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<jats:title>Building Blocks of Earth</jats:title> <jats:p> Earth formed from an explosive and energetic series of collisions that accreted material over millions of years. Comparisons between rocks from Earth's interior and more primitive extraterrestrial samples can help tease apart the composition of Earth's starting material; however, discrepancies between the abundance of certain elements or their isotope ratios often obscure their origin. <jats:bold>Fitoussi and Bourdon</jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1477" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1219509">1477</jats:related-article> , published online 1 March) analyzed the silicon isotopes of a suite of rocks from chondritic meteorites and the Moon to reconcile some of the previous models. By tuning Earth accretion models to account for these Si isotope signatures, enstatite chondrites could be ruled out as the sole end-member composition for bulk Earth. Instead, a heterogeneous mixture of several types of chondritic meteorites is more likely. </jats:p>

<jats:title>The Thin Melt Line</jats:title> <jats:p> Pronounced and persistent variations in the speed of seismic waves as they travel through Earth's interior imply the presence of a physical or chemical boundary layer. Near the lithosphere-asthenosphere boundary, these seismic discontinuities are often patchy and their origins are disputed. By analyzing a set of short-period seismic waves across the Pacific Ocean, <jats:bold>Schmerr</jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1480" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1215433">1480</jats:related-article> ; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6075" page="1448" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1219658">Kawakatsu</jats:related-article> </jats:bold> ) relates the intermittency of this seismic discontinuity to a layer of partial melt below the oceanic lithosphere. The melt may arise from a number of geodynamic processes, including upwellings from the mantle at hot spots like Hawaii, or small-scale thermal convection in the mantle. </jats:p>

<jats:title>Human Impact?</jats:title> <jats:p> Following the arrival of humans in Australia 40- to 50,000 years ago, many species of large vertebrates rapidly became extinct. By analyzing sediment cores from a site in northeastern Australia, <jats:bold> Rule <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6075" page="1483" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1214261">1483</jats:related-article> ; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6075" page="1452" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1220176">McGlone</jats:related-article> </jats:bold> ) show that the extinction of the Australian megafauna caused important ecosystem shifts. Prominent among these were a shift from rainforest vegetation to sclerophyllous vegetation and a sustained increase in the incidence of fire. The cores also provide evidence of the cause of megafaunal extinction in Australia, ruling out climate and anthropogenic fire as possible causes while confirming that the extinctions closely followed human arrival. These findings show how landscapes sometimes have been fundamentally changed by the indirect effects of early humans—which underscores the impact that even prehistoric human societies had on natural systems. </jats:p>

<jats:title>Untangling the Web</jats:title> <jats:p> Interspecific interactions link species within complex trophic and nontrophic webs (see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6075" page="1449" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1220138">Lewinsohn and Cagnolo</jats:related-article> </jats:bold> ). Theoretical work has suggested that certain characteristics of species, or even interactions, may predispose them to extinction from a network. <jats:bold> Aizen <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1486" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1215320">1486</jats:related-article> ) provide empirical evidence that plant-pollinator interactions are lost nonrandomly following habitat reduction in isolated hills in the Argentine pampas. Some types of interaction were more vulnerable to disruption than others, particularly when the specialization of the interacting was high and when the interactions were infrequent. <jats:bold> Stouffer <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1489" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1216556">1489</jats:related-article> ) applied network theory to predict the dynamical importance of species across different food webs. Characteristic three-node motifs were identified, and species were characterized according to the relative frequencies with which they occupied unique positions within the motifs. These relative frequencies and the dynamic importance of the motifs were then used to identify a species-level importance within a food web. </jats:p>

<jats:title>Untangling the Web</jats:title> <jats:p> Interspecific interactions link species within complex trophic and nontrophic webs (see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6075" page="1449" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1220138">Lewinsohn and Cagnolo</jats:related-article> </jats:bold> ). Theoretical work has suggested that certain characteristics of species, or even interactions, may predispose them to extinction from a network. <jats:bold> Aizen <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1486" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1215320">1486</jats:related-article> ) provide empirical evidence that plant-pollinator interactions are lost nonrandomly following habitat reduction in isolated hills in the Argentine pampas. Some types of interaction were more vulnerable to disruption than others, particularly when the specialization of the interacting was high and when the interactions were infrequent. <jats:bold> Stouffer <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1489" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1216556">1489</jats:related-article> ) applied network theory to predict the dynamical importance of species across different food webs. Characteristic three-node motifs were identified, and species were characterized according to the relative frequencies with which they occupied unique positions within the motifs. These relative frequencies and the dynamic importance of the motifs were then used to identify a species-level importance within a food web. </jats:p>

<jats:title>Donuts Dissociate</jats:title> <jats:p> In <jats:italic>Arabidopsis</jats:italic> , the UVR8 protein responds to ultraviolet-B (UV-B) light by dissociating into monomers, which are then available to interact with downstream factors that enact the plant's response to light. <jats:bold> Christie <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1492" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1218091">1492</jats:related-article> , published online 9 February; see the cover and see the Perspective by <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6075" page="1451" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1220248"> <jats:bold>Gardner and Correa</jats:bold> </jats:related-article> ) have now determined the crystal structure of UVR8. Without ultraviolet-B light, UVR8 dimerizes, with two donut-shaped monomers joined by a network of salt bridges. Close-packing of a pyramid of tryptophan residues permits exciton coupling that is key to UV-B perception. Electron transfer after UV-B perception could dissociate the salt bridges that hold the dimer together and release monomeric UVR8 to initiate light-induced signaling. </jats:p>

<jats:title>Mistress of Meiosis</jats:title> <jats:p> Meiosis is essential for proper distribution of maternal chromosomes to eggs during oogenesis. <jats:bold> Su <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1496" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1214680">1496</jats:related-article> ) identified a gene, meiosis arrest female 1 ( <jats:italic>Marf1</jats:italic> ), which is indispensable for meiosis and other oogenic processes. In mice, <jats:italic>Marf1</jats:italic> mutations resulted in meiotic arrest and an increase in nuclear DNA double-strand breaks, phenotypes linked to up-regulated levels of specific messenger RNAs (mRNA). These findings place MARF1 as a key regulator of mammalian female fertility through its integration of oocyte mRNA homeostasis, meiosis, and maintenance of genomic integrity. </jats:p>

<jats:title>Trimprinting the Genome</jats:title> <jats:p> Reprogramming the parental genomes during the oocyte-to-embryo transition requires highly controlled epigenetic mechanisms. Although resetting the genome to a ground state is essential, conservation of inheritable marks is equally important. Now, <jats:bold> Messerschmidt <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1499" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1216154">1499</jats:related-article> ) demonstrate that maternal deletion of the epigenetic modifier Trim28 in mice results in a strongly variable, yet ultimately embryonic, lethal phenotype. Aberrant loss of DNA methylation at imprinting control regions and thus partial loss of epigenetic memory was responsible for the phenotype. The stochastic time and mode of embryonic death reflect the exquisitely balanced interplay of maternal and zygotic factors in the early mammalian embryo. </jats:p>

<jats:title>Reversing Decline?</jats:title> <jats:p> Apolipoprotein E (apoE) normally helps in the clearance of β-amyloid from the brain, a process that is compromised in Alzheimer's disease. <jats:bold> Cramer <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1503" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1217697">1503</jats:related-article> , published online 9 February; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6075" page="1447" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1220725">Strittmatter</jats:related-article> </jats:bold> ) now show that a drug that increases apoE expression rapidly promoted soluble β-amyloid clearance in a mouse model of Alzheimer's disease. The drug also improved cognitive, social, and olfactory performance and rapidly improved neural circuit function. Similar therapeutics may potentially help to ameliorate the symptoms of Alzheimer's disease and its prodromal states. </jats:p>

<jats:title>Inhibiting the Inhibitors</jats:title> <jats:p> Excitatory neurons in the hippocampus and entorhinal cortex are under control of local γ-aminobutyric acid–releasing (GABAergic) interneurons that are the major source of inhibition in the adult brain. In contrast to the wealth of knowledge regarding connectivity in the hippocampus, much less is known about the cross-talk between the hippocampus and other brain regions. <jats:bold> Melzer <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1506" related-article-type="in-this-issue" vol="335" xlink:href="10.1126/science.1217139">1506</jats:related-article> ) investigated long-range GABAergic projections from the hippocampal formation to the medial entorhinal cortex and vice versa. These GABAergic neurons bidirectionally couple the hippocampus and medial entorhinal cortex and preferentially target local inhibitory neurons, forming disinhibitory loops that could help synchronize neuronal activity. </jats:p>