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Evolving multicellularity is easy, especially in phototrophs and osmotrophs whose multicells feed like unicells. Evolving animals was much harder and unique; probably only one pathway via benthic ‘zoophytes’ with pelagic ciliated larvae allowed trophic continuity from phagocytic protozoa to gut-endowed animals. Choanoflagellate protozoa produced sponges. Converting sponge flask cells mediating larval settling to synaptically controlled nematocysts arguably made Cnidaria. I replace Haeckel's gastraea theory by a sponge/coelenterate/bilaterian pathway: Placozoa, hydrozoan diploblasty and ctenophores were secondary; stem anthozoan developmental mutations arguably independently generated coelomate bilateria and ctenophores. I emphasize animal origin's conceptual aspects (selective, developmental) related to feeding modes, cell structure, phylogeny of related protozoa, sequence evidence, ecology and palaeontology. Epithelia and connective tissue could evolve only by compensating for dramatically lower feeding efficiency that differentiation into non-choanocytes entails. Consequentially, larger bodies enabled filtering more water for bacterial food and harbouring photosynthetic bacteria, together adding more food than cell differentiation sacrificed. A hypothetical presponge of sessile triploblastic sheets (connective tissue sandwiched between two choanocyte epithelia) evolved oogamy through selection for larger dispersive ciliated larvae to accelerate benthic trophic competence and overgrowing protozoan competitors. Extinct Vendozoa might be elaborations of this organismal grade with choanocyte-bearing epithelia, before poriferan water channels and cnidarian gut/nematocysts/synapses evolved. This article is part of the themed issue ‘Evo-devo in the genomics era, and the origins of morphological diversity’.

In recent years, scientists have made remarkable progress reconstructing the animal phylogeny. There is broad agreement regarding many deep animal relationships, including the monophyly of animals, Bilateria, Protostomia, Ecdysozoa, and Spiralia. This stability now allows researchers to articulate the diminishing number of remaining questions in terms of well-defined alternative hypotheses. These remaining questions include relationships at the base of the animal tree, the position of Xenacoelomorpha, and the internal relationships of Spiralia. Recent progress in the field of animal phylogeny has important implications for our understanding of the evolution of development, morphology, genomes, and other characters. A remarkable pattern emerges-there is far more homoplasy for all these characters than had previously been anticipated, even among many complex characters such as segmentation and nervous systems. The fossil record dates most deep branches of the animal tree to an evolutionary radiation in the early Cambrian with roots in the Late Neoproterozoic.

Diverse bilatería clades emerged apparently within a few million years during the early Cambrian, and various environmental, developmental, and ecological causes have been proposed to explain this abrupt appearance. A compilation of the patterns of fossil and molecular diversification, comparative developmental data, and information on ecological feeding strategies indicate that the major animal clades diverged many tens of millions of years before their first appearance in the fossil record, demonstrating a macroevolutionary lag between the establishment of their developmental toolkits during the Cryogenian (850 to 635 million years ago Ma) and the later ecological success of metazoans during the Ediacaran (635 to 541 Ma) and Cambrian (541 to 488 Ma) periods. We argue that this diversification involved new forms of developmental regulation, as well as innovations in networks of ecological interaction within the context of permissive environmental circumstances.

Abstract The organization of genomes into chromosomes is critical for processes such as genetic recombination, environmental adaptation, and speciation. All animals with bilateral symmetry inherited a genome structure from their last common ancestor that has been highly conserved in some taxa but seemingly unconstrained in others. However, the evolutionary forces driving these differences and the processes by which they emerge have remained largely uncharacterized. Here, we analyze genome organization across the phylum Annelida using 23 chromosome-level annelid genomes. We find that while many annelid lineages have maintained the conserved bilaterian genome structure, the Clitellata, a group containing leeches and earthworms, possesses completely scrambled genomes. We develop a rearrangement index to quantify the extent of genome structure evolution and show that, compared to the last common ancestor of bilaterians, leeches and earthworms have among the most highly rearranged genomes of any currently sampled species. We further show that bilaterian genomes can be classified into two distinct categories—high and low rearrangement—largely influenced by the presence or absence, respectively, of chromosome fission events. Our findings demonstrate that animal genome structure can be highly variable within a phylum and reveal that genome rearrangement can occur both in a gradual, stepwise fashion, or rapid, all-encompassing changes over short evolutionary timescales.

The origin of motility in bilaterian animals represents an evolutionary innovation that transformed the Earth system. This innovation probably occurred in the late Ediacaran period—as evidenced by an abundance of trace fossils (ichnofossils) dating to this time, which include trails, trackways and burrows 1 – 3 . However, with few exceptions 4 – 8 , the producers of most of the late Ediacaran ichnofossils are unknown, which has resulted in a disconnection between the body- and trace-fossil records. Here we describe the fossil of a bilaterian of the terminal Ediacaran period (dating to 551–539 million years ago), which we name Yilingia spiciformis (gen. et sp. nov). This body fossil is preserved along with the trail that the animal produced during a death march. Yilingia is an elongate and segmented bilaterian with repetitive and trilobate body units, each of which consists of a central lobe and two posteriorly pointing lateral lobes, indicating body and segment polarity. Yilingia is possibly related to panarthropods or annelids, and sheds light on the origin of segmentation in bilaterians. As one of the few Ediacaran animals demonstrated to have produced long and continuous trails, Yilingia provides insights into the identity of the animals that were responsible for Ediacaran trace fossils. Yilingia spiciformis , a bilaterian dating to the Ediacaran period, is described from body fossils associated with trails produced by the animal, shedding light on the origins of segmentation and motility in bilaterian animals.

Recent molecular clock studies date the origin of Metazoa to 750–800 million years ago (Ma), roughly coinciding with evidence from geochemical proxies that oxygen levels rose from less than 0.1% present atmospheric level (PAL) to perhaps 1–3% PAL O2. A younger origin of Metazoa would require greatly increased substitution rates across many clades and many genes; while not impossible, this is less parsimonious. Yet the first fossil evidence for metazoans (the Doushantuo embryos) about 600 Ma is followed by the Ediacaran fossils after 580 Ma, the earliest undisputed bilaterians at 555 Ma, and an increase in the size and morphologic complexity of bilaterians around 542 Ma. This temporal framework suggests a missing 150–200 Myr of early metazoan history that encompasses many apparent novelties in the early evolution of the nervous system. This span includes two major glaciations, and complex marine geochemical changes including major changes in redox and other environmental changes. One possible resolution is that animals of these still unknown Cryogenian and early Ediacaran ecosystems were relatively simple, with highly conserved developmental genes involved in cell-type specification and simple patterning. In this model, complex nervous systems are a convergent phenomenon in bilaterian clades which occurred close to the time that larger metazoans appeared in the fossil record.

A richly illustrated celebration of the mysterious world of worms in science and culture. This book celebrates the mysterious world of worms from gardens to toothaches and beyond. Kevin Butt introduces all manner of worms, including many that bear only superficial resemblance to our limbless, sinuous friends in the dirt. To trace the intimate history between worms and people, he discusses worms that live in bodies, soil, and water as well as worms from literature and mythology. Throughout the ages, worms have been portrayed as benign, even beautiful, yet at other times spitefully ostracized as deadly creatures. This richly illustrated book looks at the microscopic and the very large indeed, asking what the future holds for both human- and worm-kind.

There is more than one way to develop neuronal complexity, and animals frequently use different molecular toolkits to achieve similar functional outcomes. Genomics and metabolomics data from basal metazoans suggest that neural signalling evolved independently in ctenophores and cnidarians/bilaterians. This polygenesis hypothesis explains the lack of pan-neuronal and pan-synaptic genes across metazoans, including remarkable examples of lineage-specific evolution of neurogenic and signalling molecules as well as synaptic components. Sponges and placozoans are two lineages without neural and muscular systems. The possibility of secondary loss of neurons and synapses in the Porifera/Placozoa clades is a highly unlikely and less parsimonious scenario. We conclude that acetylcholine, serotonin, histamine, dopamine, octopamine and gamma-aminobutyric acid (GABA) were recruited as transmitters in the neural systems in cnidarian and bilaterian lineages. By contrast, ctenophores independently evolved numerous secretory peptides, indicating extensive adaptations within the clade and suggesting that early neural systems might be peptidergic. Comparative analysis of glutamate signalling also shows numerous lineage-specific innovations, implying the extensive use of this ubiquitous metabolite and intercellular messenger over the course of convergent and parallel evolution of mechanisms of intercellular communication. Therefore: (i) we view a neuron as a functional character but not a genetic character, and (ii) any given neural system cannot be considered as a single character because it is composed of different cell lineages with distinct genealogies, origins and evolutionary histories. Thus, when reconstructing the evolution of nervous systems, we ought to start with the identification of particular cell lineages by establishing distant neural homologies or examples of convergent evolution. In a corollary of the hypothesis of the independent origins of neurons, our analyses suggest that both electrical and chemical synapses evolved more than once.

Ecosystem-based management (EBM) is a framework for managing marine resources. EBM strategies can be evaluated with ecosystem models that represent functional components of ecosystems, including anthropogenic factors. Foodwebs are at the core of ecosystem models, but because dietary data can be difficult to obtain, they are often coarsely characterised. High-throughput DNA sequencing (HTS) of diets is a rapid way to parameterise foodwebs at enhanced taxonomic resolution, and potentially, to optimise the functioning of ecosystem models. We evaluated the relative merits of microscopic and HTS analyses of the diets of 8 fish species harvested in Australia’s most intensive fishery, viz. the southeast trawl fishery. We compare the taxonomic resolution and phylogenetic breadth of diets yielded by these methods and include a comparison of 3 DNA barcoding markers (mtDNA COX1 Minibar, mtDNA 16S Chord-cephA, nDNA 18S Bilateria). Using paired individual gut samples (n = 151), we demonstrate that HTS typically identified similar taxon richness but at significantly higher taxonomic resolution than microscopy. However, DNA barcode selection significantly affected both the resolution and phylogenetic breadth of estimated diets. Both COX1 Minibar and 16S Chord-cephA barcodes provided higher taxonomic resolution than morphological analysis, but the resolution varied between taxonomic groups primarily due to availabilities of reference data. However, neither barcode recovered the full dietary spectrum revealed by the 18S Bilateria barcode. HTS also revealed the presence of dietary items not previously recorded for target species, and diverse parasite assemblages. We conclude that HTS has the potential to improve structure and function of ecosystem models and to facilitate best-practice EBM.

Comparative analysis of the genomes of one mollusc ( Lottia gigantea ) and two annelids ( Capitella teleta and Helobdella robusta ) enable a more complete reconstruction of genomic features of the last common ancestors of protostomes, bilaterians and metazoans; against this conserved background they provide the first glimpse into lineage-specific evolution and diversity of the lophotrochozoans. Genomes record emergence of bilaterals This paper presents the draft genome sequences of two annelids — a freshwater leech ( Helobdella robusta ) and a bristly, segmented marine worm ( Capitella teleta ) — and a mollusc, the owl limpet ( Lottia gigantea ). These two phyla account for nearly one-third of known marine species and are of importance both ecologically and as experimental systems, yet they have not been served well by genomics efforts. Comparison of these genomes with those already available reveal some of the genetic changes linked to the origin and diversification of bilateral animals that are thought to have evolved during the 'Cambrian explosion' of multicellular life around 500 million years ago. Current genomic perspectives on animal diversity neglect two prominent phyla, the molluscs and annelids, that together account for nearly one-third of known marine species and are important both ecologically and as experimental systems in classical embryology 1 , 2 , 3 . Here we describe the draft genomes of the owl limpet ( Lottia gigantea ), a marine polychaete ( Capitella teleta ) and a freshwater leech ( Helobdella robusta ), and compare them with other animal genomes to investigate the origin and diversification of bilaterians from a genomic perspective. We find that the genome organization, gene structure and functional content of these species are more similar to those of some invertebrate deuterostome genomes (for example, amphioxus and sea urchin) than those of other protostomes that have been sequenced to date (flies, nematodes and flatworms). The conservation of these genomic features enables us to expand the inventory of genes present in the last common bilaterian ancestor, establish the tripartite diversification of bilaterians using multiple genomic characteristics and identify ancient conserved long- and short-range genetic linkages across metazoans. Superimposed on this broadly conserved pan-bilaterian background we find examples of lineage-specific genome evolution, including varying rates of rearrangement, intron gain and loss, expansions and contractions of gene families, and the evolution of clade-specific genes that produce the unique content of each genome.