Eukaryotic organisms radiated in Proterozoic oceans with oxygenated surface waters, but, commonly, anoxia at depth. Exceptionally preserved fossils of red algae favor crown group emergence more than ...1200 million years ago, but older (up to 1600-1800 million years) microfossils could record stem group eukaryotes. Major eukaryotic diversification ~800 million years ago is documented by the increase in the taxonomic richness of complex, organic-walled microfossils, including simple coenocytic and multicellular forms, as well as widespread tests comparable to those of extant testate amoebae and simple foraminiferans and diverse scales comparable to organic and siliceous scales formed today by protists in several clades. Mid-Neoproterozoic establishment or expansion of eukaryophagy provides a possible mechanism for accelerating eukaryotic diversification long after the origin of the domain. Protists continued to diversify along with animals in the more pervasively oxygenated oceans of the Phanerozoic Eon.
Cyanobacteria are the only prokaryotes to have evolved oxygenic photosynthesis, transforming the biology and chemistry of our planet. Genomic and evolutionary studies have revolutionized our ...understanding of early oxygenic phototrophs, complementing and dramatically extending inferences from the geologic record. Molecular clock estimates point to a Paleoarchean origin (3.6–3.2 billion years ago, bya) of the core proteins of Photosystem II (PSII) involved in oxygenic photosynthesis and a Mesoarchean origin (3.2–2.8 bya) for the last common ancestor of modern cyanobacteria. Nonetheless, most extant cyanobacteria diversified after the Great Oxidation Event (GOE), an environmental watershed ca. 2.45 bya made possible by oxygenic photosynthesis. Throughout their evolutionary history, cyanobacteria have played a key role in the global carbon cycle.
The core proteins of PSII involved in oxygenic photosynthesis originated during the early Archean, well before the GOE occurring around 2.3 billion years ago.Most extant cyanobacterial taxa, including the lineage leading to chloroplasts, diversified after the GOE.The evolution of modern planktonic cyanobacteria and phytoplanktonic algae reached global prominence at the end of the Precambrian, and they continue to significantly contribute to the carbon cycle.Prior to the origin of complex life, cyanobacteria were the main primary producers during most of the Proterozoic Eon.
The early evolutionary history of the chloroplast lineage remains an open question. It is widely accepted that the endosymbiosis that established the chloroplast lineage in eukaryotes can be traced ...back to a single event, in which a cyanobacterium was incorporated into a protistan host. It is still unclear, however, which Cyanobacteria are most closely related to the chloroplast, when the plastid lineage first evolved, and in what habitats this endosymbiotic event occurred. We present phylogenomic and molecular clock analyses, including data from cyanobacterial and chloroplast genomes using a Bayesian approach, with the aim of estimating the age for the primary endosymbiotic event, the ages of crown groups for photosynthetic eukaryotes, and the independent incorporation of a cyanobacterial endosymbiont by Paulinella. Our analyses include both broad taxon sampling (119 taxa) and 18 fossil calibrations across all Cyanobacteria and photosynthetic eukaryotes. Phylogenomic analyses support the hypothesis that the chloroplast lineage diverged from its closet relative Gloeomargarita, a basal cyanobacterial lineage, ∼2.1 billion y ago (Bya). Our analyses suggest that the Archaeplastida, consisting of glaucophytes, red algae, green algae, and land plants, share a common ancestor that lived ∼1.9 Bya. Whereas crown group Rhodophyta evolved in the Mesoproterozoic Era (1,600–1,000 Mya), crown groups Chlorophyta and Streptophyta began to radiate early in the Neoproterozoic (1,000–542 Mya). Stochastic mapping analyses indicate that the first endosymbiotic event occurred in low-salinity environments. Both red and green algae colonized marine environments early in their histories, with prasinophyte green phytoplankton diversifying 850–650 Mya.
...their ready availability as a food source can then limit the picoplankton population because they are eaten by other organisms, providing an opportunity for larger primary producers to grow ...instead7. ...Brocks and colleagues' observation of the Neoproterozoic expansion of nanoplanktonic green algae (those of 2-20 micrometres in diameter) probably reflected an increase in nutrient levels in the oceans of that time. Ecological models10 predict that, as a shift towards larger primary producers occurs, more energy and carbon should become available to organisms higher in the food chain, thereby providing a resource boost for early animals. ...food-source changes might have helped to pave the way for Ediacaran animal radiation. ...oxygen, long recognized as a metabolic gatekeeper for animal evolution, might also have mediated the availability of nutrients in ancient marine ecosystems.
Life: the first two billion years Knoll, Andrew H.; Bergmann, Kristin D.; Strauss, Justin V.
Philosophical transactions of the Royal Society of London. Series B. Biological sciences,
11/2016, Volume:
371, Issue:
1707
Journal Article
Peer reviewed
Open access
Microfossils, stromatolites, preserved lipids and biologically informative isotopic ratios provide a substantial record of bacterial diversity and biogeochemical cycles in Proterozoic (2500–541 Ma) ...oceans that can be interpreted, at least broadly, in terms of present-day organisms and metabolic processes. Archean (more than 2500 Ma) sedimentary rocks add at least a billion years to the recorded history of life, with sedimentological and biogeochemical evidence for life at 3500 Ma, and possibly earlier; phylogenetic and functional details, however, are limited. Geochemistry provides a major constraint on early evolution, indicating that the first bacteria were shaped by anoxic environments, with distinct patterns of major and micronutrient availability. Archean rocks appear to record the Earth's first iron age, with reduced Fe as the principal electron donor for photosynthesis, oxidized Fe the most abundant terminal electron acceptor for respiration, and Fe a key cofactor in proteins. With the permanent oxygenation of the atmosphere and surface ocean ca 2400 Ma, photic zone O2 limited the access of photosynthetic bacteria to electron donors other than water, while expanding the inventory of oxidants available for respiration and chemoautotrophy. Thus, halfway through Earth history, the microbial underpinnings of modern marine ecosystems began to take shape.
This article is part of the themed issue ‘The new bacteriology’.
Mesozoic and Early Cenozoic marine animals across multiple phyla record secular trends in morphology, environmental distribution, and inferred behaviour that are parsimoniously explained in terms of ...increased selection pressure from durophagous predators. Another systemic change in Mesozoic marine ecosystems, less widely appreciated than the first, may help to explain the observed animal record. Fossils, biomarker molecules, and molecular clocks indicate a major shift in phytoplankton composition, as mixotrophic dinoflagellates, coccolithophorids and, later, diatoms radiated across shelves. Models originally developed to probe the ecology and biogeography of modern phytoplankton enable us to evaluate the ecosystem consequences of these phytoplankton radiations. In particular, our models suggest that the radiation of mixotrophic dinoflagellates and the subsequent diversification of marine diatoms would have accelerated the transfer of primary production upward into larger size classes and higher trophic levels. Thus, phytoplankton evolution provides a mechanism capable of facilitating the observed evolutionary shift in Mesozoic marine animals.
Living animals display a variety of morphological, physiological, and biochemical characters that enable them to live in low-oxygen environments. These features and the organisms that have evolved ...them are distributed in a regular pattern across dioxygen (O
2
) gradients associated with modern oxygen minimum zones. This distribution provides a template for interpreting the stratigraphic covariance between inferred Ediacaran-Cambrian oxygenation and early animal diversification. Although Cambrian oxygen must have reached 10-20% of modern levels, sufficient to support the animal diversity recorded by fossils, it may not have been much higher than this. Today's levels may have been approached only later in the Paleozoic Era. Nonetheless, Ediacaran-Cambrian oxygenation may have pushed surface environments across the low, but critical, physiological thresholds required for large, active animals, especially carnivores. Continued focus on the quantification of the partial pressure of oxygen (
p
O
2
) in the Proterozoic will provide the definitive tests of oxygen-based coevolutionary hypotheses.
Fossils of macroscopic eukaryotes are rarely older than the Ediacaran Period (635-541 million years (Myr)), and their interpretation remains controversial. Here, we report the discovery of ...macroscopic fossils from the 1,560-Myr-old Gaoyuzhuang Formation, Yanshan area, North China, that exhibit both large size and regular morphology. Preserved as carbonaceous compressions, the Gaoyuzhuang fossils have statistically regular linear to lanceolate shapes up to 30 cm long and nearly 8 cm wide, suggesting that the Gaoyuzhuang fossils record benthic multicellular eukaryotes of unprecedentedly large size. Syngenetic fragments showing closely packed ∼10 μm cells arranged in a thick sheet further reinforce the interpretation. Comparisons with living thalloid organisms suggest that these organisms were photosynthetic, although their phylogenetic placement within the Eukarya remains uncertain. The new fossils provide the strongest evidence yet that multicellular eukaryotes with decimetric dimensions and a regular developmental program populated the marine biosphere at least a billion years before the Cambrian Explosion.
Resolving how Earth surface redox conditions evolved through the Proterozoic Eon is fundamental to understanding how biogeochemical cycles have changed through time. The redox sensitivity of cerium ...relative to other rare earth elements and its uptake in carbonate minerals make the Ce anomaly (Ce/Ce*) a particularly useful proxy for capturing redox conditions in the local marine environment. Here, we report Ce/Ce* data in marine carbonate rocks through 3.5 billion years of Earth’s history, focusing in particular on the mid-Proterozoic Eon (i.e., 1.8 – 0.8 Ga). To better understand the role of atmospheric oxygenation, we use Ce/ Ce* data to estimate the partial pressure of atmospheric oxygen (pO2) through this time. Our thermodynamics-based modeling supports a major rise in atmospheric oxygen level in the aftermath of the Great Oxidation Event (~ 2.4 Ga), followed by invariant pO2 of about 1% of present atmospheric level through most of the Proterozoic Eon (2.4 to 0.65 Ga).
Sedimentary rocks deposited across the Proterozoic-Phanerozoic transition record extreme climate fluctuations, a potential rise in atmospheric oxygen or re-organization of the seafloor redox ...landscape, and the initial diversification of animals. It is widely assumed that the inferred redox change facilitated the observed trends in biodiversity. Establishing this palaeoenvironmental context, however, requires that changes in marine redox structure be tracked by means of geochemical proxies and translated into estimates of atmospheric oxygen. Iron-based proxies are among the most effective tools for tracking the redox chemistry of ancient oceans. These proxies are inherently local, but have global implications when analysed collectively and statistically. Here we analyse about 4,700 iron-speciation measurements from shales 2,300 to 360 million years old. Our statistical analyses suggest that subsurface water masses in mid-Proterozoic oceans were predominantly anoxic and ferruginous (depleted in dissolved oxygen and iron-bearing), but with a tendency towards euxinia (sulfide-bearing) that is not observed in the Neoproterozoic era. Analyses further indicate that early animals did not experience appreciable benthic sulfide stress. Finally, unlike proxies based on redox-sensitive trace-metal abundances, iron geochemical data do not show a statistically significant change in oxygen content through the Ediacaran and Cambrian periods, sharply constraining the magnitude of the end-Proterozoic oxygen increase. Indeed, this re-analysis of trace-metal data is consistent with oxygenation continuing well into the Palaeozoic era. Therefore, if changing redox conditions facilitated animal diversification, it did so through a limited rise in oxygen past critical functional and ecological thresholds, as is seen in modern oxygen minimum zone benthic animal communities.
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