The evolution of Earth's biota is intimately linked to the oxygenation of the oceans and atmosphere. We use the isotopic composition and concentration of molybdenum (Mo) in sedimentary rocks to explore this relationship. Our results indicate two episodes of global ocean oxygenation. The first coincides with the emergence of the Ediacaran fauna, including large, motile bilaterian animals, ca. 550-560 million year ago (Ma), reinforcing previous geochemical indications that Earth surface oxygenation facilitated this radiation. The second, perhaps larger, oxygenation took place around 400 Ma, well after the initial rise of animals and, therefore, suggesting that early metazoans evolved in a relatively low oxygen environment. This later oxygenation correlates with the diversification of vascular plants, which likely contributed to increased oxygenation through the enhanced burial of organic carbon in sediments. It also correlates with a pronounced radiation of large predatory fish, animals with high oxygen demand. We thereby couple the redox history of the atmosphere and oceans to major events in animal evolution.T he concentration of O 2 in the Earth's atmosphere and oceans has increased over time from negligible levels early in Earth history to the 21% we have in the atmosphere today (1-3). Our understanding of this history is indirect, based mainly on a series of geochemical proxies reflecting chemical interactions between O 2 and other oxidation-reduction (redox) sensitive elements. These proxies include the isotopic compositions and concentrations of elements such as S, Fe, and Mo preserved in sedimentary rocks (3-7). Several of these proxies (4,5,8,9) indicate an increase in oceanic O 2 during the Ediacaran Period (635 to 542 million years ago, Ma), roughly synchronous with the emergence of large, motile bilaterian animals and, therefore, suggestive of a physiological link between Ediacaran evolution and environmental change. Despite this, the distribution of organic-rich shales (10, 11), the ratio of pyrite sulfur to organic carbon in shales (12), and modeling of the sulfur isotope record (13,14) all indicate that large tracts of subsurface ocean remained anoxic well into the early Phanerozoic Eon (the "age of visible animals," since 542 million years ago). Levels of ocean and atmospheric oxygenation, however, are unquantified from these proxies. Indeed, the most comprehensive history of Phanerozoic oxygenation has been inferred from biogeochemical models. These models diverge, however, on their predictions for the Paleozoic, suggesting either low (15) or high levels of atmospheric oxygen (16,17). Here, we present an independent record of ocean oxygenation history derived from the isotopic composition and concentration of Mo in black shales.The geochemical behavior of Mo is controlled by the relative availability of dissolved H 2 S and O 2 in the oceans. In oxic waters, Mo is soluble and exists as the molybdate anion, MoO 4 2− . In sulfidic waters, molybdate reacts with H 2 S to form particlereactive oxythiomo...

The progressive oxygenation of the Earth's atmosphere was pivotal to the evolution of life, but the puzzle of when and how atmospheric oxygen (O 2 ) first approached modern levels (∼21%) remains unresolved. Redox proxy data indicate the deep oceans were oxygenated during 435-392 Ma, and the appearance of fossil charcoal indicates O 2 >15-17% by 420-400 Ma. However, existing models have failed to predict oxygenation at this time. Here we show that the earliest plants, which colonized the land surface from ∼470 Ma onward, were responsible for this mid-Paleozoic oxygenation event, through greatly increasing global organic carbon burialthe net long-term source of O 2 . We use a trait-based ecophysiological model to predict that cryptogamic vegetation cover could have achieved ∼30% of today's global terrestrial net primary productivity by ∼445 Ma. Data from modern bryophytes suggests this plentiful early plant material had a much higher molar C:P ratio (∼2,000) than marine biomass (∼100), such that a given weathering flux of phosphorus could support more organic carbon burial. Furthermore, recent experiments suggest that early plants selectively increased the flux of phosphorus (relative to alkalinity) weathered from rocks. Combining these effects in a model of long-term biogeochemical cycling, we reproduce a sustained +2‰ increase in the carbonate carbon isotope (δ 13 C) record by ∼445 Ma, and predict a corresponding rise in O 2 to present levels by 420-400 Ma, consistent with geochemical data. This oxygen rise represents a permanent shift in regulatory regime to one where fire-mediated negative feedbacks stabilize high O 2 levels.oxygen | Paleozoic | phosphorus | plants | weathering

DNA molecules are continuously released through decomposition of organic matter and are ubiquitous in most environments. Such DNA becomes fragmented and damaged (often <100 bp) and may persist in the environment for more than half a million years. Fragmented DNA is recognized as nutrient source for microbes, but not as potential substrate for bacterial evolution. Here, we show that fragmented DNA molecules (≥20 bp) that additionally may contain abasic sites, cross-links, or miscoding lesions are acquired by the environmental bacterium Acinetobacter baylyi through natural transformation. With uptake of DNA from a 43,000-y-old woolly mammoth bone, we further demonstrate that such natural transformation events include ancient DNA molecules. We find that the DNA recombination is RecA recombinase independent and is directly linked to DNA replication. We show that the adjacent nucleotide variations generated by uptake of short DNA fragments escape mismatch repair. Moreover, doublenucleotide polymorphisms appear more common among genomes of transformable than nontransformable bacteria. Our findings reveal that short and damaged, including truly ancient, DNA molecules, which are present in large quantities in the environment, can be acquired by bacteria through natural transformation. Our findings open for the possibility that natural genetic exchange can occur with DNA up to several hundreds of thousands years old. microbial evolution | horizontal gene transfer | DNA degradation | early life | anachronistic evolution