Emergence of eukaryotes to the Cambrian explosion

Emergence of eukaryotes

The divide between prokaryote and eukaryote cells can be regarded as one of the largest discontinuities within the living world. Eukaryotes differ from prokaryotes in the presence of a proper membrane-bound nucleus (to contain the DNA in genes arranged on chromosomes) plus a generally larger cell size and presence of cell organelles such as mitochondria and chloroplasts. Reproduction in eukaryotes may be asexual, involving strictly controlled cell division by mitosis, or sexual, involving strictly controlled cell division by meiosis.

The Serial Endosymbiotic Theory of cell evolution (e.g. Margulis 1981; Fig. 7.1a) argues that the remarkable complexity of the eukaryotic cell was assembled over a long time period by symbiotic associations between different kinds of prokaryotes and an amito-chondriate protozoa host. Purple bacteria were perhaps acquired first to provide the mitochondrial organelles, while photosynthetic prokaryotes such as coccoid cyanobacteria and their relatives were probably acquired last to form chloroplasts.

The Neomuran Hypothesis (Cavalier-Smith 2002) modifies the serial endosymbiotic theory and argues for the aggregation of the DNA and the formation of a primitive nuclear membrane in an ancestral gramnegative eubacterium, to form a nucleate pre-eukaryote (Fig. 7.1b). This form had the key evolutionary innovation of a flexible cell wall which separated it from the Archaea and allowed a phagotrophic (feeding by engulfing) mode of life. Through phagocytosis the symbiotic acquisition of mitochondria in ciliate and aciliate pre-eukaryotic forms led to the Amoebozoa. Secondary symbiotic acquisition of chloroplasts in an aciliate amoebozoan produced the common ancestor of all plants. This hypothesis predicts that mitochondria were present in the common ancestors of all living eukaryotes and that anaerobic eukaryotes must have lost their mitochondria. It also allows for the relatively rapid acquisition of the eukaryotic grade of organization.

A billion years of environmental stability

Extremely long periods of nutrient and climatic stability may have been needed for the host-symbiont relationship to become fused into a single eukaryotic organism. This is because the relationship between symbiont and host can be easily destroyed by strong physical perturbations. Indeed, there appears to have been nearly a billion years of environmental stability between at least 2 Ga and 1 Ga ago, when ice ages are unknown and 513Corg isotopic values barely departed from the mean. It was during this interval that the complex organization of the eukaryotes was evolving (Brasier 2000; Fig. 7.2).

Evidence for the earliest eukaryotes

Controversial evidence for the emergence of eukaryotes is provided by macroscopic carbonaceous compression fossils, interpreted as the remains of an algal megaflora. Spiral ribbons of Grypania have been reported from rocks supposedly as old as 2.1 Ga (Fig. 7.3a; Han & Runnegar 1992), although comparable remains do not reappear for another 700 Ma. These structures grade from ribbons to large sack-shaped structures that some regard as the envelopes of cyanobacterial colonies such as Nostoc.

The gradual emergence of eukaryote organization by about 1.8 Ga is suggested by acritarchs of >60 |im

(a) Serial Endosymbiosis hypothesis

Cyanobacterium

Neomuran hypothesis

Plants

Cyanobacterium

Mitochondria

Aerobic bacterium

Aerobic bacterium

Neomuran hypothesis

Plants

Aclllate amoebozoan

Aquisition of mitochondria

Aclllate amoebozoan

Ciliate amoebozoan

Flagellum

Aquisition of mitochondria

Flagellum

Spirochaete bacterium

Spirochaete bacterium

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