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<jats:p> How did the continents form, and how long ago? These questions have been the subject of debate for 30 years. As Hofmann discusses in his Perspective, results reported by Sylvester <jats:italic>et al</jats:italic> . ( <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="521" related-article-type="in-this-issue" vol="275" xlink:href="10.1126/science.275.5299.521" xlink:type="simple">p. 521</jats:related-article> ) reveal that measurements of niobium and uranium isotopes indicate that ancient rocks from Western Australia are indistinguishable from modern mantle rock. This finding suggests that the mass and extent of continents today is comparable to that 2.7 billion years ago. </jats:p>

<jats:p> A report by Dobson <jats:italic>et al</jats:italic> . ( <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="499" related-article-type="in-this-issue" vol="275" xlink:href="10.1126/science.275.5299.499" xlink:type="simple">p. 550</jats:related-article> ) in this week's issue of <jats:italic>Science</jats:italic> brings together data from many sources on the distribution of endemic plants and animals in the United States and identifies three “hot spots” that contain a particularly high number of endangered species. Hawaii, California, and Florida contain concentrations of endangered organisms. In their Perspective, Pulliam and Interior Secretary Babbitt describe how these data can be profitably used in improving the fate of endangered species and making policy decisions. </jats:p>

<jats:p> Messenger RNAs are translated into protein—a complex enterprise. Now, in work from a number of laboratories, one small adapter protein is shown to be a central player in several variations on the process. In his Perspective, Hentze describes how eIF4G serves as an adapter that binds the small ribosome subunit to the messenger RNA, working in different ways when the message has a <jats:sup>7</jats:sup> mG cap at its 5 <jats:italic>'</jats:italic> end or a polyadenylated tail at its 3 <jats:italic>'</jats:italic> end. </jats:p>

<jats:p>Atmospheric general circulation models used for climate simulation and weather forecasting require the fluxes of radiation, heat, water vapor, and momentum across the land-atmosphere interface to be specified. These fluxes are calculated by submodels called land surface parameterizations. Over the last 20 years, these parameterizations have evolved from simple, unrealistic schemes into credible representations of the global soil-vegetation-atmosphere transfer system as advances in plant physiological and hydrological research, advances in satellite data interpretation, and the results of large-scale field experiments have been exploited. Some modern schemes incorporate biogeochemical and ecological knowledge and, when coupled with advanced climate and ocean models, will be capable of modeling the biological and physical responses of the Earth system to global change, for example, increasing atmospheric carbon dioxide.</jats:p>

<jats:p>San Carlos olivine crystals under laboratory conditions of 26 gigapascals and 973 to 1473 kelvin (conditions typical of subducted slabs at a depth of 720 kilometers) for periods of a few minutes to 19 hours transformed to the phase assemblage of perovskite and magnesiowüstite in two stages: (i) the oxygen sublattice transformed into a cubic close-packed lattice, forming a metastable spinelloid, and (ii) at higher temperatures or longer run durations, this spinelloid broke down to perovskite and magnesiowüstite by redistributing silicon and magnesium while maintaining the general oxygen framework. The breakdown was characterized by a blocking temperature of 1000 kelvin, below which olivine remained metastable, and by rapid kinetics once the reaction was activated.</jats:p>

<jats:p> High-pressure in situ x-ray diffraction and transmission electron microscopy on quenched samples show that natural garnet transforms to orthorhombic perovskite (and minor coexisting phases) containing increasing amounts of aluminum with increasing pressure. This suggests that the perovskite is the dominant host mineral for aluminum in Earth's lower mantle. Orthorhombic perovskite is quenched from ∼35 gigapascals but, because of the increased aluminum content, transforms to the LiNbO <jats:sub>3</jats:sub> structure upon quenching from ∼60 gigapascals. </jats:p>

<jats:p> A new paradigm for oxygen activation is required for enzymes such as methane monooxygenase (MMO), for which catalysis depends on a nonheme diiron center instead of the more familiar Fe-porphyrin cofactor. On the basis of precedents from synthetic diiron complexes, a high-valent Fe <jats:sub>2</jats:sub> (μ-O) <jats:sub>2</jats:sub> diamond core has been proposed as the key oxidizing species for MMO and other nonheme diiron enzymes such as ribonucleotide reductase and fatty acid desaturase. The presence of a single short Fe-O bond (1.77 angstroms) per Fe atom and an Fe-Fe distance of 2.46 angstroms in MMO reaction intermediate Q, obtained from extended x-ray absorption fine structure and Mössbauer analysis, provides spectroscopic evidence that the diiron center in Q has an Fe <jats:sub>2</jats:sub> <jats:sup>IV</jats:sup> O <jats:sub>2</jats:sub> diamond core. </jats:p>