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Hamburg defining ten parts per million of Such fuels react with oxidizing species at problems, such as leakage through the poly- nitrogen oxides as the upper level of effluent lower temperatures without carbon forma- mer membrane or cracking of the ceramic from -fuel burners. This is quite a diffi- tion and may be ideal power supplies for electrolyte, remain to be sorted out. It is cult limit to achieve because ordinary flame computers or mobile phones7. But such worth recalling that Nernst, the inventor of burners give off roughly ten times this intermediates, as with hydrogen or pure the zirconia electrolyte in the 1890s, praised amount of nitrogen oxides. methane, might not be readily available in the simultaneous invention of the telephone The logical answer is to imitate nature the quantities and economics we expect for because it enabled him to call home to switch and to oxidize the hydrocarbon fuel catalyti- regular fuels. the device on, the warm-up time being so cally, while extracting the electrons and use- Of course, the fuel reactions in the device long. Solutions to these problems are now ful energy directly through a membrane. are not the only areas of ignorance. Fuel- being found8. Only when this happens will But our knowledge and expertise of such cell systems are already semi-commercial the expected applications in domestic processes is shockingly weak, despite the but a number of issues remain to be resolved heaters, hybrid vehicles and distributed early discoveries of catalysis and fuel cells if the expected billion-dollar consumer power become reality. ■ by Davy4 and Grove5. A fuel cell works by market is to emerge in 2004 — the date set Kevin Kendall is in the School of Chemical ǁ reducing oxygen to O2 at the cathode, while for car manufacturers in California to pro- Engineering, University of Birmingham, oxidizing fuel, ideally hydrogen, at the anode duce 5% zero-emission vehicles. One prob- Edgbaston, Birmingham B15 2TT, UK. + (Fig. 1). Ions created by this process (H lem is the cost of electrolyte membranes, e-mail: [email protected] 2ǁ or O ) diffuse through the electrolyte for both solid polymer and ceramic fuel 1. Grove Symposium J. Power Sources March (2000). membrane to balance the reactions. At the cells, so the cost of materials will need to 2. Park, S., Vohs, J. M. & Gorte, R. J. Nature 404, 265–267 same time, electrons flow from the anode to fall by an of magnitude. The perfor- (2000). 3. Houghton, J. Global Warming Ch. 3 (, Oxford, 1994). the cathode through an external circuit. If mance of a fuel cell may also be limited by 4. Davy, H. Phil. Trans. R. Soc. Lond. 107, 45–85 (1817). the fuel is hydrogen, the only waste product peripheral equipment, which may represent 5. Grove, W. R. Phil. Mag. 14, 127–130 (1839). is water. Other fuels, such as methane, can be 75% of the total device. Existing systems, 6. Murray, E. P., Tsai, T. & Barnett, S. A. Nature 400, 649–651 used, but they have to be converted to hydro- such as the d.c./a.c. converter and heat (1999). 7. Büchi, F. N. (ed.) Portable Fuel Cells Conference (Bossel, gen before entering the cell. exchanger, may not be well matched to the Switzerland, 1999). Lately, evidence has emerged that new fuel-cell technology. And particular 8. Kendall, K. US Patent No. 5,827,620 (1998). methane can be converted directly by a solid- oxide fuel cell, without any flame, using cerium oxide as the electrode catalyst6. A typ- Evolution ical solid-oxide fuel cell uses a hard ceramic material such as zirconia (which conducts O2ǁ ions) as the electrolyte, and has nickel- In search of the whales’ sisters based anodes. Park et al.2 now suggest that Zhexi Luo copper with cerium or samarium oxide may be a better anode catalyst for direct oxidation of more complex hydrocarbons that exist in hich are the closest rela- molecular evidence1,2. They show that both realistic fuels such as kerosene or diesel. The tives to cetaceans (, por- issues hinge on the long-extinct fossil groups challenge is to find pathways through which Wpoises and whales)? That is, which that were split early from extant cetaceans. ethane and butane, or even aromatics such as mammalian group is their ‘sister taxon’? The topology, or branching sequence, of the toluene, can be reacted without fouling up There is a wide gulf between the morpholog- tree of living artiodactyls and the process through damaging side reactions ical and molecular evolutionary studies on cetaceans depends on inclusion of these that tend to produce tar or carbon. the question, for they give conflicting divergent lineages of cetaceans and their This is the challenge that has been picked answers. In a paper in Systematic Biology, putative extinct relatives. up by Park and colleagues. They used a O’Leary and Geisler1 highlight the impor- Among those possible fossil relatives are copper catalyst intimately in contact with tance of the early divergent lineages, or the that lived from 60 to 30 cerium oxide to produce a porous anode extinct fossil taxonomic groups, in resolving million years ago. Their feet were even-toed material. When this operated on pure this intriguing problem. and adapted for , like those of butane for 48 hours there was no sign of car- The descent of whales from land-dwelling modern artiodactyls, and their fish-eating bon deposition, but good evidence of com- mammals is a compelling example of evolu- or carrion teeth closely resemble those of plete chemical conversion and electron tion. It is documented by a rich fossil record the earliest whales. Since the 1960s3, several transfer through a zirconia electrolyte of intermediate forms spanning the morphological studies1,2,4,5 (with one excep- membrane. An ordinary cell would die in land–water transition; and by morphological tion6) have considered that extinct mesony- minutes under the same operating condi- and molecular lines of evidence, both of chids and cetaceans are sister taxa (Fig. 1a). tions because of carbon fouling. When the which testify that cetaceans have a close affin- The combined –cetacean group copper–ceria–samaria catalyst was used, ity to the artiodactyl mammals, or is, in turn, related to living artiodactyls. In even toluene showed reasonable reactivity with even-toed feet and double-pulley ankle, this scheme, living artiodactyls are recog- and electron conversion. This is very such as today’s cow, and hippopota- nized as a clade1,2,5,7, a genealogical group encouraging because aromatics are usually mus. But two central issues have yet to be set- that includes all descendants of a common prone to rapid graphite deposition, which tled. One is the identity of the cetacean sister ancestor. destroys the catalyst. taxon. The other is whether the cetaceans are In contrast, molecular studies8–12 have The hope is that even gasoline or diesel ‘nested’ within the group of artiodactyls as generally concluded that fuel may soon be catalytically converted to a branch on the family tree of the living artio- and cetaceans are more closely related to energy and heat in a clean and powerful dactyls — in other words, whether cetaceans each other than either group is to other liv- manner by such devices. But it may be easier represent some ancient artiodactyls that ing artiodactyls (Fig. 1b). Consequently, the to use partly oxidized fuels such as methanol became adapted to aquatic life. artiodactyls would be a grade that has similar or formic acid because these will work in What O’Leary and Geisler have done is to evolutionary adaptations, but not a genea- aqueous solutions with catalytic electrodes. evaluate all the available morphological and logical . Moreover, some molecular

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studies12 suggest that mesonychids are not suggest), then cetacean adaptations such as closely related to cetaceans; this is because the underwater nursing of offspring and nearly mesonychid–cetacean relationship implies a hairless could have originated in the larger gap in the fossil record9, and is there- most recent common ancestor of both fore less plausible than the hippo–whale groups (Fig. 1b); this implies that certain connection12. So, for all their congruence on aquatic features had evolved before the ori- the broad picture of –cetacean gin of cetaceans9. The similar ear regions4,5 100 YEARS AGO evolution, there is a big disagreement and the fish-eating and carrion teeth must On reading the letter of Prof. Dexter on between morphological and molecular stud- have evolved independently in mesonychids “Assaults and Drunkenness” (p. 365), I ies over these two phylogenetic issues. and in the earliest cetaceans because these notice that there is one great fallacy in the What does that mean in terms of the ori- derived features are absent in living hippos argument. When a man is intoxicated and gin of cetacean adaptations to aquatic life? If and their artiodactyl allies. commits an assault, the result is entered in hippos and whales are sister taxa to the exclu- Alternatively, if mesonychids and ceta- police reports as “assault”, the more serious sion of mesonychids (as molecular studies ceans are sister taxa (as the best morphologi- offence overshadowing the less. So that, in all probability, many of the cases of assault referred to in the statement were also Other mammals drunkenness, but were not tabulated as such. The temperature is an important element; for its variations are probably the cause of the change of character of the Rhino offences recorded. The same quantity of alcohol will, as has often been noticed, have very different effects in the summer and in Camel the winter. In hot weather alcohol has a Even-toed stimulating influence; this is much less Bull marked in the winter, and during this season the sedative effect is certainly more Cetartiodactyls

noticeable. Studying Prof. Dexter’s curves in Artiodactyl clade Artiodactyl 'grade' this light, and assuming the absence of any Even-toed Hippo other fallacies, we may reasonably conclude foot Aquatic that the number of those arrested in drunkenness or its results varies but little throughout the year. Probably the same Cetartiodactyls people supply the cases of drunkenness in

winter and assaults in summer. Mesonychids From Nature 15 March 1900. Land-dwelling

50 YEARS AGO Pakicetids In 1921 birds described as tits were Aquatic observed to prise open the wax-board tops of bottles on the doorsteps in Cetaceans Swaythling, near Stoneham, Southampton, of Early divergent and drink the milk. This is the first known record of an act which has now become a Remingtonocetus widespread habit in many parts of England and some parts of Wales, Scotland and Basilosaurus Ireland, and which has to date been Aquatic practised by at least eleven species of birds. The spread of the habit is interesting, because of the problems of behaviour involved. How far did the individual birds learn the habit from each other, or invent it abMorphological tree Molecular tree for themselves? If most of them learnt it, by what process did they do so? How did, and Figure 1 Different interpretations of cetacean evolutionary history. Central players in both how do, they detect the presence of food phylogenetic trees are the even-toed ungulates, the artiodactyls (the other, odd-toed, group of inside the bottle?… Without experimental ungulates includes such as the horse). a, A phylogenetic tree based on morphological evidence it is impossible to decide which evidence, including O’Leary and Geisler’s extensive sampling of early divergent lineages1. This senses are of use to the bird in indicating the scheme supports the traditional hypothesis that mesonychids are the sister taxon of cetaceans, and presence of food. Several correspondents that artiodactyls are a monophyletic clade (a genealogical group that includes all descendants of a have found that bottles filled with water or common ancestor) to the exclusion of living cetaceans. b, A tree based exclusively on molecular data even empty bottles are still attacked; but from living ungulates and cetaceans. Here hippopotamuses and cetaceans are sister taxa, and this conveys nothing if the previous history artiodactyls are an characterized by herbivory and even-toed feet for running. of the birds is not known. But they are not a good genealogical group unless cetaceans are considered to be a part of it. Living From Nature 18 March 1950. taxa are shown as silhouettes; early divergent taxa as skeletons; and incomplete fossils in grey. (Modified from refs 1, 2, 9 and 10.)

236 © 2000 Macmillan Magazines Ltd NATURE | VOL 404 | 16 MARCH 2000 | www.nature.com news and views cal evidence has it), then the aquatic adapta- characters. In recent years, more genes in level of homoplasy12,13, but mutational decay tions of hippos and living cetaceans must be larger sequence samples have been added of the flanking region of retroposons may convergences that occurred well after the to the arsenal for estimating ungulate and make them difficult to detect in older lin- split of their respective lineages. This is cetacean phylogeny9–12. And the poor taxo- eages. This means that retroposon-based because certain primitive cetaceans (paki- nomic sampling of some of the earlier mol- estimates may not be effective for resolving cetids) have many ear4 and ankle6 structures ecular work has at least in part been lineages that go back more than 50 million typical of a land , and mesonychids redressed10. All in all, because molecular years14. Cetaceans were already diversified by were fully terrestrial and adapted to running characters can vastly outnumber morpho- 53.5 million years ago, and their divergence (Fig. 1a). logical features, they often prevail in simulta- from extant artiodactyls goes back much - What about the relative strengths of the neous analyses of conflicting data sets. That ther15,16 than that. two lines of evidence? Morphological stud- is, in such analyses they effectively swamp A way to untie this Gordian knot may be ies of ungulate–cetacean phylogeny take in a the morphological estimates of ungulate– to seek out compatible aspects of the molec- much wider range of taxa than the molecular cetacean phylogeny. ular and morphological data sets. Measure- studies. This is because the living artio- Both morphological and molecular data ment of the hidden support and conflict dactyls and cetaceans available for molec- are vulnerable to the problem of homo- between them10 can help extract additional ular analyses represent only a few twigs of plasies — reversals to ancestral conditions or information. Also, morphological features their bushy family trees that have survived parallel changes in different lineages that can should be better analysed for some living pruning by extinction. As O’Leary and camouflage the true phylogeny. In this sense, taxa for which extensive molecular data are Geisler1 point out, 90% of ungulate genera neither approach is better than the other. For available. Such studies may be just as helpful and more than 86% of the cetacean genera instance, the ear region of the , tradi- as discoveries of new fossils and genes in are extinct. Taking the mesonychid and tionally considered to be a good source of resolving details of the cetaceans’ phyloge- cetacean fossil taxa into account produces a highly stable characters, shows some glaring netic tree — most particularly, the question markedly different evolutionary history homoplasies among the ungulates and of which group is their sister taxon. ■ (Fig. 1a). From this, O’Leary and Geisler cetaceans4,5. Moreover, the fossil record of Zhexi Luo is in the Section of Vertebrate conclude that artiodactyls are a clade to the many early divergent fossil taxa is incom- , Carnegie Museum of Natural History, exclusion of cetaceans. Here we have a good plete, resulting in ambiguities in morpho- Pittsburgh, Pennsylvania 19213, USA. example of the principle that including data logical estimates. e-mail: [email protected] from early divergent fossil lineages can On the molecular side, DNA and protein 1. O’Leary, M. A. & Geisler, J. H. Syst. Biol. 48, 455–490 (1999). shake, and reshape, the trees of living taxa sequences have parallel and back mutations. 2. O’Leary, M. A. 15, 315–330 (1999). 3. Van Valen, L. M. Bull. Am. Mus. Nat. Hist. 132, 1–126 based solely on molecular evidence. Even the newest studies using retroposons, (1966). However, for living taxa, use of sequen- which are the RNA-mediated insertion 4. Luo, Z. & Gingerich, P. D. Papers on Paleontology (Univ. Michigan) 31, 1–98 (1999). ces of genes and proteins is in some ways sequences interspersed in the genome, have 5. Geisler, J. H. & Luo, Z. in Evolutionary Emergence of Whales (ed. more powerful than use of morphological their limitations. Retroposons show a low Thewissen, J. G. M.) 161–212 (Plenum, New York, 1998).

Fibre optics Transparent talk

Modern telecommunications rely on optical fibres involves heating silica the transmission of light signals rods with ultra-pure glass cores to along fibre-optic cables — fast, but very high temperatures. Once how to minimize signal degradation softened, the rods can be drawn out over large distances? The secret is into fibres some tens of kilometres to eliminate contaminants that long. To achieve the necessary cause unwanted absorption. These temperatures — in excess of intruders tend to creep into the 2,000 °C — it is common to use fibres during the manufacturing torches that burn hydrogen and stages. Elsewhere in this issue oxygen: the perfect recipe for water. (Nature 404, 262–264; 2000), But the incorporation of water Gordon Thomas and colleagues into the silica rods is a more subtle reveal how the main culprit, water, matter. To investigate this process, sneaks in. Putting this knowledge Thomas and colleagues measured into practice allows the manufacture the transmission of infrared light diffuses from the outside in. process. This parameter was of dry fibres with near maximal through a small section of rod that Given that the signals in optical much higher than expected from transparency and improved had been cooled before drawing out, fibres are confined to a narrow core low-temperature diffusion, probably bandwidth. thereby ‘freezing in' the water. They region, the distribution of water in reflecting an increased mobility of The problem with water, in found that the absorption, and the rod might appear to be good hydroxyl groups in the hot, molten particular the hydroxyl (OH) group, hence the hydroxyl concentration, is news. But Thomas and colleagues state. is that it gets excited when much stronger in the outer layers of found that the drawing process — There is no doubt that irradiated by certain infrared the rod. This can be seen in the which increases the aspect ratio by absorption by hydroxyl groups wavelengths. As this band is used figure, which indicates the varying a factor of about a hundred million contributes significantly to for telecommunications, precious hydroxyl concentrations through a — lets the water in much further. transparency loss in optical fibres. signal power can be lost in vibration cross-section of the rod. (The colour They confirmed this by calculating Uncovering the physical origin of the of the water molecules. scale is logarithmic, blue the diffusion coefficient, a quantity contamination points to an obvious So where does the water come representing the highest that describes the flow of both water solution: pick a water-free heat from? One process for making concentrations.) Clearly, the water and glass during the contamination source. Karen Southwell

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6. Thewissen, J. G. M. & Madar, S. J. Syst. Biol. 48, 21–30 (1999). 12.Nikaido, M., Rooney, A. & Okada, N. Proc. Natl Acad. Sci. USA of articles published in this exclusive physics 7. Luckett, W. P. & Hong, N. J. Mamm. Evol. 5, 127–182 (1998). 99, 10261–10266 (1999). journal on a particular topic tapers off as 8. Graur, D. & Higgins, D. Mol. Biol. Evol. 11, 357–364 (1994). 13.Hillis, D. M. Proc. Natl Acad. Sci. USA 96, 9979–9981 our fundamental knowledge of the subject 9. Gatesy, J., Hayashi, C., Cronin, M. & Arcander, P. Mol. Biol. (1999). increases. So why were references 1, 3 and 4 Evol. 13, 954–963 (1996). 14.Miyamoto, M. M. Curr. Biol. 9, R816–R819 (1999). 10. Gatesy, J., O’Grady, P. & Baker, R. H. Cladistics 15, 271–313 (1999). 15.Bajpai, S. & Gingerich, P. D. Proc. Natl Acad. Sci. USA 95, the only ones in the past 15 years to enjoy this 11.Ursing, B. M. & Arnason, U. Proc. R. Soc. Lond. B 265, 15464–15468 (1998). privilege? As the authors readily admit, it 2251–2255 (1998). 16.Kumar, S. & Hedges, S. B. Nature 392, 917–920 (1997). seems less to do with what they tell us about photofission than with the tantalizing prospect that lasers can now be used to Photonuclear physics induce nuclear processes and the myriad applications that may follow. The most familiar laser-driven nuclear Laser light splits atom process is fusion (the inverse of fission), in Donald Umstadter which lasers are used to heat hydrogen nuclei to temperatures greater than that of the Sun in order to fuse them together to make asers have become ubiquitous, being helium and release energy. Indeed, one of ) Relativistic protons used in everything from a bar-code 10 15 1 petawatt the main reasons for the current interest in Lreader to a compact disk player. Who Relativistic electrons the fission papers3,4 is that the relativistic would have thought that they might be used electrons produced by these experiments to split the atom? A few scientists proposed 10 Nonlinear optics might some day be used as a spark plug to to do just that more than a decade ago1. But ignite a fusion reaction. accomplishing it in the laboratory had to 5 Another laser-driven nuclear reaction 2 await the maturity of new technology , 1 kilowatt that might soon be possible is the creation of which enabled the construction of the 0 an ionized gas (plasma) of positrons as well world’s most powerful lasers at the Lawrence (log Power of light sources 1900 2000 as electrons. (Positrons are anti-electrons.) Livermore National Laboratory in the Year High-density antimatter is difficult to pro- United States and at the Rutherford Apple- Figure 1 As the power of laboratory light sources duce on Earth because it annihilates upon ton Laboratory in the United Kingdom. increases, the energy of accelerated particles encountering ordinary matter, but it is of Now two independent research teams have increases accordingly, making laser-induced interest to astrophysicists because it might used these lasers to split the uranium atom3,4. nuclear reactions possible. The most powerful exist elsewhere in the Universe, for example This work, reported in Physical Review lasers have now reached petawatt (1015 watts) in pulsars, quasars and black holes. Remark- Letters, is just the latest milestone in the race peak powers, allowing them to accelerate ably, with compact powerful lasers, dense to discover what happens when matter inter- electrons to relativistic speeds. The gamma rays electron–positron plasmas might soon be acts with the highest electromagnetic field produced by such electrons have been used in available for study in university laboratories. strengths of light. recent experiments to split the uranium atom3,4. Several groups have used electrons accel- Over the past century, the peak power of erated by high-power lasers to accelerate ions artificial light sources has increased expo- trons accelerate rapidly during a collision to high velocities6,7. The ions can be used to nentially, from a kilowatt (103 watts) to a with neighbouring ions. If these gamma rays produce high concentrations of short-lived petawatt (1015 watts). This corresponds to are energetic enough, they can then perturb radioactive isotopes to tag cancer cells in an increase of more than a factor of ten each the delicate stability of heavy nuclei such as nuclear medicine. More energetic protons decade (Fig. 1). These dramatic advances uranium, causing it to split into lighter from larger and more expensive radio- have led to new physical insights and applica- elements and neutrons in a process called frequency-driven accelerators are even being tions along the way. For instance, it was the photonuclear fission. This is exactly what used to kill cancer cells. invention of the cathode ray tube, used to the two groups of laser scientists have just As the power of lasers continues to make X-rays at the turn of the last century demonstrated. increase, the day will come when protons can (and still used to this day for most television The fission of uranium usually brings to even be accelerated to relativistic velocities displays), that led to the discovery of elec- mind nuclear reactors. But before you get and, possibly, directly by the laser field itself. trons. Later, the invention of the laser your hopes up that splitting atoms with (Protons are more difficult to accelerate than increased the power of light to the point that lasers will solve the world’s energy prob- electrons because they are thousands of it could accelerate an electron to close to the lems, it should be pointed out that more times more massive.) This next era of speed it naturally has as it orbits the nucleus energy is required to power the laser than is laser–matter interactions will involve all of the atom. Remarkably, the distance released through the fission process. sorts of other nuclear reactions that could required to reach this speed is but a millionth Besides, there are much more efficient ways lead to novel applications, some of which are of a metre, the light’s own wavelength. Hav- to accelerate electrons and produce gamma anyone’s guess. ■ ing laser fields almost comparable to atomic rays than by using optical light — for Donald Umstadter is at the Center for Ultrafast fields has led to the discipline of physics instance, with accelerators driven by radio- Optical Science and the Department of Nuclear known as nonlinear optics. frequency waves. Engineering and Radiological Science, University With the latest increase in laser power, Because photofission has been studied of Michigan, Ann Arbor, Michigan 48109, USA. electrons can now be accelerated to even with radio-frequency accelerators since they e-mail: [email protected] higher speeds, close to the speed of light were invented decades ago, it is now quite 1. Boyer, K., Luk, T. S. & Rhodes, C. K. Phys. Rev. Lett. 60, itself, again in a distance of just the laser well understood. In fact, a simple search of 557–559 (1988). 2. Strickland, D. & Mourou, G. Opt. Commun. 56, 219–221 (1985). wavelength. At such high speeds, electrons a database of physics journals for articles 3. Ledingham, K. W. D. et al. Phys. Rev. Lett. 84, 899–902 (2000). are governed by the laws of special relativity, published since 1967 using the keyword 4. Cowan, T. E. et al. Phys. Rev. Lett. 84, 903–906 (2000). leading to new types of nonlinear motion5. “photofission” yields a total of 463 hits. 5. Chen, S.-Y., Maksimchuk, A. & Umstadter, D. Nature 396, 653–655 (1998). Gamma rays (energetic X-rays) are emitted Only seven of these appeared in the journal 6. Sarkisov, G. et al. Phys. Rev. E 59, 7042–7054 (1999). when the highly energetic ‘relativistic’ elec- Physical Review Letters. Usually, the number 7. Krushelnick, K. et al. Phys. Rev. Lett. 83, 737–740 (1999).

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