ESRF News June 2021

NEWS

June 2021 ESRFnews

Modern human brains evolved only about 1.5 million years ago, after the genus first dispersed from Africa, according to a study part conducted at the ESRF s ID17 beamline.

Until now, scientists had assumed that the structure of modern human brains (albeit not the size) emerged when the first populations of the genus Homo appeared in Africa, 2.5 million years ago, making it possible for them to disperse into Southeast Asia in the centuries following. An international team led by Christoph Zollikofer and Marcia Ponce de León from the Department of Anthropology at the University of Zurich put that assumption to the test by comparing the fossilised brain cases of early Homo from Africa, Georgia and Southeast Asia, in part using ID17 computed tomography. Key changes in the frontal lobe associated with planning and complex patterns of thought and action, including language, appeared only in the specimens younger than 1.5 million years; older specimens, even those from outside Africa, were relatively primitive (Science372 6538).

Modern Homo brains evolved post-dispersal

Supercooled liquids form molecular mosaics ESRF users from Spain and Italy have found that supercooled liquids develop a mosaic of mobile and less mobile molecules, providing evidence for the random phase order transition theory of glasses. In simple terms, glasses are liquids

that can no longer flow, because they have been cooled fast enough to prevent crystallisation. Quite how this arrest occurs at a microscopic level has long been a mystery. One promising area of study involves the relaxation of molecules that occurs when a liquid attempts to restore equilibrium. Relaxations in a supercooled liquid are complex, heterogeneous processes, spanning many orders of magnitude in time. For instance, in a liquid above its melting temperature the local structure changes on a timescale of a few picoseconds, whereas close to the glassy state, where the dynamic arrest occurs, the structure relaxes on the order of 100 seconds. There is a faster process known as the Johari- Goldstein relaxation, which begins

just before the dynamic arrest and continues below the glass transition. The role of this relaxation has still been debated, however, because the scales involved are still difficult to access: interatomic lengths, and times of the order of hundreds of nanoseconds. Using time-domain X-ray

interferometry at the ESRF s ID18 beamline, Federico Caporaletti and colleagues from the University of Trento and the University of Pisa in Italy, and the Polytechnic University of Catalonia in Spain, have characterised the dynamical properties of the Johari-Goldstein relaxation in the liquid close to its glass-transition temperature. Combining their data with those from the literature, the researchers were able to provide a new picture for the microscopic dynamics in the supercooled state, in which molecules form an infinite, percolating cluster spanning the whole sample, with pockets of less mobile molecules (fig. 1). According to Caporaletti and colleagues, the cluster supports the random phase order transition theory of the glass transition, which was first developed in the 1980s (Nature Commun. 12 1867).

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Figure 1: At any given time during the Johari-Goldstein relaxation, which anticipates the glass transition, one group of molecules (red) are highly mobile and perform wide spatial excursions, while the rest of the molecules (white) remain spatially connected.