence for surface system evolution comes from a fantastic suite of geochemical analyses, but these are necessarily limited in time and often tell an incomplete and inherently local story. We can strengthen these analyses by introducing global biogeochemical models: these systems take information about past tectonic and biological processes and produce independent and detailed estimates for changes to the surface environment, which can then be related to the existing analytical databases. Professor Mills will introduce our latest results for the changes in surface temperature and atmospheric oxygen through the Neoproterozoic, question how we might expect marine oxygen levels to change, and discuss the coupling between marine oxygenation and biodiversity.

09:30-09:45 Discussion

09:45-10:15 Microbial mats and oxygen oases as hotspots for evolution

Dr Kurt Konhauser, University of Alberta, Canada

Abstract

It is generally accepted that animals first evolved during the Ediacaran Period when the oceans were just becoming fully oxygenated. The best evidence of the first animals include in situ trace fossils that are commonly associated with microbially induced sedimentary structures. The trace fossils generally were formed parallel to the surface of the seabed, at or below the sediment–water interface. This evidence suggests that the earliest mobile animals inhabited settings with high microbial populations, and may have mined microbially bound sediments for food resources, and possibly O2 if the mats were comprised of cyanobacteria. Numerous studies of modern cyanobactertially-dominated mats have further documented that during the day O2 levels in the mats are several times higher than in the overlying water column, in effect serving as oxygen-rich oases. These findings raise the possibility that animal evolution during the Ediacaran could have arisen in similarly O2-rich microbial mats that inhabited an otherwise poorly oxygenated or even anoxic environments. If true, then oxygen availability in sea water may not have played a direct role in the appearance of the first animals, especially considering the apparent existence of cyanobacterial mats as far back in time as 3.2 billion years ago. 

10:15-10:30 Discussion

10:30-11:00 Coffee break

11:00-11:30 Sponges and the origin of complex multicellularity

Professor Maja Adamska, Australian National University, Australia

Abstract

Sponges, morphologically simple animals and likely descendants of the oldest surviving animal lineage, are the key phylum to study the origin of animal multicellularity. Sponge body plan is based on a system of chambers built of collared cells (choanocytes), which propel water and capture food particles. Water, entering the canal system through small pores, is expelled by a larger apical opening (osculum). The outermost epithelium is composed of flat pinacocytes, with diverse motile cells inhabiting the space between the two layers. Morphology and function of choanocytes are reminiscent of choanoflagellates, colonial and single-cell protists, which are the nearest relatives of animals. The combination of similarity and phylogeny implies a choanoflagellate-like ancestor of all animals, and a sponge-like ancestor of all complex animals. The bi-layered body plan of sponges, with the major apical opening, is similar to cnidarian polyps, suggesting transition between poriferan and cnidarian grades of organization, with the endoderm derived from choanoderm, the ectoderm from pinacoderm, and the polyp mouth from the osculum. Recently, gene expression studies – initially of candidate genes, then involving massive single cell transcriptome sequencing – have been used to test these long-standing hypotheses. These studies produced unprecedented insights into cell and developmental biology of sponges, as well as several lineages of protists closely related to animals. However, interpretations of these insights vary widely, and we are far from understanding the events leading to emergence of complex animal multicellularity. Will improvement of sequencing technologies and inclusion of additional model species help shed light on our beginnings?

11:30-11:45 Discussion

11:45-12:15 Tracking the evolution of the Neoproterozoic marine biosphere using molecular fossils

Professor Gordon Love, University of California, Riverside, USA

Abstract

Sterane biomarkers preserved in immature ancient sedimentary rocks hold promise for tracking the diversification and ecological expansion of eukaryotes. Bacterial markers dominate biomarker assemblages until the early Neoproterozoic and eukaryotic steranes do not leave a detectable record till ca. 800 Ma and younger, probably due to low environmental abundance of microbial eukaryotes in most marine settings. The earliest proposed animal biomarkers from demosponges (Demospongiae) are recorded in a ca. 100-Myr-long sequence of Neoproterozoic-Cambrian marine sedimentary strata from the Huqf Supergroup, South Oman Salt Basin (Love et al., 2009). This C30 sterane biomarker, informally known as 24-isopropylcholestane (24-ipc), possesses the same carbon skeleton as sterols found in some modern-day demosponges. However, this evidence is controversial because 24-ipc is not exclusive to demosponges since 24-ipc sterols are found in trace amounts in some pelagophyte algae. We recently detected a new fossil sterane biomarker that co-occurs with 24-ipc in a suite of late Neoproterozoic-Cambrian sedimentary rocks and oils, which possesses a rare hydrocarbon skeleton that is uniquely found within extant demosponge taxa. This sterane is informally designated as 26-methylstigmastane (26-mes), reflecting the very unusual methylation at the terminus of the steroid side-chain. These new findings strongly suggest that demosponges, and hence multicellular animals, were prominent in some late Neoproterozoic marine environments at least extending back to the Cryogenian Period.

12:15-12:30 Discussion

13:30-14:00 Assessing the role of oxygen in early animal ecosystems

Dr Rosalie Tostevin, University of Cape Town, South Africa

Abstract

A hypothesised rise in oxygen levels around the Neoproterozoic–Palaeozoic Boundary has been repeatedly linked to the origin and radiation of early animals. Given that oxygen is required by all extant animals, this hypothesis seems intuitive and has proved rather attractive. But a large body of recent work has shown that the role of oxygen in early animal ecosystems is more complex and dynamic than previously thought. One issue is that geochemical proxies often don’t provide the information needed to address ecologically relevant questions. For example, it is perhaps more important to know which regions of the ocean were anoxic, than how much of the ocean was anoxic. It is also important to know how much oxygen was available in the oxygenated parts of the ocean. Waters containing 100 µM or 1 µM O₂ would be indistinguishable in many proxy systems, but the first could host a complex ecosystem containing skeletal animals and motile predators, and the second would be largely uninhabitable. These issues can be partly resolved by considering the systematics of each geochemical proxy, and exactly what information it provides about the redox structure of ancient environments. This talk will focus on the questions we are able to ask using geochemical proxies, and evaluate our current ability to assess the role of oxygen in early animal evolution.

14:00-14:15 Discussion

14:15-14:45 Real and fake planktonic ecology in silico: using numerical models to explore the marine environmental impacts of the rise of complex life

Professor Andy Ridgwell, University of California Riverside, USA

Abstract

Species do not live in isolation, but adapt and ultimately, evolve, in relationship with other species as well as with their chemical and physical environment. In the ocean, the surface biogeochemical environment modulates the makeup of the pelagic ecosystem, while the ecological assemblage, in turn, regulates the carbon and nutrient cycles in the ocean and concentration of CO2 in the atmosphere, thus influencing the environment. Evolutionary feedbacks, both negative and positive, must therefore exist between plankton, and global biogeochemical cycles and climate. This has important implications for understanding the rise of complex life and presents a significant challenge to understanding the rock record (what is the geological chicken vs. the egg?).

In this talk, Professor Ridgwell will illustrate a theoretical way forward via toy (‘fake’) worlds – aka, numerical models. Specifically, he will present some initial results of some contrasting modelling philosophies, both based on what happens if you incorporate a diverse marine ecology in an Earth system model. In the first example, idealized evolutionary changes in ecosystem structure are prescribed, and the attendant impacts on the marine environment are then simulated – (the chicken is prescribed and the eggs follow?). In the second, the structure and diversity of the ecosystem is allowed to ‘evolve’ in association with the environment. The resulting ecological successions exhibit some real world behavior, and in fact mimic observations of planktic evolutionary recovery in the aftermath of the end Cretaceous impact. The technical challenge is to apply such an approach to ~720-520 million years ago.

14:45-15:00 Discussion

15:00-15:30 Coffee break

15:30-16:00 Insights into eukaryogenesis from the fossil record

Professor Susannah Porter, University of California, Santa Barbara, USA

Abstract

Eukaryogenesis—the steps by which the eukaryotic cell emerged—has long intrigued scientists. The recent discovery of the closely related Asgard archaea has provided new insights into how this might have occurred, but fossils are the only way to reconstruct evolutionary transitions in clades now extinct. Though many cell characters are not preserved, there are at least four crown group characters for which proxy records exist: sterol synthesis (steranes), mitochondria (occurrences of fossils in different redox habitats), the cytoskeleton (fossils with spiny ornamentation), and the capacity to form resistant-walled cysts (the body fossils themselves). We could thus use these records to infer the relative order in which these characters evolved, and the environmental context of their evolution. In this talk I review these records and propose a different scenario for early eukaryote evolution than what is widely assumed. Rather than crown group eukaryotes originating in the late Paleoproterozoic and remaining ecologically minor components for more than half a billion years in a prokaryote-dominated world, the fossil record may point instead to a late emergence of the eukaryote crown group, with cyst formation and the cytoskeleton appearing early, and sterol synthesis evolving late. The origin of mitochondria cannot as easily be pinned down, in part because it’s difficult to reconstruct redox habitats of ancient eukaryotes. However, it is clear from the handful of studies we have that it cannot be assumed early Proterozoic eukaryotes were aerobic.

16:00-16:15 Discussion

16:15-16:45 Reconstructing Earth’s oxygenation

Dr Noah Planavsky, Yale University, USA

Abstract

Deciphering the role—if any—that oxygen levels played in controlling the timing and tempo of the radiation of complex life is one of the most fundamental questions in Earth and life sciences. Accurately reconstructing Earth’s redox history is an essential part of tackling this question.  Over the past few decades, there has been a flood of research applying geochemical redox proxies in an effort to tell the story of Earth’s oxygenation. However, many of these studies have led to conflicting interpretations of the timing and intensity of oxygenation events, even when considering the same isotopic systems. There are two potential explanations for conflicting redox reconstructions—1) that oxygen levels were incredibly dynamic in both time and space or 2) that as a community we have often studied rocks affected by secondary alteration (particularly secondary oxidation). It is an unpopular view to suggest that as a community we have failed to police our own work and it is even more unpopular to directly question the fidelity of individual studies. However, I will make a case that we—researchers interested in understanding the factors controlling the rise of complex life—are currently facing a quality control crisis. I will make a case that secondary alteration is likely widespread in previously published geochemical studies and highlight examples were other communities have faced similar quality control calamities. Lastly, I will argue that proper sample archiving in publicly assessable museums needs to be a prerequisite for publication in all paleoredox studies.

16:45-17:00 Discussion

17:00-18:00 Poster session

09:00-09:30 The environmental and ecological context of the rise of the Ediacara Biota

Professor Mary Droser, University of California, Riverside, USA

Abstract

The advent and evolution of complex life on Earth is interpreted largely from the fossils of the Precambrian soft-bodied Ediacara Biota, which appeared and evolved during a time of dynamic biogeochemical and environmental fluctuation in the global ocean. The Ediacara Biota is historically divided into three successive Assemblages—the Avalon, the White Sea, and the Nama—which are marked by the appearance of novel biological traits and ecological strategies.  Recent research on all three assemblages at multiple localities has begun to clarify the significance of the Ediacara biota to our understanding of the development of Phanerozoic and Modern ecosystems.  Heterogeneous seafloors – or patchiness – was on par with modern oceans during the reign of the Avalon assemblage and continued through the White Sea Assemblage. This unusually variable diversity-abundance structure is likely due both to their preservation as near-snapshots of benthic communities and to original ecological differences, in particular the paucity of motile taxa and the near-lack of predation and infaunalization. The younger White Sea and Nama Assemblages further record a “second wave” of ecological innovations, including the development of bilaterian-grade animals and Phanerozoic-style ecological innovations, such as scavenging, complex reproductive strategies, increased ecospace utilization and motility. Evidence from both the fossil record as well as geochemical data suggests that there was an extinction of some taxa between the White Sea and Nama assemblages. However, emerging data suggests that a number of Ediacaran body plans survived into the Cambrian.

09:30-09:45

09:45-10:15 Clay minerals and the fossilisation of early complex life

Dr Ross P Anderson, University of Oxford, UK

Abstract

Proterozoic fossils provide the only direct evidence of early eukaryotic life. Yet they are rare, restricted to rocks where non-biomineralised remains are conserved. Compilations of Proterozoic eukaryotic fossil occurrences suggest most are found in mudstones—a clay-rich lithology known for iconic Cambrian soft-tissue fossilisation (Burgess Shale-type [BST]). Here we compare the role of clays in Cambrian BST and Proterozoic fossilisation. Experimental data suggest both berthierine and kaolinite are toxic to decay-bacteria and may promote fossilisation. X-ray diffraction (XRD) confirms BST fossils are found in rocks rich in berthierine. Moreover, novel selected-area XRD shows kaolinite to be intimately associated with BST fossil tissues. Its association with these tissues hints at early clay-organic interactions that likely promoted organic polymerisation. XRD of a similar compilation of Proterozoic fossil bearing shales reveals a contrasting pattern. Fossils with the highest preservation quality are associated with high illite content versus berthierine, suggesting that the main fossilisation control may be burial rate rather than clay mineralogy, and that most Proterozoic microfossils do not require BST conditions for fossilisation—likely a function of algae and protists being more resistant to decay than Cambrian animals. However, elemental/mineral distributions over cross-sections of fragile eukaryotic fossils and surrounding matrix from three Proterozoic localities reveal kaolinite enrichments adjacent to fossil cell-walls, similar to the association between kaolinite and BST fossils. These data suggest the conditions for Proterozoic fossilisation might be more ubiquitous than previously thought. However, to fossilise delicate forms, a small subset of Proterozoic fossil localities exhibit characteristics of BST fossilisation.

10:15-10:30 Discussion

10:30-11:00 Coffee break

11:00-11:30 The rise of bioturbation: tracking and modelling the development of the sedimentary mixed layer

Dr Lidya Tarhan, Yale University, USA

Abstract

Bioturbation—sediment mixing by burrowing animals—critically shapes seafloor ecology and sediment properties, as well as global marine biogeochemical cycling. Observation of strong bioturbation-biogeochemical feedbacks in modern marine environments suggests that the evolutionary development of bioturbation should have profoundly impacted contemporaneous biogeochemical (e.g., C, P, O and S) cycling. Stratigraphic archives indicate that the early Palaeozoic development of bioturbation was a protracted process, and that the appearance of intensively and deeply mixed sediments lagged significantly behind relatively early advances in infaunal seafloor colonization. Recent modelling work has suggested that even limited bioturbation may nonetheless have initiated an early Palaeozoic productivity crisis and ocean-wide deoxygenation. However, the precise biogeochemical impact of early Palaeozoic bioturbation has remained debated. To further address this question, I explore a new and more fully parameterized multi-component reaction-transport diagenetic model. This approach indicates that the relationship between bioturbation and both C-P-O and S cycling is complex and non-linear, and that not only intensity but style of bioturbation (e.g., biodiffusion vs. bioirrigation) influence the magnitude of P recycling and S oxidation. In this light, early Palaeozoic bioturbation—which was likely bioirrigation-dominated and characterized by relatively muted and shallow biodiffusional sediment mixing—may have initially only weakly influenced net S oxidation, while simultaneously mediating increased P recycling. Moreover, porosity—a parameter that, although rarely explored in diagenetic models, is substantially impacted by bioturbation—strongly influences both these systems. Lastly, in contrast to previous studies, I find that bioturbation amplifies the sensitivity of the coupled C-P-O cycle to environmental perturbations.

11:30-11:45 Discussion

11:45-12:15 Survivorship and selection bias in the Cambrian explosion and their role in its structure

Professor Graham Budd, Uppsala University, Sweden

Abstract

Big evolutionary events such as the Cambrian Explosion have inspired many attempts at explanation – why do they happen when they do? What shapes them, and why do they eventually come to an end? Or, more generally, simply what causes them? However, much less attention has been paid to the idea of a “null hypothesis” – that certain features of such diversifications arise simply through their statistical structure. When we look back from our own perspective to the origins of large groups such as the arthropods, or even the animals themselves, we will see many features that look causal but are in fact inevitable. For example, such large clades tend to be characterised by a burst of morphological innovation at their base, which has then often (and arguably invalidly) been used as an explanation for the subsequent success of the group (the “key innovations” concept). This is not necessarily to say that such events do not have causes, but that we need to be rather careful in trying to understand what it is we can actually determine merely from the patterns we see in the fossil record.

Typical sorts of features that might be affected by such biases include the early rates of diversification (the so-called “push of the past”), the rate of establishment of “body plans” and the overall timing of such events.  The Cambrian explosion exemplifies many of these issues, and understanding them is therefore essential to perceiving what its fossil record may (and may not) be telling us.

12:15-12:30 Discussion

13:30-14:00 Ediacaran stratigraphy and global correlation

Professor Maoyan Zhu, Chinese Academy of Sciences, China

Abstract

The Ediacaran Period (635-541 Ma) was a critical time interval witnessed of unprecedented change in the Earth system from the late Cryogenian snowball Earth glaciation to the Cambrian explosion of animals. Despite intense studies in past two decades, efforts to subdivide the Ediacaran Period has faltered due to a lack of robust global correlation tools, thus different chronostratigraphic models have proposed. Consequently, the uncertainty of the Ediacaran chronostratigraphic framework has severely hampered our understanding of the causal relations between the evolution of complex life and changes of the global climates and seawater chemistry in this critical interval of the Earth history.

Beginning with an introduction of the current status of the global Ediacaran time scale, this presentation will review recent advances and problems in the investigation of the Ediacaran stratigraphy of South China, one of the key and intensively investigated areas in the world, focusing on correlation and age constrains of the carbon isotope excursions from sections in various facies, in particular the debate about the DOUNCE excursion. Through integrate approach, the presentation will attempt to testify different correlation models of the global Ediacaran stratigraphy, particularly the synchronicity of the DOUNCE/Shuram/Wonoka excursions and diachronous occurrence of the late Ediacaran glaciations worldwide.

14:00-14:15 Discussion

14:15-14:45 Developing a spatio-temporal scaffolding for the integration of data and data-model comparison

Dr Daniel Condon, British Geological Survey, UK

14:45-15:00 Discussion

15:00-15:30 Coffee break

15:30-16:00 Everything about Neoproterozoic environmental changes and animal evolution (except for oxygen)

Professor Erik Sperling, Stanford University, USA

Abstract

Animals originated and evolved during a unique time in Earth history – the Neoproterozoic Era. Considering environmental conditions, geochemical data have long suggested that animals evolved in a relatively low oxygen ocean. Here, we present new analyses of sedimentary total organic carbon contents in shales suggesting that the Neoproterozoic ocean may also have had lower primary productivity – or at least lower quantities of organic carbon reaching the seafloor – compared with the Phanerozoic. Indeed, recent modeling efforts suggest that low primary productivity is an expected corollary of a low-O2 world. Comparing animal ecological responses across natural gradients of both food supply and oxygen levels in the modern ocean reveals obvious similarities, suggesting that precise causality could be difficult to infer if both food supply and oxygen changed in lockstep. In this light, we propose the fire triangle metaphor for environmental influences on early animal evolution. Moving toward consideration of all metabolically important aspects of the Cambrian radiation (fuel, heat, and oxidant) will ultimately lead to a more holistic view of the event. In particular, given the effects of temperature on animal metabolism (‘temperature-dependent hypoxia’), the role of oxygen in early animal evolution cannot be considered independent of temperature. Trait-based ecophysiological frameworks are consequently explored as a pathway to understand the impact of multiple environmental parameters on early animal communities. 

16:00-16:15 Discussion

16:15-17:00 Panel discussion