The cell

The cell is a dynamic entity. Cells are not simply building blocks that are linked together to create an organism: each cell comprises a dynamic network of interacting macromolecules. Just how dynamic has been brought home by recent advances in cell imaging technologies. A host of multisubunit molecular structures must assemble and disassemble in a highly coordinated, exquisitely regulated, and beautifully choreographed manner to ensure the integrity of the cell and provide its ability to function correctly as a single unit within a large multicellular organism. This chapter explains how the cell is the fundamental unit of all forms of independent life on this planet, from the simplest single-celled prokaryote to the most complex multicellular eukaryote.

Organically-preserved multicellular eukaryote from the early Ediacaran Nyborg Formation, Arctic Norway

Eukaryotic multicellularity originated in the Mesoproterozoic Era and evolved multiple times since, yet early multicellular fossils are scarce until the terminal Neoproterozoic and often restricted to cases of exceptional preservation. Here we describe unusual organically-preserved fossils from mudrocks, that provide support for the presence of organisms with differentiated cells (potentially an epithelial layer) in the late Neoproterozoic. Cyathinema digermulense gen. et sp. nov. from the Nyborg Formation, Vestertana Group, Digermulen Peninsula in Arctic Norway, is a new carbonaceous organ-taxon which consists of stacked tubes with cup-shaped ends. It represents parts of a larger organism (multicellular eukaryote or a colony), likely with greater preservation potential than its other elements. Arrangement of open-ended tubes invites comparison with cells of an epithelial layer present in a variety of eukaryotic clades. This tissue may have benefitted the organism in: avoiding overgrowth, limiting fouling, reproduction, or water filtration. C. digermulense shares characteristics with extant and fossil groups including red algae and their fossils, demosponge larvae and putative sponge fossils, colonial protists, and nematophytes. Regardless of its precise affinity, C. digermulense was a complex and likely benthic marine eukaryote exhibiting cellular differentiation, and a rare occurrence of early multicellularity outside of Konservat-Lagerstätten.

REAVALIAÇÃO DE FÓSSEIS DO GRUPO BAMBUÍ: IMPLICAÇÕES PALEOBIOLÓGICAS PARA O NEOPROTEROZOICO TARDIO DO BRASIL

Fósseis do Neoproterozoico tardio têm recebido grande atenção na última década por representarem profundas mudanças na biota. Tais mudanças incluem a passagem de uma biosfera dominada por formas procariontes unicelulares para formas eucariontes multicelulares. No Brasil, tem-se testemunhado muitos avanços no conhecimento sobre os fósseis desta idade, o que coloca o país na vanguarda das pesquisas paleontológicas do Neoproterozoico. Dentre as unidades brasileiras que figuram entre as que possuem este importante registro está o Grupo Bambuí, aflorante na porção central do Brasil. Fósseis têm sido identificados neste grupo desde o século XIX através de notas sobre o que hoje é conhecido como microbialitos, porém, foi na metade do século passado que o conhecimento sobre o registro fossilífero aumentou consideravelmente e passou a incluir possíveis icnofósseis, microfósseis e algas macroscópicas. No entanto, o significado destes fósseis tornou-se obsoleto, sobretudo mediante aos avanços da Paleontologia do Pré-Cambriano, ocorrida nas últimas duas décadas. Baseado na importância do registro fóssil do Grupo Bambuí e frente à eminente necessidade de sua contextualização no atual cenário de fósseis do Neoproterozoico, realizou-se uma reavaliação de fósseis descritos entre as décadas de 70 e 80 do século passado. Dos quatro táxons revistos, Kinneyia lucianoi Sommer 1970, Bambuilithos hectoris Sommer 1981 e Bambuilithos teixeranus Sommer 1982 passam a serem considerados pseudofósseis, enquanto que Bambuites erichsenii Sommer 1971 permanece classificado como morfofóssil, porém é posto em sinonímia com Leiosphaeridia jacutica (Timofeev, 1966), emend. Mikhailova & Jankauskas, 1989. A reanálise desse material traz uma nova visão sobre a paleobiologia registrada no Grupo Bambuí e atualiza seu registro no panorama mundial de fósseis do Neoproterozoico tardio.Palavras-chave: Grupo Bambuí, microfóssil, pseudofósseis, paleobiologia. Abstract: RE-EVALUATION OF FOSSILS FROM BAMBUÍ GROUP: PALEOBIOLOGICAL IMPLICATIONS FOR THE LATE NEOPROTEROZOIC OF BRAZIL. Fossils of Late Neoproterozoic have received great attention in the last decade once they represent profound changes in biota. Such changes include the passage of a prokaryote-dominated biosphere to a multicellular, eukaryote-dominated biosphere. In Brazil, one has witnessed advances in knowledge concerning fossils of this age, placing the country at the forefront of paleontological research of the Neoproterozoic. Among the Brazilian units that comprise this important record is the Bambuí Group, outcropping in the central part of Brazil. Fossils have been identified in this unit since the eighteenth century via short notes about microbialites. However, it was in the middle of the 19th century that the knowledge of the fossil record has considerably increased, and then, included possible trace fossils, microfossils and macroscopic algae. Nonetheless, such record has become obsolete, mainly by the advances of Precambrian Paleontology, occurred in the last two decades. Based on the importance of the fossil record of the Bambuí Group and through the imminent need for its context in the current Neoproterozoic fossils scenario, a re-evaluation of fossils described from the 70s and 80s of last century was performed. Of the four groups reviewed, Kinneyia lucianoi Sommer 1970, Bambuilithos hectoris Sommer 1981 and Bambuilithos teixeranus Sommer 1982 are considered pseudofossils, and Bambuites erichsenii Sommer 1971 remained as a morphofossil, and was placed in synonymy with Leiosphaeridia jacutica (Timofeev, 1966), emend. Mikhailova & Jankauskas, 1989. The re-analysis of this material provides new insight into the paleobiology recorded in the Bambuí Group and updates its record in the world panorama of the Late Neoproterozoic fossils. Keywords: Bambuí Group, microfossil, pseudofossils, paleobiology

Introduction

The environments of multicellular eukaryote organisms, including mammals, contain an extraordinary variety of extracellular signalling molecules that regulate body form and function by orchestrating cell and tissue growth, cell differentiation and the behaviour of mature cells. To control themselves properly, eukaryote cells must mount correct responses to this abundance of external chemical influences. How much of their genome of ~ 10 5 genes they commit to this task remains uncertain, but it must be substantial: maybe ~ 10 4 genes in a mammal?