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Birth of the jawed vertebrates.

Birth of the jawed vertebrates.
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   variation of the presumed orbital cause (Fig. 1). And it was only 14,000–10,000 years ago that the major (and apparently irreversible) collapse of the ice sheets occurred. The interplay of the bipolar see-saw and CO 2   can provide a coherent explanation for thesepuzzles 8,9 . In this view, the abrupt onset of Antarctic warming 18,000 years ago canbe attributed to the bipolar see-saw, due toa switch-off of the Atlantic circulation inresponse to the crossing of some threshold by  the increasing injection of meltwater from the northern ice sheets. Hard on the heels of this warming, about 300 years later 10 , CO 2 levels inthe atmosphere started to rise as the deep ocean warmed and released CO 2 owing to decreased stratification 11 or decreased Antarctic sea ice 12 . This increase is in accord with observations of  a simultaneous rise in the abundances of theassociated krypton and xenon markers   in the atmosphere 13 . Carbon dioxide then warmed the entireplanet through the greenhouse effect andcontributed to further melting of the greatice sheets. This melting ice in turn kept thebipolar see-saw in its ‘warm south’ polarity in a positive feedback loop, wherein the sustainedsupply of meltwater ensured continued release of CO 2 and thereby continued warming and melting 8,9 . Extreme seasonality owing to winter sea-ice cover on the North Atlantic explainswhy Greenland’s mean annual temperaturerecords seem cold at this time, despite warm summers and abundant melting 14 . Only when the bipolar see-saw switchedback to its ‘warm north’ polarity 14,700 yearsago, perhaps due to de-densification of thedeep ocean caused by the Antarctic’s warm-ing and the addition of fresh water from rain and melting ice 15 , did the increase of CO 2 come to a temporary halt. At this point, the triplewhammy of switched-on Atlantic Ocean cir- culation in the presence of high CO 2 and strong June sunshine provided the coup de grâce for the northern ice sheets. If this hypothesized link between the bipolar see-saw and CO 2 is supported by furtherresearch, there are implications of potentialrelevance to future climate. First, it is difficult to explain the demise of the ice sheets without the added heating from CO 2 , confirming that this gas has killed ice sheets in the past and may  do so again. Second, the predicted slowdownof Atlantic circulation in the coming century may cause an additional release of CO 2 from the ocean that adds to the human-made CO 2 , abiogeochemical feedback that is not considered in current climate projections.   To sum up, the chain of causality for the lastdeglaciation began with Earth’s spin-axis. But this gradual change was amplified in an unsteady cascade of events by ice cover, meltwater, ocean circulation and CO 2 in the atmosphere. Barker and colleagues’ identification 6 of the see-saw in records of past climate helps make sense of thesometimes asynchronous sequence of changes in these disparate phenomena. ■   PALAEONTOLOGY Birth of the jawed vertebrates Per E. Ahlberg The discovery of embryos in certain fossil fishes not only shows thatinternal fertilization and live birth evolved early in vertebrate history, butalso raises questions about the srcin of jawed vertebrates. Every once in a while, a discovery comes alongthat puts our biological understanding of some extinct group of organisms on a much firmerfooting. On page 1124 of this issue, Long andcolleagues 1 present such a discovery, one thatmay prove to have far-reaching implicationsfor our understanding of early vertebrateevolution. For the second time in less than ayear 2 , they have found preserved embryos in the body cavity of a placoderm fish. Placoderms are extinct jawed fishes that lived from approximately 430 million years ago to the end of the Devonian 360 million years ago. Their heads and shoulder girdles are covered inbony armour; plates of bone also form the bitingsurfaces of the jaws, sometimes flat and covered Figure 1 | Gnathostomes and their modes of reproduction. a , Reproductive modes mapped onto agnathostome phylogeny, simplified from ref. 7. ‘C’ marks the gnathostome crown-group node: the partof the tree that lies below this node is the gnathostome stem group; that above it is the crown group. Inthis scheme, both osteichthyans and chondrichthyans are clades (groups comprising all descendantsof a single common ancestor), but the ‘placoderms’ and ‘acanthodians’ are not — hence the invertedcommas. Examples of placoderm groups are the antiarchs, ptyctodonts and arthrodires, the latterincluding Incisoscutum 1 (the antiarchs are placed as the lowest placoderm branch and the arthrodiresas the highest in ref. 7).   b , A conventional consensus phylogeny: placoderms and acanthodians areinterpreted as clades. (Animal images by M. D. Brazeau, reproduced with permission.) a b Osteichthyans: spawning is the ancestralmode; some have evolvedinternal fertilization andlive birth. ‘Acanthodians’: mode of reproductionunknown. Chondrichthyans: all have internalfertilization; manyare live-bearers. ‘Placoderms’: at least somewere live-bearers. C Jeffrey P. Severinghaus is at the ScrippsInstitution of Oceanography, La Jolla,California 92093-0244, USA.e-mail: jseveringhaus@ucsd.edu 1. Broecker, W. S. & Denton, G. H. Geochim. Cosmochim. Acta   53, 2465–2501 (1989).2. Blunier, T. & Brook, E. J. Science   291, 109–112 (2001).3. Broecker, W. S. Paleoceanography   13, 119–121 (1998).4. Stocker, T. F. & Johnsen, S. J. Paleoceanography  doi:0.1029/2003PA000920 (2003).5. Keeling, R. F. & Visbeck, M. Quat. Sci. Rev.   24,   1809–1820(2005).6. Barker, S. et al. Nature 457,   1097–1102 (2009).7. Yokoyama, Y., Lambeck, K., DeDeckker, P., Johnston, P. &Fifield, L. K. Nature   406, 713–716 (2000).8. Paillard, D. Rev. Geophys.   39, 325–346 (2001).9. Denton, G. H. et al.   Pages News   14, 14–16 (2006).10. Ahn, J. & Brook, E. J. Science   322, 83–85 (2008).11. Toggweiler, J. R. Paleoceanography   14, 571–588 (1999).12. Stephens, B. B. & Keeling, R. F. Nature   404, 171–174 (2000).13. Headly, M. A., Kawamura, K. & Severinghaus, J. P. Eos Trans. AGU 89 (53), Fall Mtg Suppl., Abstr. PP41F-04 (2008).14. Denton, G. H., Alley, R. B., Comer, G. C. & Broecker, W. S. Quat. Sci. Rev.   24, 1159–1182 (2005).15. Knorr, G. & Lohmann, G. Nature   424, 532–536 (2003).16. Jouzel, J. et al.   Science   317, 793–797 (2007).17. Cuffey, K. M. & Clow, G. D.  J. Geophys. Res.   102,  26383–26396 (1997). 1094 NATURE | Vol 457 | 26 February 2009 NEWS & VIEWS © 2009 Macmillan Publishers Limited. All rights reserved  with small bumps, sometimes developed into nasty, self-sharpening scissor blades. The largest of these fishes equalled the size of a great white shark and must have been formidable predators, but most were less than a metre in length. Working with three-dimensional placoderm fossils from Gogo in Western Australia, Longand colleagues have discovered specimensof three different placoderms,  Materpiscis 2 ,  Austroptyctodus 2 and Incisoscutum 1 , that con-tain minute but perfectly preserved armourplates of the same species in their body cavi- ties. When first discovered, they were thought to be stomach contents 3 . But the plates show no bite marks or etching by stomach acids, and are not mixed with bones from other species;they are the remains of unborn embryos. In  Materpiscis , a curving tubular structure associ- ated with one of them has been interpreted as an umbilical cord 2 . Embryos in the body cavity imply inter-nal fertilization. It was noted long ago 4 that ptyctodonts, the placoderm subgroup to which  Materpiscis and  Austroptyctodus belong, have sexually dimorphic pelvic fins, somewhat like the ‘claspers’ used for internal fertilization in sharks. Arthrodires, the placoderm group thatincludes Incisoscutum , lack the sexually dimor- phic external bones present on the pelvic fins of ptyctodonts. However, Long et al. argue that the partially preserved internal fin skeletons of their specimens indicate a shark-like struc- ture, probably implying sexual dimorphismand internal fertilization. Ptyctodonts andarthrodires seem to be closely related, and so internal fertilization, and possibly live birth of  young, are probably shared features retained from their common ancestor. The living jawed vertebrates, or gnatho- stomes, fall into two groups, the Chondrichthyesand the Osteichthyes (Fig. 1). The Chondrich-thyes (sharks, rays and ratfishes) all have inter- nal fertilization, and many give birth to live young, whereas the ancestral condition for the Osteichthyes (ray-finned fishes, lobe-finnedfishes and land vertebrates) is to spawn small eggs that are fertilized externally. Live-bearers tend to produce much fewer young than exter- nal spawners and have lower potential rates of  population growth. This contrast in reproduc- tion puts a new perspective on the ecology of  the Gogo environment, a tropical reef  5 , where a wide diversity of placoderms coexisted with lungfishes and primitive ray-finned fishes thatwere probably externally fertilizing spawners. Itis also interesting to note that the extinction of the placoderms at the end of the Devonian was followed by a major diversification of chon-drichthyans. But it is to the study of gnatho- stome interrelationships that the discoveries of  Long et al. may prove to be most pertinent. Ideas about the srcin of gnathostomes arecurrently in a state of flux. For much of thetwentieth century, placoderms were regarded as relatives or possibly ancestors of chondrich- thyans 4 , partly because they seemed to use internal fertilization. But recently the majority   view has placed them in the gnathostomestem group 6 — that is, the common ances-tral lineage of the living jawed vertebrates. Anew analysis by Brazeau 7 suggests that placo-derms may not be a natural group at all, but a‘paraphyletic array’ spread out along thegnathostome stem (Fig. 1a; contrast with Fig. 1b).   If that is correct, placoderms become extremely informative about the srcin of  jawed vertebrate morphology. This is where the evidence for internal fertilization and live- bearing in placoderms becomes important. The ancestral mode of reproduction for osteichthyans seems to be external fertilization. The distribution of live-bearing among living vertebrates strongly suggests that internally fertilizing live-bearers are unlikely to give riseto externally fertilizing spawners, so we would not expect the osteichthyan stem lineage, or thegnathostome stem lineage below it, to contain asegment characterized by live-bearing. Brazeau’sanalysis 7 places the ptyctodonts and arthrodiresas successive branches off the gnathostome stem,implying the existence of such a segment unless the two groups have evolved live-bearing inde- pendently. However, only a minor change in the tree would be needed to join ptyctodonts and arthrodires together in a clade (that is, forming a single side branch), and thus make the offend-ing stem segment disappear. A more important question is whether the most primitive placo-derms, such as the antiarchs (bottom-feedingfishes with armoured pectoral fins), were also live-bearers, because this would undermine thecase for the placoderms forming a paraphyletic segment of the gnathostome stem. Long and colleagues 1 argue that the antiarchshad external fertilization. They lack pelvic finsaltogether, and fossils have been found of free- living juveniles that are small and undevel- oped enough to correspond to the embryos of   Materpiscis ,  Austroptyctodus and Incisoscutum . It may thus be that both internal fertilizationand live-bearing evolved within the placo- derms. Perhaps this was a unique innovation inone placoderm clade. Alternatively, could someplacoderms be stem gnathostomes and others, those with internal fertilization, stem chon-drichthyans? Possibly, but this conflicts withnew evidence that the acanthodians (vaguely shark-like fishes, with fin spines and tiny  scales, which became extinct about 250 million years ago) form a paraphyletic array encom-passing the bases of the chondrichthyan and osteichthyan lineages (Fig. 1a) 6 . The tangled skein of jawed-vertebrate srcins continues to challenge researchers. But discoveries such as the placoderm embryosof Gogo are giving us the tools to gradually untangle it — as well as showing us intimate glimpses of life in a lost world. ■ Per E. Ahlberg is in the Subdepartment ofEvolutionary Organismal Biology, Department ofPhysiology and Developmental Biology, UppsalaUniversity, 752 36 Uppsala, Sweden.e-mail: per.ahlberg@ebc.uu.se 1. Long, J. A., Trinajstic, K. & Johanson, Z. Nature   457,  1124–1127 (2009).2. Long, J. A., Trinajstic, K., Young, G. C. & Senden, T. Nature   453, 650–653 (2008).3. Dennis, K. & Miles, R. S. Zool. J. Linn. Soc.   73, 213–258(1981).4. Moy-Thomas, J. A. & Miles, R. S. Palaeozoic Fishes  (Chapman & Hall, 1971).5. Long, J. A. Swimming in Stone: The Amazing Gogo Fossils of the Kimberley (Fremantle, 2008).6. Janvier, P. Early Vertebrates (Oxford Sci. Publ., 1996).7. Brazeau, M. D. Nature   457, 305–308 (2009).     D .    K    I    N    G    /    D    O    R    L    I    N    G    K    I    N    D    E    R    S    L    E    Y    /    G    E    T    T    Y    I    M    A    G    E    S Eat more, move less and loseweight — sounds too good tobe true. Yet this is exactlywhat Julia Fischer and hercolleagues observed in micelacking one specific gene(J. Fischer et al.   Nature advanceonline publication doi:10.1038/nature07848; 2009).Obesity is a complexdisorder because, as well asenvironmental factors, manygenes seem to be involved. Onesuch gene is FTO , as severalstudies have indicated thatdifferent versions of FTO arestrongly correlated with bodymass index: individuals carryingthe high-risk version weighroughly 3 kilograms more thanthose with the low-risk version.Fischer et al. studied micelacking Fto ( Fto −  / − mice) andcompared themwith normal mice andwith those carrying only onecopy of the gene ( Fto +/ − mice).The absence of Fto did not affectembryonic development, but bysix weeks after birth, Fto −  / − miceweighed on average 30–40%less than normal or Fto +/ − mice.This reduction in weight wasassociated with a marked lossof white fat tissue, with near-complete loss by 15 months.The lower weight of themutant mice doesn’t seemto be due to reduced calorificintake. In fact, these mice atemore in proportion to theirbody weight than normal mice.Moreover, on a high-fat diet,both groups of mutant micegained much less weight thannormal animals. Instead, Fto −  / −  mice used more energy,while not moving much. Theauthors suggest that thisincreased energy expendituremight be due to higher activityof the sympathetic nervoussystem — that is, to enhancedcirculating levels of adrenalineand noradrenaline.Fischer and colleagues’ dataindicate that variations in thehuman FTO gene might affectits levels of expression, eitherputting individuals at risk ofobesity or protecting them fromit. It remains to be seen how FTO  might regulate the activity of thesympathetic nervous system. Sadaf Shadan OBESITY Fat chance 1095 NATURE | Vol 457 | 26 February 2009 NEWS & VIEWS © 2009 Macmillan Publishers Limited. All rights reserved
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