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Message in a bottle


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Message in a bottle

  1. 1. Are we alone? Although the Search forExtraterrestrial Intelligence (SETI)has yet to detect a signal, the effortscontinue because so little of the possibleparameter space has been searched so far.These projects have almost all followed thedominant paradigm — launched 45 yearsago by Cocconi and Morrison in the pagesof this journal1— of using radio telescopesto look for signs of extraterrestrial life. Thisfocus on electromagnetic waves (primarilyat radio wavelengths, but also at opticalones) was based on various arguments fortheir efficiency as a means of interstellarcommunication. On page 47 of this issue,however, Rose and Wright2make the casethat, if speedy delivery is not required, longmessages are in fact more efficiently sent inthe form of material objects — effectivelymessages in a bottle. Although the sugges-tion itself is not new3,4, it had never beforebeen backed up by quantitative analysis.A fundamental problem in searching forextraterrestrial intelligence is to guess thecommunications set-up of the extraterrestri-alswhomightbetryingtocontactus.Inwhichdirection should we look for their transmit-ter? At which frequencies? How might themessage be coded? How often is it broadcast?(For this discussion I am assuming that thesignals are intentional, setting aside thea priori equally likely possibility that the firstsignal found could be merely leakage arisingfrom their normal activities.) Conventionalwisdom holds that they would set up a beamof electromagnetic waves,just as we could dowith, for example, the 305-metre Areciboradio telescope in Puerto Rico, Earth’s mostpowerful radio transmitter, or a pulsed laseron the 10-metre Keck optical telescope inHawaii. Rose and Wright conclude, however,that the better choice would be to send pack-ageslacedwithinformation.Unless the messages are short or theextraterrestrials are nearby, this ‘write’ strat-egyrequireslessenergyperbitof transmittedinformation than the ‘radiate’ strategy does.Cone-shaped beams of radiation necessarilygrow in size as they travel outwards,meaningthatthegreatmajorityoftheenergyiswasted,even if some of it hits the intended target. Apackage,on the other hand,is not‘diluted’asittravelsacrossspace(Fig.1),presumingthatit’s correctly aimed at its desired destination.For short messages, however, electromag-neticwaveswinoutbecauseof theoverheadsinvolved in launching, shielding and thendecelerating a package, no matter howsmall it is — FedEx charges you just asmuchtosendamicrogramas100grams.Fora two-way conversation with extraterres-trials, the light-speed of electromagneticwavesisfarsuperior.As an example of a large message, con-sider all of the written and electronic infor-mationnowexistingonEarth:it’sestimated5to amount to about one exabyte (that’s 1018bytes,or1019bits).RoseandWrightcalculatethat, using scanning tunnelling microscopy,these bits could be inscribed (in nanometresquares) within one gram of material! Butthis precious package would still require acocoon of 10,000 kilograms to accelerate itfrom our planet to a speed of 0.1% of thespeed of light, protect it from radiationdamage along a 10,000-light-year route,andthendecelerateituponarrival.Rose and Wright’s arguments are opento various criticisms. We do not understandhoweconomicsworksonthisplanet,letalonefor an extraterrestrial intelligence, so it is notclear that the key criterion in choosing a mes-sage’smediumwouldbeenergyexpendedpernews and viewsNATURE|VOL431|2SEPTEMBER2004| 27bit,astheseauthorsassume.Furthermore,wedonotknowifsuchpackages,evenifefficient-ly sent, would ever in fact be recognized andopened. Of course, we also do not knowwhether electromagnetic signals intendedfor us — and which may in fact be nowbathing the Earth — will ever be recognizedassuch.Inbothcases,repetitionanddiversityof communications media would seem toincreasetheextraterrestrials’andourchancesof success.So how should these results influencetoday’s SETI strategy? Short “we are here”messages would still seem to be most effici-ently sent by electromagnetic waves, and weshould continue looking for the same. Butperhaps some attention should be paid to thepossibilityof onedayfindinginourSolarSys-tem an information-drenched artefact, sentby an extremely advanced extraterrestrialcivilization interested only in one-way com-munication. This intruder might be orbitingtheSunoraplanet,orrestingsomewhereonaplanet,moonorasteroid.Thescenarioisrem-iniscent of Arthur C. Clarke’s 2001: A SpaceOdyssey, in which a monolith discovered onthe Moon has been left by extraterrestrials. IfMessage in a bottleWoodruff T. Sullivan IIIExtraterrestrial civilizations may find it more efficient to communicate bysending material objects across interstellar distances rather than beams ofelectromagnetic radiation.Figure 1 Is there anybody out there? Launched in1977 and now at the edge of the Solar System, theVoyager spacecraft (one shown here inprototype) carry phonographic records ofsounds and images from Earth, including themathematical and biological information shownon the right. Rose and Wright2suggest thatsending such packages of information out intospace is the most effective way for anextraterrestrial intelligence to send longmessages to us — and that we should thus bealert to the possibility of finding similar objects,sent from another civilization, in the vicinity ofEarth.JPL/NASA;J.LOMBERG/SPL2.9 n&v NEW MH 27/8/04 5:28 pm Page 27©2004 NaturePublishing Group
  2. 2. once shared,and continue to share,regulato-ry elements. But although this idea mightaccount for the preservation of some degreeof organization, it seems inadequate toexplain the extent to which the complexhas been maintained. Another possibility isthat the mechanism that allows the genes tobe expressed in a strict anterior–posteriorexpression pattern requires some type ofhigher-level organization, involving the pro-gressive chemical or structural modificationof alargecontiguousstretchof DNA.Theworkof Dubouleandcolleaguesoverthe past few years has added an extra dimen-sion to the issue of collinearity. They haveshown that the vertebrate Hox genes shownot just spatial but also temporal collineari-ty3;thatis,genesatoneendofthecomplexareexpressed not only in the anterior of theembryo, but also relatively early in develop-ment. Hox genes located further along thecomplex are expressed both more posterior-ly and later. Duboule and colleagues4haveprovidedevidencethatitmaybetherequire-ment to maintain temporal collinearity thatis responsible for keeping the complextogether. A Hox gene experimentally movedaroundwithinthecomplexcanretainspatialinformation,but will have an altered tempo-ralexpressionprofile.Continuing this theme, Seo et al.1providea fascinating example of an animal in whichthe Hox complex has not stayed together yetappears to maintain some degree of orderedspatialexpressionalongtheanterior–posteri-or axis. Their studies focus on Oikopleuradioica(Fig.1).Oikopleuraisatypeof tunicate,but is quite distant from Ciona, the otherwell-studied representative of this group ofanimals. Tunicates are evolutionarily primi-tive relatives of vertebrates, and comparisonsbetween living tunicates and vertebrates mayhelp researchers to piece together the featuresof the common invertebrate ancestor thatgave rise to vertebrates. Oikopleura also has aremarkable genome — it is very small (at60–70 megabases) and compact (with onegeneevery4kilobases)5.Seo et al. find that Oikopleura has a com-plement of nine Hox genes. As expected,Oikopleura counterparts of the vertebrateanterior Hox genes are expressed in anteriorregions of the developing animal,and coun-terparts of progressively more posteriorvertebrate Hox genes are expressed in corre-spondingly more posterior regions. Whatis fascinating, however, is that the Oiko-pleura Hox genes retain this pattern ofexpressioneventhoughtheyarenolongerinany sort of complex. Seo et al. show that forat least eight of these genes, no other Hoxgene is found within 250 kilobases on eitherside. It is not that these Hox genes are in agene-poor region, however; each is sur-rounded by other genes at the usual highdensityfoundinthisanimal.These results, then, would seem to indi-cate that spatial collinearity can be main-tained without requiring the organization ofthe Hox genes into a complex. Oikopleura,however, appears not to maintain temporalcollinearity. The expression of its Hox genesdoes not seem to begin in a progressivetemporal order, but rather at roughly thesame time. Extensive splits within the Hoxcomplex are also seen in the roundwormCaenorhabditis elegans and in Ciona, twoothercasesinwhichtemporalcollinearitynolonger applies. Even in fruitflies, the Hoxcomplexissplitintotwo,andmanynon-Hoxnews and views28 NATURE|VOL431|2SEPTEMBER2004| were to find such anobject, it would hardly be the first time thatsciencefictionhadbecomesciencefact. ■Woodruff T. Sullivan III is in the Department ofAstronomy, University of Washington, Seattle,Washington 98195, USA.e-mail: woody@astro.washington.edu1. Cocconi, G. & Morrison, P. Nature 184, 844–846 (1959).2. Rose, C. & Wright, G. Nature 431, 47–49 (2004).3. Bracewell, R. Nature 187, 670–671 (1960).4. Papagiannis, M. Q. J. R. Astron. Soc. 19, 277–281(1978).5. Murphy, C. Atlantic 277, No. 5, 20–22 (1996).Evolutionary biologyTime, space and genomesNipam H. PatelIn most animals, the Hox genes — which control development — areclustered together. But why? New evidence supports the idea that therequirement for a temporal order of expression keeps the cluster intact.Some of the most striking discoveriesin developmental biology over thepast century concern the set of genescalled homeotic (Hox) genes. Genetic studiesin fruitflies first showed that these genes havea major role in producing the head-to-tail(anterior–posterior) pattern of tissues alongthe body axis. Then came the startlingfinding that the order of these genes along achromosome correlates with the anterior–posterior position of the body regions theycontrol, and with the domains in which thegenes are expressed. It soon became apparentthat the same relationship exists in other ani-mal groups, including vertebrates. Intrigu-ingly, however, it seems that somewhere inthe evolutionary lineage leading to the tuni-cate Oikopleura dioica, the Hox complex hasdisintegrated, as Seo and colleagues reporton page 67 of this issue1.Evolutionary analyses have suggestedthatthecommonancestorofbilaterallysym-metrical creatures — which include mostanimals, the main exceptions being cnidari-ans and sponges — probably possessed atleastsevenHoxgenes,organizedintoasinglecomplex.Withinthelineageleadingtoverte-brates,geneduplicationsledtoanexpansionof the cluster, and then the cluster itselfunderwent duplications, leading to the fourcopies of the Hox complex now found inhumans and mice.All along, the collinearityof the genes — the correspondence betweentheirphysicalorderalongchromosomesandtheir domains of expression and function —wasmaintained.But why has collinearity been preserved?The ancestral bilaterian complex itself prob-ably arose through several rounds of localduplications, explaining how the genes firstbecame organized as a cluster. In general,however, gene order is constantly shuffledby chromosomal rearrangements such asinversions and movements of large DNAsegments.Giventherateatwhichthisprocessoccurs, the maintenance of collinear organi-zation over at least 600 million years ofevolution must not just be due to chance2.One possibility is that different Hox genesFigure 1 Distant relative. Oikopleura dioica has a simple body plan reminiscent of that of a tadpole,hinting at its close affinity to vertebrates. Remarkably, the generation time for this organism is onlyfour days.R.RUDOLF2.9 n&v NEW MH 27/8/04 5:28 pm Page 28©2004 NaturePublishing Group