NEWS & VIEWS RESEARCH present in the sea-gooseberry genome but, surprisingly, are expressed not in nerve cells but in other tissues. And of the 10 major common neurotransmitter molecules used by animals, Pleuro­brachia employs only two (glutamate and its antagonist GABA) in the operation of its nervous system. On the basis of these characteristics, it would seem that an animal with such a small number of traditionally neu­ ral proteins would have a simple nerve net, as opposed to a central nervous system, and would not show any complex behaviour. On the contrary, however, comb jellies perform complex actions such as predation and hori­ zontal diurnal migrations in the water column, so they must use different molecules in their nervous system. The phylogenetic position of comb jellies at the base of the animal tree of life and the findings of Moroz and co-workers suggest a fascinating scenario — that comb jellies evolved a nervous system that is unrelated to that of other animals. Heretical hypoth­ eses such as this strike a blow against the anthropocentric view that complex animals emerged gradually along one lineage only, culminating in humans, and that complex organ systems did not evolve twice. But such views do not reflect how evolution really works. Evolution does not follow a chain of events in which one lineage progresses con­ tinuously towards complexity while other branches stagnate7. Instead, it is an ongoing process in all lineages. When the animal tree branched more than 500 million years ago, one lineage gave rise to ctenophores and the other to all remaining animals alive today, and it seems that the two lineages independently evolved a rapid internal communication system. However, the last word has not yet been said on this issue, because the branching sequence of the earliest animal groups is still hotly debated. Some researchers have expressed doubt that ctenophores are at the base, and claim that the lack of many genes in comb jel­ lies can be explained by massive gene loss that mimics a simple genome8. Regardless of where ctenophores finally end up on the tree, the development and evolution of the complex nervous system of these creatures will be an enigma for some time. If it turns out that comb jellies are not at the base of the tree and that animal neurons indeed originated only once, someone must figure out why the molecular biology under­ lying the comb-jelly nervous system is so different from that of other animals. ■ Andreas Hejnol is at the Sars International Centre for Marine Molecular Biology, University of Bergen, 5008 Bergen, Norway. e-mail: [email protected] 1. Moroz, L. L. et al. Nature 510, 109–114 (2014). 2. Dunn, C. W. et al. Nature 452, 745–749 (2008). 3. Ryan, J. F. et al. Science 342, 1242592 (2013).

4. Maxwell, E. K., Ryan, J. F., Schnitzler, C. E., Browne, W. E. & Baxevanis, A. D. BMC Genom. 13, 714 (2012). 5. Ryan, J. F. et al. EvoDevo 1, 9 (2010). 6. Jager, M. et al. J. Exp. Zool. B 316B, 171–187 (2011).

7. Maxmen, A. Nautilus No. 9; http://nautil.us/ issue/9/time/evolution-youre-drunk (2014). 8. Dohrmann, M. & Wörheide, G. Integr. Comp. Biol. 53, 503–511 (2013). This article was published online on 21 May 2014.


How Antarctic ice retreats New records of iceberg-rafted debris from the Scotia Sea reveal episodic retreat of the Antarctic Ice Sheet since the peak of the last glacial period, in step with changes in climate and global sea level. See Letter p.134 TREVOR WILLIAMS


bout 19,000 years ago, the ice sheets that covered large areas of North America and Eurasia began to melt. The ice on Antarctica also melted, but to a lesser extent — most of it still exists today. Understand­ ing past ice-sheet instability and melting is important for predicting future ice behaviour in a warming world. On page 134 of this issue, Weber et al.1 present new well-dated records of iceberg-rafted debris from two marine sedi­ ment cores from the Scotia Sea that reveal at least eight episodes of ice loss from Antarctica between 20,000 and 9,000 years ago. These records allow examination of inter­actions between temperature, ice melt and the water masses of the Southern Ocean, which are

central to the carbon cycle and to climate change between glacial and interglacial periods. Since the Last Glacial Maximum (LGM) — the peak of the most recent glacial period  — which occurred between about 26,000 and 19,000 years ago2, melting ice sheets have raised global sea level by about 130 metres (ref. 3). But there are significant uncertainties in the timing and amount of ice lost from Ant­ arctica. For example, estimates of sea-level rise resulting from Antarctic ice melt have ranged from about 8 m to 30 m, with the most recent estimates around the lower end of this range4. Currently, Antarctica loses ice by two main processes: melting of the underside of floating ice shelves and calving of icebergs. The ice­ bergs melt slowly as they are carried westwards along the coast of Antarctica, and icebergs

Figure 1 | Iceberg-rafted debris from marine sediment offshore of Porpoise Bay, East Antarctica.  Weber et al.1 describe similar iceberg-rafted debris from two marine sediment cores from the southern Scotia Sea that document several episodes of ice loss from Antarctica between 20,000 and 9,000 years ago. Scale bar, 2 mm. 5 J U N E 2 0 1 4 | VO L 5 1 0 | NAT U R E | 3 9

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RESEARCH NEWS & VIEWS that reach the Weddell Sea are blocked by the Antarctic Peninsula and turn northwards, where they melt more rapidly in the warmer waters of the Scotia Sea. Icebergs themselves are ephemeral, but they carry mineral grains and rock fragments scoured from Antarctic bedrock. As icebergs melt, this iceberg-rafted debris (IBRD; Fig. 1) falls to the seabed and is steadily buried in marine sediments to form a record of iceberg activity. Some icebergs are more debris-rich than others: large tabular icebergs calved from floating ice shelves have already lost much of their debris-rich bases to melting, whereas icebergs that calve close to the line between grounded and floating ice tend to retain their debris, and are probably more common during ice-sheet retreat than during times of ice-sheet stability. Nevertheless, the main interpretive link is sound — more IBRD is a sign of more icebergs and greater ice loss from the Antarctic Ice Sheet. The two IBRD records reported by Weber and colleagues are similar, even though the sediment-core sites are separated by 2° of latitude and are subject to different local oceanographic conditions. This simi­larity increases confidence that the IBRD records represent an iceberg signal. Until now, the timing of ice retreat has been constrained by radiocarbon dating of marine sediments and by dating of land surfaces that were uncovered as the ice sheets thinned5,6. Here, the eight episodes of iceberg discharge were dated by matching the record of windblown dust in the same sediment cores to windblown dust in an already-dated Antarctic ice core. This approach results in a continuously dated record, which provides a significant advance in knowledge of when the Antarctic Ice Sheet retreated over the time since the LGM. Weber et al. find that the first of the Antarctic ice discharges took place 20,000–19,000 years ago and was followed by a series of larger epi­ sodes between 17,000 to 9,000 years ago. The largest iceberg release lasted from 14,800 to 14,400 years ago and overlapped, within dating uncertainty, with a period of sealevel rise known as meltwater pulse 1A (MWP1A), which occurred 14,650 to 14,310 years ago7. During this period, sea levels rose by about 14–18 m at the astonishing rate of 4 m or more per century. Weber and colleagues’ iceberg-discharge data clearly show a contri­ bution to MWP-1A from Antarctica, but how much meltwater does this represent? Recent work4 puts the total budget for sea-level rise from Antarctic ice melt since the LGM at about 9 m, and this melt budget has to be shared among the eight iceberg-discharge events, including MWP-1A. By this reckoning, the Antarctic contribution to MWP-1A is rela­ tively minor compared with the contribution from Northern Hemisphere ice, which must provide the balance of the roughly 14–18-m rise in sea level. This result contrasts with modelling of differences in amplitude of the

MWP-1A sea-level rise at different locations (caused by the gravitational and rotational effects of removing the ice mass), which esti­ mates that half or more of MWP-1A comes from Antarctica7. This mismatch has yet to be fully resolved. Questions remain about which areas of the Antarctic Ice Sheet became unstable and produced these iceberg-discharge events. Did major ice-drainage sectors retreat simulta­ neously, perhaps in response to external triggering such as an initial sea-level rise from the north or southward migration of relatively warm Circumpolar Deep Water, as modelled by the authors? Or did different sectors retreat independently, as individual thresholds for instability were crossed in each sector? Prin­ cipal sources of icebergs were probably the nearby Antarctic Peninsula and Weddell Sea embayment, where ice streams drain about a quarter of Antarctic ice by area. Icebergs are also likely to have travelled from other ice out­ lets around East Antarctica. The provenance of the IBRD, and the icebergs that carried it, can be found by matching the geochemical finger­print (such as characteristic argonisotope ages) of individual mineral grains in the IBRD to the corresponding geochemical fingerprint of the different source areas8 — a topic for future study. The episodic iceberg discharges described by Weber and colleagues shed light on the question of how the Antarctic ice sheets melt. Will there be similar iceberg releases in the future? Ice streams in the Amundsen Sea sec­ tor of the West Antarctic Ice Sheet are already

in the early stages of retreat9,10. During the last interglacial period, about 125,000 years ago, sea levels reached 6–9 m higher than today11,12, much of this attributable to an Antarctic melt­ water source, at global temperatures only 1–2 °C warmer than those of today. The planet is on course for a temperature rise exceeding this value, so we can expect similar ice-sheet instability and retreat to that described by Weber et al. in the future. ■ Trevor Williams is at the Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964, USA. e-mail: [email protected] 1. Weber, M. E. et al. Nature 510, 134–138 (2014). 2. Clark, P. U. et al. Science 325, 710–714 (2009). 3. Austermann, J., Mitrovica, J. X., Latychev, K. & Milne, G. A. Nature Geosci. 6, 553–557 (2013). 4. Whitehouse, P. L., Bentley, M. J. & Le Brocq, A. M. Quat. Sci. Rev. 32, 1–24 (2012). 5. Heroy, D. C. & Anderson, J. B. Quat. Sci. Rev. 26, 3286–3297 (2007). 6. Hillenbrand, C.-D. et al. Quat. Sci. Rev. http://dx.doi. org/10.1016/j.quascirev.2013.07.020 (2013). 7. Deschamps, P. et al. Nature 483, 559–564 (2012). 8. Pierce, E. L. et al. Paleoceanography 26, PA4217 (2011). 9. Joughin, I., Smith, B. E. & Medley, B. Science http:// dx.doi.org/10.1126/science.1249055 (2014). 10. Rignot, E., Mouginot, J., Morlighem, M., Seroussi, H. & Scheuchl, B. Geophys. Res. Lett. http://dx.doi. org/10.1002/2014GL060140 (2014). 11. Kopp, R. E., Simons, F. J., Mitrovica, J. X., Maloof, A. C. & Oppenheimer, M. Nature 462, 863–867 (2009). 12. Dutton, A. & Lambeck, K. Science 337, 216–219 (2012). This article was published online on 28 May 2014.


Short RNAs and shortness of breath The simultaneous deletion of six RNA molecules in mice has been found to cause respiratory and fertility defects, owing to improper assembly of structures called cilia on the cell surface. See Article p.115 IRMA SÁNCHEZ & BRIAN D. DYNLACHT


efects in the assembly of cilia — fine projections of the cell surface — are the cause of a plethora of human dis­ eases1. One such disease is primary ciliary dyskinesia, a syndrome that severely com­ promises the function of the respiratory tract and reproductive system, resulting in airway infections and infertility. Identification of fac­ tors that control the assembly of cilia would constitute a major scientific advance. In this issue, Song et al.2 (page 115) implicate clusters of non-protein-coding RNA molecules in the

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regulation of cilium assembly, and show that deletion of these regulators in mice leads to a syndrome reminiscent of primary ciliary dyskinesia. There are two types of cilium, non-motile and motile, both of which are essential for normal growth and development. Single, nonmotile primary cilia are present on diverse cell types and function as sensors that provide the cell with information about the external environment. Motile cilia, meanwhile, func­ tion to coordinate fluid flow, and are found in large numbers on multiciliated cells (MCCs) in the respiratory tract and the Fallopian tubes

Climate science: How Antarctic ice retreats.

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