Editors' Choice

Science  24 Dec 2010:
Vol. 330, Issue 6012, pp. 1725
  1. Physics

    Medium-Sized Bang

    1. Jelena Stajic
    CREDIT: COURTESY THE ALICE EXPERIMENT AT CERN

    Most models that strive to describe the state of our universe after the Big Bang predict the existence of the quark-gluon plasma microseconds after the beginning of time. High temperatures are thought to have supported a state wherein the constituents of atomic nucleons—quarks and gluons—existed unbound. In an effort to recreate such conditions, researchers earlier collided gold ions using the Relativistic Heavy Ion Collider (RHIC) and found that the state of matter they created behaved much more like a liquid than a gas. Now, similar experiments have been carried out using the Large Hadron Collider (LHC) to collide lead ions at even higher energies than those achieved at RHIC. The results were collected and analyzed by two groups. Aamodt et al. (ALICE collaboration) found that at these higher energies and temperatures, the quark-gluon plasma still behaves like a (nearly perfect) liquid, implying that it is a strongly interacting system. This conclusion was further corroborated by Aad et al. (ATLAS collaboration), who determined that jets of particles produced by the collisions in the plasma are strongly quenched by their interaction with the surrounding medium.

    Phys. Rev. Lett. 105, 252302; 252303 (2010).

  2. Evolution

    Dark Phase Dating

    1. H. Jesse Smith

    Molecular phylogenies indicate that grasses that transform CO2 through the C4 photosynthetic pathway developed around 30 million years ago, long after the appearance of the first C3 grasses 60 million years ago or earlier. Fossil and isotopic records do not show the presence of C4 grasses until around 20 million years ago, however, leaving their date of origin poorly constrained. It has been suggested that the C4 pathway evolved in response to the rapid decrease of atmospheric CO2 from 1000 ppm to 500 ppm that occurred between 30 and 25 million years ago, as C4 photosynthesis confers a competitive advantage over C3 photosynthesis in low-CO2 conditions. Urban et al. present evidence that C4 grasses already were abundant in southwestern Europe 34 million years ago, before the concentration of atmospheric CO2 began to fall in the early Oligocene. The authors measured the carbon isotopic composition of morphologically indistinguishable C3 and C4 pollen grains in order to determine to which metabolic group they belonged, taking advantage of the large difference in the carbon isotopic signatures that characterize the two photosynthetic pathways. The results thus indicate that factors other than decreasing atmospheric CO2 concentrations must have driven the evolution of C4 photosynthesis.

    Geology 38, 1091 (2010).

  3. Psychology

    Making Up Is Hard

    1. Gilbert Chin

    The trust game—(i) player 1 gives €10 to player 2; (ii) that amount of money is tripled; and (iii) player 2 decides how much of the €30 is given back to player 1—provides an experimental setting in which the aftermath of a breach of trust can be studied. De Cremer et al. show that when students in the role of player 1 were treated unfairly (for instance, when receiving only €5), they judged an apology from player 2 as being less valued and less apt to induce reconciliation when the apology was actually received in comparison to a scenario where they only imagined receiving one. This disconnect also influenced their behavior; the students who received the apology were less trusting in a second round of the game, whereas those who had imagined the receipt of an apology were more willing to resume a trusting stance. Why? One possibility is that mental simulation may have enabled a more effective repair of social status via a public acknowledgment of the transgression.

    Psychol. Sci. 10.1177/0956797610391101 (2010).

  4. Biomedicine

    Travel Assistance

    1. Paula A. Kiberstis

    Stromal cells (green) accompany metastasizing tumor cells (red).

    CREDIT: DUDA ET AL., PROC. NATL. ACAD. SCI. U.S.A. 107, 21677 (2010)

    Tumor metastases are a major cause of death from solid tumors. Evidence from preclinical models suggests that tumor cells do not metastasize alone but rather are assisted by specific host cells that modify the microenvironment of the target organ so that it can support the survival and growth of newly arriving tumor cells. Two independent studies of lung metastasis in mice converge on this theme. Kowanetz et al. show that tumor cells secrete granulocyte colony-stimulating factor, a protein that expands and mobilizes bone marrow cells of a specific type called Ly6G+Ly6C+ granulocytes and facilitates their homing into the lung before the arrival of tumor cells. Upon accumulation in the lungs, these granulocytes then secrete proteins that enhance the invasive properties of tumor cells, including matrix metalloproteinases and Bv8, a protein that stimulates tumor cell migration. Duda et al. provide evidence that the stability of circulating metastatic tumor cells is enhanced when they “cotravel” with stromal cells derived from the primary tumor, such as fibroblasts. Once these cellular clumps reach the lung, the stromal cells appear to provide an early growth advantage to the tumor cells. Further exploration of the cells and signaling molecules identified in these studies could lead to therapies that prevent or inhibit metastases.

    Proc. Natl. Acad. Sci. U.S.A. 107, 21248; 21677 (2010).

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