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Maintainingthe plausibleA N T O N I O L A Z C A N O

Stimulated by controversies2 that arose overthe evidence for extraterrestrial life in the

Martian meteorite ALH 81004 in the mid-1990s, NASA reorganized its programmeson exobiology and planetary science aspart of an attempt to integrate its researchon life sciences into its space-explorationefforts. Thus was born astrobiology, an all-encompassing effort that was expanded toinclude the study of extremophiles, the occur-rence of planets and their habitability beyondour Solar System, and research on the originand evolution of life on Earth, among otherdisciplines3. But can a funding programme betransformed into a new science?

The astrobiology programme has indeedsupported such studies, and has created jobsfor young researchers, endorsed the teachingof evolutionary biology, and promoted majorefforts in outreach following NASA’s gener-ous tradition of sharing its scientific resultsfor free. The creation of the NASA Astrobiol-ogy Institute1 was followed by the launch ofspecialized journals and scientific societies,university courses, graduate programmes andbooks, as well as by a handful of centres andnetworks of varying scope and uneven aca-demic standards in countries other than theUnited States. The commitment of such efforts

to understanding evolutionary perspectives is

a major accomplishment, but I feel that manyof them tend to place far too much weight on ahandful of loose analogies between extremo-philic microbes and the potential habitabilityof other worlds in our Solar System.

In the absence of unambiguous proof for itsexistence, almost nothing can be said aboutextraterrestrial life about which the oppositeis not also true. The scarcity of evidence givesconsiderable latitude, and, in certain circles,astrobiology has become a resounding butmeaningless catchword in the competition forgrant money. It has been argued that the poten-

tial discovery of aterrestrial ‘shadowbiosphere’ — thatis, organisms thathave an alternativechemical compo-sition to that ofall known organ-isms — would

imply that life hasappeared morethan once on our

planet and therefore that it could also havedeveloped in other worlds.

In spite of the fanfare4 that advertised theexistence of ‘arsenic-based DNA’ in a microbeisolated from a saline lake in California5, meredistilled water and additional laboratory con-trols have washed away the speculation aboutsuch alien biochemistries6,7. And some of theattempts by other researchers to extrapolateto other parts of the Universe the ability ofmicrobes to adapt to extreme environments

may be due more to the struggle for funding

than to the desire to study habitable planets.The search for life beyond Earth is a legiti-

mate scientific question and an alluring intel-lectual endeavour that can best be served bykeeping a healthy distance from science-fictionscenarios and from the theological musings

that somewhat surprisingly find their way intoastrobiology meetings. Depending on who youspeak to, astrobiology seems to include every-thing from the chemical composition of theinterstellar medium to the origin and evolutionof intelligence, society and technology — as ifthe Universe is following an inevitable upwardlinear path leading from the Big Bang to theappearance of life and civilizations capable ofcommunication.

Neither the formation of planets nor theorigin of life is seen today as the result ofinscrutable events; rather, they are consideredas natural outcomes of evolutionary processes.However, this does not mean that such out-comes are inevitable, and it is still to be shownthat life exists — or has existed — in placesother than Earth (Fig. 1). The lack of evidenceshould not inhibit us in the slightest, but unlesswe are bound by the highest academic stand-ards and critical attitudes, astrobiological dis-cussions will become nothing more than emptyspeculation laced with a formidable disregardfor scientific plausibility.

Antonio Lazcano is a biologist in theFacultad de Ciencias, Universidad Nacional Autónoma de México, Ciudad Universitaria,04510 Mexico DF, Mexico.

e-mail: alar@ciencias.unam.mx 

The last greatexperimentK E V I N P . H A N D

Of the four basic sciences — physics,chemistry, geology and biology — only

one has yet to prove its functionality beyondEarth. In the centuries since Galileo, we have

come to learn that the laws of physics, and the

FORUM: Astrobiology

Frontier or fictionAstrobiology, the study of life in the Universe, is sometimes criticized as being a fashionable label with which to rebrandexisting research fields. Its practitioners, however, argue that the discipline provides a broad framework for developing abetter understanding of the frontiers of biology. A biologist and a planetary scientist offer their views.

THE TOPIC IN BRIEF

 ●  Space-exploration programmes in the

1960s powered a new branch of biology

called exobiology, which was focused on the

search for life beyond Earth.

●  Today, the term exobiology has been

almost abandoned, to be replaced by a

broader one, astrobiology — the study of the

origin, evolution, distribution and future of

life in the Universe1.

 ●  However, the nature of astrobiology as

a distinct scientific discipline has been

challenged because of the lack of proof for

extraterrestrial life.

 ●  Also, astrobiology is sometimes criticized

as being a buzzword added to certain

research topics in fields such as planetary

science or biology — for example, in the

study of microbes known as extremophiles,

which live under extreme conditions.

“In certain circles,astrobiologyhas become aresounding butmeaninglesscatchword in thecompetition forgrant money.”

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principles of chemistry and of geology, workbeyond our planet. Yet when it comes to thisbizarre phenomenon known as life, we have yetto make that leap. The interdisciplinary field

 

ofastrobiology seeks to address that goal throughthe exploration of other worlds and of the lim-its of life on Earth.

A common misconception is that astro-biology is equivalent to the search for life else-where. Some people have even gone so far as tosay that it is a science without a subject becausewe don’t yet have any evidence for extraterres-trial life8,9. But that is a flawed argument thathas been put to rest on several occasions (see,for example, ref. 10). Many experiments inscience target hypothetical particles or objects;biology is simply handicapped by the fact thatfirst principles and mathematics provide lim-ited predictive power. In fact, the search for lifebeyond Earth is just one subset of astrobiology.As Knoll et al. have written11: “astrobiologycan be thought of as the application of geobio-logical principles to the study of planets andmoons beyond the Earth.”

The heavily debated detection of methaneon Mars serves as a useful example12. Becausemethane is rapidly destroyed by ultraviolet

light from the Sun, the alleged presence ofthis molecule in Mars’s atmosphere wouldnecessitate an active source from geologicalor biological processes. Astronomers, chem-ists, geologists and biologists have all weighedin to address the issue from different perspec-tives and have worked together in an effort totriangulate on the major strengths and weak-nesses of the data and models. Considerablesupport for that interdisciplinary work comesfrom NASA’s Astrobiology Institute. This sum-mer, the Mars Science Laboratory (MSL; a newNASA mission to explore the red planet) mayhelp to resolve this debate, and the astrobiol-

ogy community will be able to put the MSL

   E   A   R   T   H  :   N   A   S   A  ;   M   A   R   S  :   J   P   L   /   N   A   S   A

  ;   G   A   N   Y   M   E   D   E  :   T .

   S   T   R   Y   K   /   J   P   L   /   N   A   S   A  ;   T   I   T   A   N  :   S   P   A   C   E   S   C   I .   I   N   S   T .   /

   J   P   L   /   N   A   S   A  ;   E   U   R   O   P   A  :   N   S   S   D   C   P   H   O   T   O   G   A   L   L   E   R   Y   /   N   A   S   A  ;   E   N   C   E   L   A   D   U   S  :   J   P   L -   C

   A   L   T   E   C   H   /   S   P   A   C   E   S   C   I .   I   N   S   T .   /

   N   A   S   A

results into context in part owing to our under-standing of the geochemical and biologicalprocesses that generate methane here on Earth.

Some have argued that we know so lit-tle about life on Earth and its origin that weshould focus our limited resources on thesechallenges9. Sixty years ago I might havebeen mildly sympathetic to this viewpoint,

but now I think it’sfair to say that ourmapping of thetree of life on Earthbegs the question ofwhether other treesexist. The analogyin chemistry wouldbe as if, after creat-ing the first periodictable in the 1860s,Mendeleev and oth-ers had decided not

to search for more elements, even though thegaps in the table provided a guide for where tolook. Life on Earth serves as a guide for identi-fying potentially habitable environments else-where. Yes, there is still much to learn aboutthe specifics of terrestrial organisms, but our

understanding of life as a phenomenon and ofbiology as a science will be greatly advancedby finding a second, separate origin that canhelp to put what we observe here on Earth incontext.

And we now know where to go to conductthis great experiment. The vast, global sub-surface liquid-water ocean of Jupiter’s moonEuropa13 is arguably the best place to search forextant life in the Solar System. Furthermore,Europa’s great distance from Earth nearlyensures that the two worlds have not seededeach other. Any life on Europa would thus rep-resent a second, independent origin, even if it

happened to converge on the chemistry based

on DNA, RNA and proteins found in Earth’sorganisms.

If funding permits, within the next few dec-ades we will know the answer to whether ornot life exists elsewhere in our Solar System.We are perhaps just two or three missions awayfrom having received enough data back fromseveral potentially habitable worlds (Fig. 1) toknow whether or not life has ever taken holdas a widespread process on other planets ormoons. The combined cost of such missionswould be comparable to that of the Large Had-ron Collider, which has just revealed evidencefor the Higgs boson, completing one of theother last great experiments of our era. It’s timefor biology’s great experiment. It’s time to learnwhether we live in a biological Universe or onein which life on Earth is a singularity.■

Kevin P. Hand is a planetary scientist at the JetPropulsion Laboratory, California Institute ofTechnology, Pasadena, California 91103, USA.e-mail: kevin.p.hand@jpl.nasa.gov 

1. http://astrobiology.nasa.gov/nai2. Darling, D. Life Everywhere: The Maverick Science of

 Astrobiology  (Basic, 2001).3. Dick, S. J. & Strick, J. E.The Living Universe: NASA

and the Development of Astrobiology (Rutgers Univ.Press, 2004).4. http://blogs.discovermagazine.com/loom/

category/arsenic-life/5. Wolfe-Simon, F.et al. Science 332, 1163–1166

(2011).6. Reaves, M. L., Sinha, S., Rabinowitz, J. D.,

Kruglyak, L. & Redfield, R. J. Science 337, 470–473(2012).

7. Erb, T. J., Kiefer, P., Hattendorf, B., Günther, D. &Vorholt, J. A. Science 337, 467–470 (2012).

8. Simpson, G. G. Science 143, 769–775 (1964).9. Bada, J. L. Science 307, 46 (2005).10. Chyba, C. F.Science 308, 495–496 (2005).11. Knoll, A. H., Canfield, D. E. & Konhauser, K. O. in

Fundamentals of Geobiology  1–4 (Wiley–Blackwell,2012).

12. Kerr, R. A. Science 336, 1500–1503 (2012).13. Kivelson, M. G. et al. Science 289, 1340–1343

(2000).

Figure 1 | Life around us? Searching for extraterrestrial life is one of the aims of astrobiology, and there are several potentially habitable worlds in the SolarSystem, in addition to Earth. Two of Jupiter’s moons (Ganymede and Europa) and two of Saturn’s moons (Titan and Enceladus) harbour liquid-water oceans.And Mars probably had vast liquid-water environments during much of its early history.

“If funding permits, withinthe next fewdecades we willknow the answerto whether ornot life existselsewhere in ourSolar System.”

Earth Mars Ganymede Titan Europa Enceladus

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