Ecological drivers of the Ediacaran-Cambrian diversification of Metazoa

Emergence and maintenance of stable coexistence during a long-term multicellular evolution experiment

The evolution of multicellular life spurred evolutionary radiations, fundamentally changing many of Earth's ecosystems. Yet little is known about how early steps in the evolution of multicellularity affect eco-evolutionary dynamics. Through long-term experimental evolution, we observed niche partitioning and the adaptive divergence of two specialized lineages from a single multicellular ancestor. Over 715 daily transfers, snowflake yeast were subjected to selection for rapid growth, followed by selection favouring larger group size. Small and large cluster-forming lineages evolved from a monomorphic ancestor, coexisting for over ~4,300 generations, specializing on divergent aspects of a trade-off between growth rate and survival. Through modelling and experimentation, we demonstrate that coexistence is maintained by a trade-off between organismal size and competitiveness for dissolved oxygen. Taken together, this work shows how the evolution of a new level of biological individuality can rapidly drive adaptive diversification and the expansion of a nascent multicellular niche, one of the most historically impactful emergent properties of this evolutionary transition.

Jurassic paleosurfaces with fecal mounds reveal the last supper of arenicolid worms

Exceptional paleosurfaces preserving fecal casting mounds occur in the Upper Jurassic Lastres Formation of Spain. As in modern shorelines, these biogenic structures are associated with straight to sinuous-crested ripples showing the interplay of biological and physical processes in a low-energy marine environment. These trace fossils display characteristics, distribution, and densities like those of modern arenicolid populations (approximately 35 specimens per m2). Under close examination, these fecal casting mounds are morphologically undistinguishable from those produced by recent arenicolids (e.g. Arenicola marina, Abarenicola pacifica), providing evidence of the presence of these polychaetes in the Late Jurassic. As their modern counterparts, fossil arenicolids very likely modified their environment generating a seabed topography and impacting ancient benthic communities, sediment characteristics, and sediment biogeochemistry. Although the presence of oxic microhabitats and biogeochemical processes cannot be accurately measured in the fossil record, comparison with the work of modern populations allows to make inferences on sediment reworking and bioirrigation potential. In addition, association with grazing trails supports the idea of fertilization and modulation of food resources to other species. These paleosurfaces underscore the significance of high-fidelity snapshots in the fossil record (true substrates) to reconstruct past ecologies and sediment biogeochemistry. A new ichnotaxon, Cumulusichnus asturiensis n. igen. and n. isp., is defined.

Emergence and maintenance of stable coexistence during a long-term multicellular evolution experiment

The evolution of multicellular life spurred evolutionary radiations, fundamentally changing many of Earth’s ecosystems. Yet little is known about how early steps in the evolution of multicellularity transform eco-evolutionary dynamics, e.g., via niche expansion processes that may facilitate coexistence. Using long-term experimental evolution in the snowflake yeast model system, we show that the evolution of multicellularity drove niche partitioning and the adaptive divergence of two distinct, specialized lineages from a single multicellular ancestor. Over 715 daily transfers, snowflake yeast were subject to selection for rapid growth in rich media, followed by selection favoring larger group size. Both small and large cluster-forming lineages evolved from a monomorphic ancestor, coexisting for over ~4,300 generations. These small and large sized snowflake yeast lineages specialized on divergent aspects of a trade-off between growth rate and survival, mirroring predictions from ecological theory. Through modeling and experimentation, we demonstrate that coexistence is maintained by a trade-off between organismal size and competitiveness for dissolved oxygen. Taken together, this work shows how the evolution of a new level of biological individuality can rapidly drive adaptive diversification and the expansion of a nascent multicellular niche, one of the most historically-impactful emergent properties of this evolutionary transition.

Branching archaeocyaths as ecosystem engineers during the Cambrian radiation

The rapid origination and diversification of major animal body plans during the early Cambrian coincide with the rise of Earth's first animal-built framework reefs. Given the importance of scleractinian coral reefs as ecological facilitators in modern oceans, we investigate the impact of archaeocyathan (Class Archaeocyatha) reefs as engineered ecosystems during the Cambrian radiation. In this study, we present the first high-resolution, three-dimensional (3D) reconstructions of branching archaeocyathide (Order Archaeocyathida) individuals from three localities on the Laurentian paleocontinent. Because branched forms in sponges and corals display phenotypic plasticity that preserve the characteristics of the surrounding growth environment, we compare morphological measurements from our fossil specimens to those of modern corals to infer the surface conditions of Earth's first reefs. These data demonstrate that archaeocyaths could withstand and influence the flow of water, accommodate photosymbionts, and build topographically complex and stable structures much like corals today. We also recognize a stepwise increase in the roughness of reef environments in the lower Cambrian, which would have laid a foundation for more abundant and diverse coevolving fauna.

Assessing the expansion of the Cambrian Agronomic Revolution into fan-delta environments

The intensity, extent, and ecosystem-level impact of bioturbation (i.e. Agronomic Revolution) at the dawn of the Phanerozoic is a hotly debated issue. Middle Cambrian fan-delta deposits in southwestern Saskatchewan provide insights into the paleoenvironmental extent of the Agronomic Revolution into marginal-marine environments. The studied deposits reveal that several environmental stressors had direct impact on trace-fossil distribution and bioturbation intensities in Cambrian fan deltas. Basal and proximal subaerial deposits are characterized by very coarse grain size and absence of bioturbation. Mid-fan and fan-toe deposits were formed under subaqueous conditions and are characterized by rapid bioturbation events in between sedimentation episodes when environmental stressors were ameliorated, providing evidence of a significant landward expansion of the Agronomic Revolution. Transgressive marine deposits accumulated after the abandonment of the fan-delta system display high levels of bioturbation intensity, reflecting stable environmental conditions that favored endobenthic colonization. The presence of intense bioturbation in both subaqueous fan delta and transgressive deposits provides further support to the view that Cambrian levels of biogenic mixing were high, provided that stable environmental conditions were reached. Our study underscores the importance of evaluating sedimentary facies changes to assess the impact of environmental factors prior to making evolutionary inferences.

The interplay of environmental constraints and bioturbation on matground development along the marine depositional profile during the Ordovician Radiation

This study documents the distribution of matgrounds in a wide variety of environments recorded in the Ordovician Lashkerak and Ghelli Formations in the Alborz Mountains of northern Iran in order to evaluate controls on their distribution along the marine depositional profile. Detailed facies analysis allowed differentiating three groups of facies associations in the Lower to Upper Ordovician deposits of the Lashkerak formation: (i) estuarine system; (ii) wave-dominated shoreface-offshore complex; and (iii) mixed river- and wave-influenced deltaic system. The Middle to Upper Ordovician deposits of the Ghelli formation are divided into two groups of facies associations: (i) tide-influenced deltaic succession and (ii) deep-water fan system. Microbially induced sedimentary structures (MISS) are present in deposits formed in the central estuarine basin (Lashkerak formation) and in proximal lobes and lobe fringes of deep-water turbidite fans (Ghelli formation). On the contrary, MISS are absent in deposits from the wave-dominated shoreface-offshore complex, river- and tide-dominated deltas, and various subenvironments of the incised wave-dominated estuary (i.e., bayhead delta and estuary mouth) and the deep-marine turbidite fan system (i.e., turbidite channel, slope, and outer lobe). The lack of evidence of mat-building microorganisms in the deltaic systems may have resulted from two factors: (1) high physico-chemical stressors caused by river-induced processes, and (2) increase in degree of sediment disturbance, biodiffusion, and bioirrigation by burrowing organisms. Formation of microbial mats in the wave-dominated shoreface-offshore complex was inhibited by the activity of an abundant and diverse infauna capable of reworking the sediment. Our analysis shows that the spatial distribution of microbial mats was controlled by an interplay of environmental factors and innovations in animal-substrate interactions, mostly expressed by secular changes in bioturbation. This study supports the notion that the agronomic revolution was diachronic, with marginal-marine and deep-sea ecosystems lagging behind shallow-marine settings.

Were the First Trace Fossils Really Burrows or Could They Have Been Made by Sediment-Displacive Chemosymbiotic Organisms?

This review asks some hard questions about what the enigmatic graphoglyptid trace fossils are, documents some of their early fossil record from the Ediacaran–Cambrian transition and explores the idea that they may not have been fossils at all. Most researchers have considered the Graphoglyptida to have had a microbial-farming mode of life similar to that proposed for the fractal Ediacaran Rangeomorpha. This begs the question “What are the Graphoglyptida if not the Rangeomorpha persevering” and if so then “What if…?”. This provocative idea has at its roots some fundamental questions about how to distinguish burrows sensu-stricto from the external molds of endobenthic sediment displacive organisms.

The co-evolution of life and biogeochemical cycles in our planet

Abstract The Earth has undergone numerous geological and biological changes over billions of years. The evolution of plants and animals had a direct relationship with the elements’ changes in the atmosphere and the development of the biogeochemical cycles on Earth. The Anthropocene is the age of the Homo sapiens leaves its geological signature on the planet. Human domination and/or interference in the biogeochemical cycles results in an environmental change that affects not only ecosystems, in general, but also the biota and global biodiversity. In this way, we are creating another mass extinction event, the “sixth extinction wave” as well as transforming the ecosystems’ functions and services.

The diverse radiodont fauna from the Marjum Formation of Utah, USA (Cambrian: Drumian)

Radiodonts have long been known from Cambrian deposits preserving non-biomineralizing organisms. In Utah, the presence of these panarthropods in the Spence and Wheeler (House Range and Drum Mountains) biotas is now well-documented. Conversely, radiodont occurrences in the Marjum Formation have remained scarce. Despite the large amount of work undertaken on its diverse fauna, only one radiodont (Peytoia) has been reported from the Marjum Biota. In this contribution we quadruple the known radiodont diversity of the Marjum fauna, with the description of the youngest members of two genera, Caryosyntrips and Pahvantia, and that of a new taxon Buccaspinea cooperi gen. et sp. nov. This new taxon can be identified from its large oral cone bearing robust hooked teeth with one, two, or three cusps, and by the unique endite morphology and organisation of its frontal appendages. Appendages of at least 12 podomeres bear six recurved plate-like endites proximal to up to four spiniform distal endites. Pahvantia hastata specimens from the Marjum Formation are particularly large, but otherwise morphologically indistinguishable from the carapace elements of this species found in the Wheeler Formation. One of the two new Caryosyntrips specimens can be confidently assigned to C. camurus. The other bears the largest spines relative to appendage length recorded for this genus, and possesses endites of variable size and unequal spacing, making its taxonomic assignment uncertain. Caryosyntrips, Pahvantia, and Peytoia are all known from the underlying Wheeler Formation, whereas isolated appendages from the Spence Shale and the Wheeler Formation, previously assigned to Hurdia, are tentatively reidentified as Buccaspinea. Notably, none of these four genera occurs in the overlying Weeks Formation, providing supporting evidence of a faunal restructuring around the Drumian-Guzhangian boundary. The description of three additional nektonic taxa from the Marjum Formation further documents the higher relative proportion of free-swimming species in this biota compared to those of the Wheeler and Weeks Lagerstätten. This could be related to a moderate deepening of the basin and/or changing regional ocean circulation at this time.

Ancient life and moving fluids

Over 3.7 billion years of Earth history, life has evolved complex adaptations to help navigate and interact with the fluid environment. Consequently, fluid dynamics has become a powerful tool for studying ancient fossils, providing insights into the palaeobiology and palaeoecology of extinct organisms from across the tree of life. In recent years, this approach has been extended to the Ediacara biota, an enigmatic assemblage of Neoproterozoic soft‐bodied organisms that represent the first major radiation of macroscopic eukaryotes. Reconstructing the ways in which Ediacaran organisms interacted with the fluids provides new insights into how these organisms fed, moved, and interacted within communities. Here, we provide an in‐depth review of fluid physics aimed at palaeobiologists, in which we dispel misconceptions related to the Reynolds number and associated flow conditions, and specify the governing equations of fluid dynamics. We then review recent advances in Ediacaran palaeobiology resulting from the application of computational fluid dynamics (CFD). We provide a worked example and account of best practice in CFD analyses of fossils, including the first large eddy simulation (LES) experiment performed on extinct organisms. Lastly, we identify key questions, barriers, and emerging techniques in fluid dynamics, which will not only allow us to understand the earliest animal ecosystems better, but will also help to develop new palaeobiological tools for studying ancient life.

The origin and rise of complex life: progress requires interdisciplinary integration and hypothesis testing

Understanding of the triggers and timing of the rise of complex life ca 2100 to 720 million years ago has expanded dramatically in recent years. This theme issue brings together diverse and novel geochemical and palaeontological data presented as part of the Royal Society ‘ The origin and rise of complex life: integrating models , geochemical and palaeontological data ’ discussion meeting held in September 2019. The individual papers offer prescient insights from multiple disciplines. Here we summarize their contribution towards the goal of the meeting; to create testable hypotheses for the differing roles of changing climate, oceanic redox, nutrient availability, and ecosystem feedbacks across this profound, but enigmatic, transitional period.

The rise and early evolution of animals: where do we stand from a trace-fossil perspective?

The trace-fossil record provides a wealth of information to track the rise and early evolution of animals. It comprises the activity of both hard- and soft-bodied organisms, is continuous through the Ediacaran (635-539 Ma)- Cambrian (539-485 Ma) transition, yields insights into animal behaviour and their role as ecosystem engineers, and allows for a more refined characterization of palaeoenvironmental context. In order to unravel macroevolutionary signals from the trace-fossil record, a variety of approaches is available, including not only estimation of degree of bioturbation, but also analysis of ichnodiversity and ichnodisparity trajectories, and evaluation of the occupation of infaunal ecospace and styles of ecosystem engineering. Analysis of the trace-fossil record demonstrates the presence of motile benthic bilaterians in the Ediacaran, mostly feeding from biofilms. Although Ediacaran trace fossils are simple and emplaced at or immediately below the sediment surface, an increase in ichnofossil complexity, predation pressure, sediment disturbance and penetration depth is apparent during the terminal Ediacaran. Regardless of this increase, a dramatic rise in trace fossil diversity and disparity took place during the earliest Cambrian, underscoring that the novelty of the Fortunian (539-529 Ma) cannot be underestimated. The Fortunian still shows the persistence of an Ediacaran-style matground ecology, but is fundamentally characterized by the appearance of new trace-fossil architectural plans reflecting novel ways of interacting with the substrate. The appearance of Phanerozoic-style benthic ecosystems attests to an increased length and connectivity of the food web and improved efficiency in organic carbon transfer and nutrient recycling. A profound reorganization of the infaunal ecospace is recorded in both high-energy sand-dominated nearshore areas and low-energy mud-dominated offshore environments, during the early Cambrian, starting approximately during Cambrian Age 2 (529-521 Ma), but continuing during the rest of the early Cambrian. A model comprising four evolutionary phases is proposed to synthetize information from the Ediacaran-Cambrian trace-fossil record. The use of a rich ichnological toolbox; critical, systematic and comprehensive evaluation of the Ediacaran-Cambrian trace-fossil record; and high-resolution integration of the ichnological dataset and sedimentological information show that the advent of biogenic mixing was an important factor in fully marine environments at the dawn of the Phanerozoic.

The origin of animal body plans: a view from fossil evidence and the regulatory genome

The origins and the early evolution of multicellular animals required the exploitation of holozoan genomic regulatory elements and the acquisition of new regulatory tools. Comparative studies of metazoans and their relatives now allow reconstruction of the evolution of the metazoan regulatory genome, but the deep conservation of many genes has led to varied hypotheses about the morphology of early animals and the extent of developmental co-option. In this Review, I assess the emerging view that the early diversification of animals involved small organisms with diverse cell types, but largely lacking complex developmental patterning, which evolved independently in different bilaterian clades during the Cambrian Explosion.

Complex marine bioturbation ecosystem engineering behaviors persisted in the wake of the end-Permian mass extinction

The end-Permian mass extinction was the most severe mass extinction event of the Phanerozoic and was followed by a several million-year delay in benthic ecosystem recovery. While much work has been done to understand biotic recovery in both the body and trace fossil records of the Early Triassic, almost no focus has previously been given to analyzing patterns in ecosystem engineering complexity as a result of the extinction drivers. Bioturbation is a key ecosystem engineering behavior in marine environments, as it results in changes to resource flows and the physical environment. Thus, the trace fossil record can be used to examine the effect of the end-Permian mass extinction on bioturbating ecosystem engineers. We present a dataset compiled from previously published literature to analyze burrowing ecosystem engineering behaviors through the Permian-Triassic boundary. We report two key observations: first, that there is no loss in bioturbation ecosystem engineering behaviors after the mass extinction, and second, that these persisting behaviors include deep tier, high-impact, complex ecosystem engineering. These findings suggest that while environmental conditions may have limited deeper burrowing, complex ecosystem engineering behaviors were able to persist in the Early Triassic. Furthermore, the persistence of deep tier bioirrigated three-dimensional network burrows implies that benthic biogeochemical cycling could have been maintained at pre-extinction states in some local environments, stimulating ecosystem productivity and promoting biotic recovery in the Early Triassic.

Tempos and modes of collectivity in the history of life

Collective integration and processing of information have increased through the history of life, through both the formation of aggregates in which the entities may have very different properties and which jointly coarse-grained environmental variables (ranging from widely varying metabolism in microbial consortia to the ecological diversity of species on reefs) and through collectives of similar entities (such as cells within an organism or social groups). Such increases have been implicated in significant transitions in the history of life, including aspects of the origin of life, the generation of pangenomes among microbes and microbial communities such as stromatolites, multicellularity and social insects. This contribution provides a preliminary overview of the dominant modes of collective information processing in the history of life, their phylogenetic distribution and extent of convergence, and the effects of new modes for integrating and acting upon information on the tempo of evolutionary change.

Increase in metazoan ecosystem engineering prior to the Ediacaran-Cambrian boundary in the Nama Group, Namibia

The disappearance of the soft-bodied Ediacara biota at the Ediacaran-Cambrian boundary potentially represents the earliest mass extinction of complex life, although the precise driver(s) of this extinction remain unresolved. The 'biotic replacement' model proposes that an evolutionary radiation of metazoan ecosystem engineers in the latest Ediacaran profoundly altered marine palaeoenvironments, resulting in the extinction of Ediacara biota and setting the stage for the subsequent Cambrian Explosion. However, metazoan ecosystem engineering across the Ediacaran-Cambrian transition has yet to be quantified. Here, we test this key tenet of the biotic replacement model by characterizing the intensity of metazoan bioturbation and ecosystem engineering in trace fossil assemblages throughout the latest Ediacaran Nama Group in southern Namibia. The results illustrate a dramatic increase in both bioturbation and ecosystem engineering intensity in the latest Ediacaran, prior to the Cambrian boundary. Moreover, our analyses demonstrate that the highest-impact ecosystem engineering behaviours were present well before the onset of the Cambrian. These data provide the first support for a fundamental prediction of the biotic replacement model, and evidence for a direct link between the early evolution of ecosystem engineering and the extinction of the Ediacara biota.

Enhanced weathering as a trigger for the rise of atmospheric O2 level from the late Ediacaran to the early Cambrian

A shift toward a higher oxygen level in both ocean and atmosphere systems during the late Ediacaran to the early Cambrian has been suggested from multiple indirect proxies. However, the mechanism and magnitude of this oxidation remain unclear. To solve this issue, we measured carbon isotopes in both carbonate and organic matter as well as their trace element compositions for an Ediacaran-Cambrian sequence in the Lower Yangtze basin, South China. The δ13Corg and δ13Ccarb excursions of this sequence are coupled and can be compared with contemporaneous global carbon isotope curves. A 2‰ rise in Δ13Ccarb-org occurred from the late Ediacaran to the early Cambrian, suggesting a substantial increase in atmospheric oxygen level from 16% to 30% of the present atmospheric level (PAL). Furthermore, the distribution pattern of rare earth elements and the concentrations of water-insoluble elements in the carbonates indicate a sudden enhancement in chemical weathering of the continental crust during the early Cambrian, which may be a trigger for the rise of atmospheric O2 level. Both the supply of a large amount of nutrients due to the enhanced continental weathering and the contemporary increase of atmospheric oxygen concentrations may have promoted the appearance of large metazoans in the early Cambrian.

The Temporal and Environmental Context of Early Animal Evolution: Considering All the Ingredients of an "Explosion"

Animals originated and evolved during a unique time in Earth history-the Neoproterozoic Era. This paper aims to discuss (1) when landmark events in early animal evolution occurred, and (2) the environmental context of these evolutionary milestones, and how such factors may have affected ecosystems and body plans. With respect to timing, molecular clock studies-utilizing a diversity of methodologies-agree that animal multicellularity had arisen by ∼800 million years ago (Ma) (Tonian period), the bilaterian body plan by ∼650 Ma (Cryogenian), and divergences between sister phyla occurred ∼560-540 Ma (late Ediacaran). Most purported Tonian and Cryogenian animal body fossils are unlikely to be correctly identified, but independent support for the presence of pre-Ediacaran animals is recorded by organic geochemical biomarkers produced by demosponges. This view of animal origins contrasts with data from the fossil record, and the taphonomic question of why animals were not preserved (if present) remains unresolved. Neoproterozoic environments demanding small, thin, body plans, and lower abundance/rarity in populations may have played a role. Considering environmental conditions, geochemical data suggest 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. Combined with an inability to inhabit productive regions in a low-O2 ocean, earliest animal communities would likely have been more food limited than generally appreciated, impacting both ecosystem structure and organismal behavior. In light of this, we propose the "fire triangle" metaphor for environmental influences on early animal evolution. Moving toward consideration of all environmental aspects of the Cambrian radiation (fuel, heat, and oxidant) will ultimately lead to a more holistic view of the event.

Flatworm mucus as the base of a food web

Background

By altering their habitats, engineering species can improve their own fitness. However, the effect of this strategy on the fitness of coexisting species or on the structure of the respective food web is poorly understood. In this study, bacteria and bacterivorous nematodes with short (Caenorhabditis elegans) and long (Plectus acuminatus) life cycles were exposed to the mucus secreted by the freshwater flatworm Polycelis tenuis. The growth, reproduction, and feeding preferences of the nematodes in the presence/absence of the mucus were then determined. In addition, confocal laser scanning microscopy (CLSM) was used to examine the structural footprint of the mucus and the mucus colonization dynamics of bacteria and protozoans.

Results

Mucus exposure resulted in a greater reproductive output in P. acuminatus than in C. elegans. In a cafeteria experiment, both nematode species were attracted by bacteria-rich patches and were not deterred by mucus. CLSM showed that the flatworms spread a layer of polysaccharide-rich mucus ca. 15 µm thick from their tails. Subsequent colonization of the mucus by bacteria and protozoans resulted in an architecture that progressively resembled a complex biofilm. The presence of protozoans reduced nematode reproduction, presumably due to competition for their bacterial food supply.

Conclusion

Animal secretions such as mucus may have broader, community-level consequences and contribute to fueling microbial food webs.

Electronic supplementary material

The online version of this article (10.1186/s12898-019-0231-2) contains supplementary material, which is available to authorized users.

Trace fossils associated with Burgess Shale non-biomineralized carapaces: bringing taphonomic and ecological controls into focus

The association of trace fossils and non-biomineralized carapaces has been reported from Cambrian Lagerstätten worldwide, but the abundance, ichnodiversity, taphonomy and ecological significance of such associations have yet to be fully investigated. Two main end-member hypotheses are explored based on the study of a relatively wide variety of trace fossils preserved associated to Tuzoia carapaces from the middle Cambrian Burgess Shale in British Columbia. In the ecological Tuzoia garden hypothesis, the bacterially enriched surface of carapaces provides opportunities for intricate ecologic interactions among trophic levels. In the taphonomic shielding hypothesis, the trace fossil-carapace association results from preferential preservation of traces as controlled by compaction independent of any association in life. In an attempt to better understand the role of the carapace as a medium for preservation of trace fossils and to evaluate the effects of mechanical stress related to burial, a numerical model was developed. Results indicate that the carapace can shield underlying sediment from mechanical stress for a finite time, differentially protecting trace fossils during the initial phase of burial and compaction. However, this taphonomic model alone fails to fully explain relatively high-density assemblages displaying a diversity of structures spatially confined within the perimeter of carapaces or branching patterns recording re-visitation.

Adaptation, chance, and history in experimental evolution reversals to unicellularity

Evolution is often deemed irreversible. The evolution of complex traits that require many mutations makes their reversal unlikely. Even in simpler traits, reversals might become less likely as neutral or beneficial mutations, with deleterious effects in the ancestral context, become fixed in the novel background. This is especially true in changes that involve large reorganizations of the organism and its interactions with the environment. The evolution of multicellularity involves the reorganization of previously autonomous cells into a more complex organism; despite the complexity of this change, single cells have repeatedly evolved from multicellular ancestors. These repeated reversals to unicellularity undermine the generality of Dollo's law. In this article, we evaluated the dynamics of reversals to unicellularity from recently evolved multicellular phenotypes of the brewers yeast Saccharomyces cerevisae. Even though multicellularity in this system evolved recently, it involves the evolution of new levels of selection. Strong selective pressures against multicellularity lead to rapid reversibility to single cells in all of our replicate lines, whereas counterselection favoring multicellularity led to minimal reductions to the rates of reversal. History and chance played an important role in the tempo and mode of reversibility, highlighting the interplay of deterministic and stochastic events in evolutionary reversals.

Ediacaran scavenging as a prelude to predation

Predation is one of the most fundamental ecological and evolutionary drivers in modern and ancient ecosystems. Here, we report the discovery of evidence of the oldest scavenging of shallowly buried bodies of iconic soft-bodied members of the Ediacara Biota by cryptic seafloor mat-burrowing animals that produced the furrow and levee trace fossil, Helminthoidichnites isp. These mat-burrowers were probably omnivorous, stem-group bilaterians that largely grazed on microbial mats but when following mats under thin sands, they actively scavenged buried Dickinsonia, Aspidella, Funisia and other elements of the Ediacara Biota. These traces of opportunistic scavengers of dead animals from the Ediacaran of South Australia represent a fundamental ecological innovation and a possible pathway to the evolution of macrophagous predation in the Cambrian. While the Ediacaran oceans may have had oxygen levels too low to support typical large predators, the Helminthoidichnites maker lived in and grazed on microbial mats, which may have provided a localized source of oxygen.

Early Palaeozoic ocean anoxia and global warming driven by the evolution of shallow burrowing

The evolution of burrowing animals forms a defining event in the history of the Earth. It has been hypothesised that the expansion of seafloor burrowing during the Palaeozoic altered the biogeochemistry of the oceans and atmosphere. However, whilst potential impacts of bioturbation on the individual phosphorus, oxygen and sulphur cycles have been considered, combined effects have not been investigated, leading to major uncertainty over the timing and magnitude of the Earth system response to the evolution of bioturbation. Here we integrate the evolution of bioturbation into the COPSE model of global biogeochemical cycling, and compare quantitative model predictions to multiple geochemical proxies. Our results suggest that the advent of shallow burrowing in the early Cambrian contributed to a global low-oxygen state, which prevailed for ~100 million years. This impact of bioturbation on global biogeochemistry likely affected animal evolution through expanded ocean anoxia, high atmospheric CO₂ levels and global warming.

The extent to which the onset of bioturbation affected global biogeochemistry during the Palaeozoic remains unclear. Here, the authors integrate bioturbation into the COPSE model, compare output with geochemical proxies, and suggest shallow burrowing contributed to a global low oxygen state during the early Cambrian.

Sediment disturbance by Ediacaran bulldozers and the roots of the Cambrian explosion

Trace fossils of sediment bulldozers are documented from terminal Ediacaran strata of the Nama Group in Namibia, where they occur in the Spitskop Member of the Urusis Formation (Schwarzrand Subgroup). They consist of unilobate to bilobate horizontal to subhorizontal trace fossils describing scribbles, circles and, more rarely, open spirals and meanders, and displaying an internal structure indicative of active fill. Their presence suggests that exploitation of the shallow infaunal ecospace by relatively large bilaterians was already well underway at the dawn of the Phanerozoic. Efficient burrowing suggests coelom development most likely linked to metazoan body-size increase. These trace fossils are the earliest clear representatives so far recorded of sediment bulldozing, an activity that may have had a negative impact on suspension-feeding and/or osmotroph communities, as well as on matgrounds, representing early examples of ecosystem engineering and trophic-group amensalism. The occurrence of sediment bulldozers may have promoted the establishment of gradients in horizontal and vertical distribution of organic material in connection with spatially heterogeneous environments on the sea floor at a critical time in Earth evolution.

Late Neoproterozoic seawater oxygenation by siliceous sponges

The Cambrian explosion, the rapid appearance of most animal phyla in the geological record, occurred concurrently with bottom seawater oxygenation. Whether this oxygenation event was triggered through enhanced nutrient supply and organic carbon burial forced by increased continental weathering, or by species engaging in ecosystem engineering, remains a fundamental yet unresolved question. Here we provide evidence for several simultaneous developments that took place over the Ediacaran–Cambrian transition: expansion of siliceous sponges, decrease of the dissolved organic carbon pool, enhanced organic carbon burial, increased phosphorus removal and seawater oxygenation. This evidence is based on silicon and carbon stable isotopes, Ge/Si ratios, REE-geochemistry and redox-sensitive elements in a chert-shale succession from the Yangtze Platform, China. According to this reconstruction, sponges have initiated seawater oxygenation by redistributing organic carbon oxidation through filtering suspended organic matter from seawater. The resulting increase in dissolved oxygen levels potentially triggered the diversification of eumetazoans.

The Ediacaran–Cambrian oxygenation of seawater is thought to have been caused by lifeforms engaging in ecosystem engineering. Here, the authors show that siliceous sponges increased seawater dissolved oxygen concentrations by redistributing organic carbon oxidation through filtering suspended organic matter.

A trophic framework for animal origins

Metazoans emerged in a microbial world and play a unique role in the biosphere as the only complex multicellular eukaryotes capable of phagocytosis. While the bodyplan and feeding mode of the last common metazoan ancestor remain unresolved, the earliest multicellular stem-metazoans likely subsisted on picoplankton (planktonic microbes 0.2-2 μm in diameter) and dissolved organic matter (DOM), similarly to modern sponges. Once multicellular stem-metazoans emerged, they conceivably modulated both the local availability of picoplankton, which they preferentially removed from the water column for feeding, and detrital particles 2-100 μm in diameter, which they expelled and deposited into the benthos as waste products. By influencing the availability of these heterotrophic food sources, the earliest multicellular stem-metazoans would have acted as ecosystem engineers, helping create the ecological conditions under which other metazoans, namely detritivores and non-sponge suspension feeders incapable of subsisting on picoplankton and DOM, could emerge and diversify. This early style of metazoan feeding, specifically the phagocytosis of small eukaryotic prey, could have also encouraged the evolution of larger, even multicellular, eukaryotic forms less prone to metazoan consumption. Therefore, the first multicellular stem-metazoans, through their feeding, arguably helped bridge the strictly microbial food webs of the Proterozoic Eon (2.5-0.541 billion years ago) to the more macroscopic, metazoan-sustaining food webs of the Phanerozoic Eon (0.541-0 billion years ago).

Cryptic Disc Structures Resembling Ediacaran Discoidal Fossils from the Lower Silurian Hellefjord Schist, Arctic Norway

The Hellefjord Schist, a volcaniclastic psammite-pelite formation in the Caledonides of Arctic Norway contains discoidal impressions and apparent tube casts that share morphological and taphonomic similarities to Neoproterozoic stem-holdfast forms. U-Pb zircon geochronology on the host metasediment indicates it was deposited between 437 ± 2 and 439 ± 3 Ma, but also indicates that an inferred basal conglomerate to this formation must be part of an older stratigraphic element, as it is cross-cut by a 546 ± 4 Ma pegmatite. These results confirm that the Hellefjord Schist is separated from underlying older Proterozoic rocks by a thrust. It has previously been argued that the Cambrian Substrate Revolution destroyed the ecological niches that the Neoproterozoic frond-holdfasts organisms occupied. However, the discovery of these fossils in Silurian rocks demonstrates that the environment and substrate must have been similar enough to Neoproterozoic settings that frond-holdfast bodyplans were still ecologically viable some hundred million years later.

Novelty and Innovation in the History of Life

The history of life as documented by the fossil record encompasses evolutionary diversifications at scales ranging from the Ediacaran-Cambrian explosion of animal life and the invasion of land by vascular plants, insects and vertebrates to the diversification of flowering plants over the past 100 million years and the radiation of horses. Morphological novelty and innovation has been a recurrent theme. The architects of the modern synthesis of evolutionary theory made three claims about evolutionary novelty and innovation: first, that all diversifications in the history of life represent adaptive radiations; second, that adaptive radiations are driven principally by ecological opportunity rather than by the supply of new morphological novelties, thus the primary questions about novelty and innovation focus on their ecological and evolutionary success; and third, that the rate of morphological divergence between taxa was more rapid early in the history of a clade but slowed over time as ecological opportunities declined. These claims have strongly influenced subsequent generations of evolutionary biologists, yet over the past two decades each has been challenged by data from the fossil record, by the results of comparative phylogenetic analyses and through insights from evolutionary developmental biology. Consequently a broader view of novelty and innovation is required. An outstanding issue for future work is identifying the circumstances associated with different styles of diversification and whether their frequency has changed through the history of life.

Biotic replacement and mass extinction of the Ediacara biota

The latest Neoproterozoic extinction of the Ediacara biota has been variously attributed to catastrophic removal by perturbations to global geochemical cycles, 'biotic replacement' by Cambrian-type ecosystem engineers, and a taphonomic artefact. We perform the first critical test of the 'biotic replacement' hypothesis using combined palaeoecological and geochemical data collected from the youngest Ediacaran strata in southern Namibia. We find that, even after accounting for a variety of potential sampling and taphonomic biases, the Ediacaran assemblage preserved at Farm Swartpunt has significantly lower genus richness than older assemblages. Geochemical and sedimentological analyses confirm an oxygenated and non-restricted palaeoenvironment for fossil-bearing sediments, thus suggesting that oxygen stress and/or hypersalinity are unlikely to be responsible for the low diversity of communities preserved at Swartpunt. These combined analyses suggest depauperate communities characterized the latest Ediacaran and provide the first quantitative support for the biotic replacement model for the end of the Ediacara biota. Although more sites (especially those recording different palaeoenvironments) are undoubtedly needed, this study provides the first quantitative palaeoecological evidence to suggest that evolutionary innovation, ecosystem engineering and biological interactions may have ultimately caused the first mass extinction of complex life.

The Neoproterozoic

The Neoproterozoic era was arguably the most revolutionary in Earth history. Extending from 1000 to 541 million years ago, it stands at the intersection of the two great tracts of evolutionary time: on the one side, some three billion years of pervasively microbial 'Precambrian' life, and on the other the modern 'Phanerozoic' biosphere with its extraordinary diversity of large multicellular organisms. The disturbance doesn't stop here, however: over this same stretch of time the planet itself was in the throes of change. Tectonically, it saw major super-continental reconfigurations, climatically its deepest ever glacial freeze, and geochemically some of the most anomalous perturbations on record. What lies behind this dramatic convergence of biological and geological phenomena, and how exactly did it give rise to the curiously complex world that we now inhabit?

Suspension feeding in the enigmatic Ediacaran organism Tribrachidium demonstrates complexity of Neoproterozoic ecosystems

The first diverse and morphologically complex macroscopic communities appear in the late Ediacaran period, 575 to 541 million years ago (Ma). The enigmatic organisms that make up these communities are thought to have formed simple ecosystems characterized by a narrow range of feeding modes, with most restricted to the passive absorption of organic particles (osmotrophy). We test between competing feeding models for the iconic Ediacaran organism Tribrachidium heraldicum using computational fluid dynamics. We show that the external morphology of Tribrachidium passively directs water flow toward the apex of the organism and generates low-velocity eddies above apical "pits." These patterns of fluid flow are inconsistent with osmotrophy and instead support the interpretation of Tribrachidium as a passive suspension feeder. This finding provides the oldest empirical evidence for suspension feeding at 555 to 550 Ma, ~10 million years before the Cambrian explosion, and demonstrates that Ediacaran organisms formed more complex ecosystems in the latest Precambrian, involving a larger number of ecological guilds, than currently appreciated.

A public goods approach to major evolutionary innovations

The history of life is marked by a small number of major transitions, whether viewed from a genetic, ecological, or geological perspective. Specialists from various disciplines have focused on the packaging of information to generate new evolutionary individuals, on the expansion of ecological opportunity, or the abiotic drivers of environmental change to which organisms respond as the major drivers of these episodes. But the critical issue for understanding these major evolutionary transitions (METs) lies in the interactions between environmental, ecologic, and genetic change. Here, I propose that public goods may serve as one currency of such interactions: biological products that are non-excludable and non-rivalrous. Such biological public goods may be involved in either the generation of new evolutionary variation, as with genetic sequences that are easily transferred between different microbial lineages, or in the construction of new ecological niches, as with the progressive oxygenation of the oceans and atmosphere. Attention to public goods emphasizes the processes by which organisms actively construct their own evolutionary opportunities. Such public goods may have facilitated some METs.

Diversity partitioning during the Cambrian radiation

The fossil record offers unique insights into the environmental and geographic partitioning of biodiversity during global diversifications. We explored biodiversity patterns during the Cambrian radiation, the most dramatic radiation in Earth history. We assessed how the overall increase in global diversity was partitioned between within-community (alpha) and between-community (beta) components and how beta diversity was partitioned among environments and geographic regions. Changes in gamma diversity in the Cambrian were chiefly driven by changes in beta diversity. The combined trajectories of alpha and beta diversity during the initial diversification suggest low competition and high predation within communities. Beta diversity has similar trajectories both among environments and geographic regions, but turnover between adjacent paleocontinents was probably the main driver of diversification. Our study elucidates that global biodiversity during the Cambrian radiation was driven by niche contraction at local scales and vicariance at continental scales. The latter supports previous arguments for the importance of plate tectonics in the Cambrian radiation, namely the breakup of Pannotia.

Biogeomorphic Impacts of Invasive Species

Invasive species, often recognized as ecosystem engineers, can dramatically alter geomorphic processes and landforms. Our review shows that the biogeomorphic impacts of invasive species are common, but variable in magnitude or severity, ranging from simple acceleration or deceleration of preexisting geomorphic processes to landscape metamorphosis. Primary effects of invasive flora are bioconstruction and bioprotection, whereas primary effects of invasive fauna are bioturbation, bioerosion, and bioconstruction. Land-water interfaces seem particularly vulnerable to biogeomorphic impacts of invasive species. Although not different from biogeomorphic impacts in general, invasive species are far more likely to lead to major geomorphic changes or landscape metamorphosis, which can have long-lasting impacts. In addition, invasive species can alter selection pressures in both macroevolution and microevolution by changing geomorphic processes. However, the differing timescales of biological invasions, landscape evolution, and biological evolution complicate assessment of the evolutionary impacts of invasive organisms.

Oxygen and animal evolution: did a rise of atmospheric oxygen "trigger" the origin of animals?

Recent studies challenge the classical view that the origin of animal life was primarily controlled by atmospheric oxygen levels. For example, some modern sponges, representing early-branching animals, can live under 200 times less oxygen than currently present in the atmosphere - levels commonly thought to have been maintained prior to their origination. Furthermore, it is increasingly argued that the earliest animals, which likely lived in low oxygen environments, played an active role in constructing the well-oxygenated conditions typical of the modern oceans. Therefore, while oxygen is still relevant to understanding early animal evolution, the relationships between the two might be less straightforward than previously thought.

Decoupling of body-plan diversification and ecological structuring during the Ediacaran-Cambrian transition: evolutionary and geobiological feedbacks

The rapid appearance of bilaterian clades at the beginning of the Phanerozoic is one of the most intriguing topics in macroevolution. However, the complex feedbacks between diversification and ecological interactions are still poorly understood. Here, we show that a systematic and comprehensive analysis of the trace-fossil record of the Ediacaran-Cambrian transition indicates that body-plan diversification and ecological structuring were decoupled. The appearance of a wide repertoire of behavioural strategies and body plans occurred by the Fortunian. However, a major shift in benthic ecological structure, recording the establishment of a suspension-feeder infauna, increased complexity of the trophic web, and coupling of benthos and plankton took place during Cambrian Stage 2. Both phases were accompanied by different styles of ecosystem engineering, but only the second one resulted in the establishment of the Phanerozoic-style ecology. In turn, the suspension-feeding infauna may have been the ecological drivers of a further diversification of deposit-feeding strategies by Cambrian Stage 3, favouring an ecological spillover scenario. Trace-fossil information strongly supports the Cambrian explosion, but allows for a short time of phylogenetic fuse during the terminal Ediacaran-Fortunian.

Niche construction theory: a practical guide for ecologists

Niche construction theory (NCT) explicitly recognizes environmental modication by organisms ("niche construction") and their legacy overtime ("ecological inheritance") to be evolutionary processes in their own right. Here we illustrate how niche construction theory provides usedl conceptual tools and theoretical insights for integrating ecosystem ecology and evolutionary theory. We begin by briefly describing NCT, and illustrating how it deifers from conventional evolutionary approaches. We then distinguish between two aspects ofniche construction--environment alteration and subsequent evolution in response to constructed environments--equating the first of these with "ecosystem engineering." We describe some of the ecological and evolutionary impacts on ecosystems of niche construction, ecosystem engineering and ecological inheritance, and illustrate how these processes trigger ecological and evolutionary feedbacks and leave detectable ecological signatures that are open to investigation. FIinally, we provide a practical guide to how NCT could be deployed by ecologists and evolutionary biologists to aeplore ecoeoolutionay dynamics. We suggest that, by highlighting the ecological and evolutionay ramifications of changes that organisms bring about in ecosystems, NCT helps link ecosystem ecology to evolutionary biology, potentially leading to a deeper understanding of how ecosystems change over time.

More on how and why: a response to commentaries

We are grateful to the commentators for taking the time to respond to our article. Too many interesting and important points have been raised for us to tackle them all in this response, and so in the below we have sought to draw out the major themes. These include problems with both the term 'ultimate causation' and the proximate-ultimate causation dichotomy more generally, clarification of the meaning of reciprocal causation, discussion of issues related to the nature of development and phenotypic plasticity and their roles in evolution, and consideration of the need for an extended evolutionary synthesis.

Before the endless forms: embodied model of transition from single cells to aggregates to ecosystem engineering

The emergence of complex multicellular systems and their associated developmental programs is one of the major problems of evolutionary biology. The advantages of cooperation over individuality seem well known but it is not clear yet how such increase of complexity emerged from unicellular life forms. Current multicellular systems display a complex cell-cell communication machinery, often tied to large-scale controls of body size or tissue homeostasis. Some unicellular life forms are simpler and involve groups of cells cooperating in a tissue-like fashion, as it occurs with biofilms. However, before true gene regulatory interactions were widespread and allowed for controlled changes in cell phenotypes, simple cellular colonies displaying adhesion and interacting with their environments were in place. In this context, models often ignore the physical embedding of evolving cells, thus leaving aside a key component. The potential for evolving pre-developmental patterns is a relevant issue: how far a colony of evolving cells can go? Here we study these pre-conditions for morphogenesis by using CHIMERA, a physically embodied computational model of evolving virtual organisms in a pre-Mendelian world. Starting from a population of identical, independent cells moving in a fluid, the system undergoes a series of changes, from spatial segregation, increased adhesion and the development of generalism. Eventually, a major transition occurs where a change in the flow of nutrients is triggered by a sub-population. This ecosystem engineering phenomenon leads to a subsequent separation of the ecological network into two well defined compartments. The relevance of these results for evodevo and its potential ecological triggers is discussed.