news and views authors ascribe to CP47 and the first six identified in the CP47 map. Daedalus helices of the PsaA/B subunits in PSI, Rhee and colleagues’ three-dimensional because only a weak sequence homology has map of the sub-core of the PSII reaction Fossil knowledge been noted between the PsaA/B helices and centre not only gives us a framework on those predicted for CP47 and CP43 of PSII. which to model this enzyme from the related What we know of ancient societies comes This match is strong evidence that PsaA/B, structural elements of the bacterial and PSI largely from written records — paper and which are essential subunits in the PSI reac- reaction centres, but also shows how the vellum documents and baked clay tablets. tion centre, have evolved from a gene fusion pigment–protein complexes that convert Clay tablets are by far the most enduring. of a CP43/47-like protein with a five-helix sunlight into chemical energy have evolved. Organic documents have mainly survived8 prototype of a bacterial reaction centre. To understand the structure and function of by preservation and repeated copying over Despite the limited resolution, the these amazing enzymes is increasingly the centuries. What does this imply for tetrapyrrole pigments can be detected in the important in view of the urgent need to find a our own torrent of data? PSII map by comparison with the bacterial regenerative replacement for fossil fuels. The obvious conclusion is that it is all reaction centre (Fig. 2). Two elongated Andreas Engel is at the Biozentrum der Universität doomed. Magnetic and optical tapes and densities that partially overlap with the Basel, Maurice Müller Institut, Klingelbergstrasse disks will delaminate and embrittle; paper tetrapyrroles of the bacterial special pair 70, CH-4056 Basel, Switzerland. and film will bleach and crumble. Worse, probably represent the two chlorophylls of e-mail: [email protected] respect for the past is now dead. Modern the primary electron donor, P680. Interest- 1. Deisenhofer, J., Epp, O., Miki, K., Huber, R. & Michel, H. short-term businesses, election-driven ingly, these densities are slightly further Nature 318, 618–624 (1985). governments and fashion-crazed media 2. Feher, G., Allen, J. P., Okamura, M. Y. & Rees, D. C. Nature 339, apart than in the bacterial reaction centre — 111–116 (1989). have no use for archives; a future Dark Age 11 Å instead of 7.6 Å — consistent with spec- 3. Schubert, W. D. et al. J. Mol. Biol. 272, 741–769 (1997). might not bother with copying at all. Only troscopic analyses6. Further densities in 4. Rhee, K. H., Morris, E. P., Barber, J. & Kühlbrandt, W. Nature a few ‘clay tablets’, in the form of the D1/D2 complexes could tentatively be 396, 283–286 (1998). commemorative statuary and crockery, 5. Moser, C. C., Keske, J. M., Warncke, K., Farid, R. S. & Dutton, assigned to the pheophytins, and densities P. L. Nature 355, 796–802 (1992). will survive to puzzle future historians. corresponding to chlorophylls have been 6. Durrant, J. R. et al. Proc. Natl Acad. Sci. USA 92, 4798–4802 (1995). But Daedalus is more optimistic. He Atmospheric chemistry points out that fossilization can preserve animal remains over millions of years. Shipping forecast is partly cloudy Their organic content is replaced by stable mineral, and with amazing fidelity. So Aircraft contrails are a familiar feature of DREADCO chemists are trying to fossilize modern life, but ships too can leave traces books, film, CDs, magnetic tapes and disks of their passage in the sky. As this satellite and so on. They are immersing them in image shows (R. J. Ferek et al., in J. pressurized carbonated or silicated water, Geophys. Res. 103, 23199–23206; 1998), and studying the rate and fidelity with diesel-powered vessels leave behind ‘ship which their organic content is replaced by tracks’ — narrow lines of perturbed the mineral. Daedalus argues that their regions in the marine atmosphere information content will be unchanged. It resembling bright bands of cloud. is either held as a spatial pattern (as in CDs Ship tracks were first reported more and gramophone records) or as a stable than 30 years ago. They are caused by the mineral itself: metallated carbon ink in interaction of particles emitted from ships printed paper, silver grains in with ambient air, which results in a higher monochrome film, magnetic oxide cloud droplet concentration and lower particles in tape and disks. Fossilized average droplet size. The consequence is decreases over time, while the cloud information media would faithfully carry increased cloud radiance at 3.7 mm, as droplets (which form in the presence of their message down the millennia. detected by the advanced very-high- activated particles known as cloud- Reading them, however, will be tricky. resolution radiometers (AVHRR) on condensation nuclei) are about six times A carbonate disk would be too brittle to be satellites of the US National Oceanic and more abundant and half as large as those in played; a silica reel of tape or film could Atmospheric Adminstration. the surrounding unperturbed clouds. not be unwound. The DREADCO team are On 26 August 1992, the region off the Surprisingly, the higher droplet studying their data-rich fossils by X-ray coast of Washington state was covered by a concentrations persist for a long time. and magnetic-resonance tomography, uniform layer of low-lying stratus clouds — The radiative properties of clouds trying to read them directly as three- the perfect weather to look for ship tracks influence the global radiation budget, and dimensional solid objects. They hope to on satellite images, and direct an aircraft to depend sensitively on droplet size and define the optimum procedure for collect microphysical and chemical data in concentration. Anthropogenic pollutants recovering data from the mineralized and around them. The two tracks spottedby such as sulphur dioxide, volatile organic remains of our time. Ferek et al., seen in this picture as a narrow, compounds and particulates can initiate the But how to inform future historians of inverted V, are caused by plumes from the formation of cloud-condensation nuclei, this crucial work? Daedalus plans to Forest Wave and Al Alamira on their way and thereby perturb these radiative encode his findings on the glass fibre read- northwest across the North Pacific Ocean. properties. Ship tracks seem an ideal setting only memory, or FROM, he devised last The data collected during repeated for tackling the challenge of quantifying the week. Fully mineral already, it is the only aircraft crossings of the tracks, at about 210 correlation between emissions and cloud truly future-proof data format. FROMs m above the sea surface, show the expected perturbations, and so may help to improve will be the key by which our descendants strong perturbation of microphysical cloud understanding of Earth’s radiation budget will unlock the copious fossilized glories properties: the initially very high total and the factors affecting it. and absurdities of our civilization. concentration of particles in the tracks Magdalena Helmer David Jones 222 Nature © Macmillan Publishers Ltd 1998 NATURE | VOL 396 | 19 NOVEMBER 1998 | www.nature.com.