2012 03 02 Палеонтология 1 Биоразнообразие

Древожизни

OnImportantStages of Geosphere and Biosphere Evolution

N. L. Dobretsov, N. A. Kolchanov, and V. V. Suslov

Scenario of the basic stagesof climateevolution, earth’s crust evolution and biosphere evolution

ПАЛЕОНТОЛОГИЧЕСКАЯ

ЛЕТОПИСЬ

Comets,Carbonaceous Meteoritesand the Originof the Biosphere

R. B. HooverAstrobiology Laboratory, NASA/NSSTC,Huntsville, AL, USA;e-mail:[e-mail]

Is lifeon Earth endogenous or exogenous?

Is liferestricted toEarth or is lifea Cosmic imperative?

Cyanobacterial filaments embedded in the

Orgueil meteorite

Evidencefor indigenousmicrofossils in carbonaceous meteorites suggests thatthe paradigmof the endogenousorigin of life on Earth should be reconsidered.

Itis now widely acceptedthat cometsand carbonaceous meteoritesplayed an importantrolein the delivery of water,organics and life-criticalbiogenic elements tothe earlyEarth and facilitatedtheorigin and evolution of theEarth’sbiosphere. However,the detection of embedded microfossilsand mats in carbonaceous meteoritesimplies that comets and meteoritesmay have played a directrolein thedelivery of intactmicroorganisms and that thebiosphere may extend far into thecosmos. Recent space observationshave foundthenucleiof comets to have verylow albedos(0.03) and these jet-black surfaces can become very hot (T 400K) nearperihelion.This chapterreviews recent observationaldataon comets and suggests thatliquidwater pools could exist in cavities and fissures between the internalices and rocks and theexterior carbonaceouscrust. The presenceof light and liquid waternear thesurfaceof thenucleus enhancesthe possibilitythat cometscould harbor prokaryoticextremophiles(e.g., cyanobacteria,sulfur bacteriaand archaea)capable of growth over a wide range of temperatures. The hypothesisthatcomets are theparentbodies of the CI1and theCM2carbonaceous meteoritesis advanced.Electron microscopy images will be presentedshowing formsinterpretedas indigenousmicrofossils embeddedin freshlyfracturedinterior surfacesof theOrgueil(CI1) and Murchison(CM2) meteorites. The size range and morphologicalcharacteristics of these forms are consistent with known representativesof morphotypesof all fivesubsections (orders) of Phylum Cyanobacteria.Energydispersive X-rayspectroscopy (EDS) elementaldatashow that theformsin themeteoriteshave anomalous C/Nand C/Sas compared with modern extremophiles and cyanobacteria.These images and spectraldata indicate thattheclearlybiogenic and embedded remains cannot be interpretedas recent biological contaminantsand thereforeareindigenous microfossilsin the meteorites.

Cyanobacterial filaments embedded in the Orgueil meteorite

Conclusions

Thediscovery of indigenousmicrofossils of cyanobacteriain carbonaceous meteorites clearly indicatesthat thebiosphereof Earthis open ratherthanclosed. The possibilitythat biological matterhas been transferredintospace bytheimpact of large comets and asteroids onto shallow marinesediments must be considered as well as the possibilitythat terrestrialmicrobial extremophilesarrived on Earthduringthe Hadeanbombardmentfrom otherregimes of the cosmos. The images providedin Figs. 1and2are unambiguouslybiological in nature.

These filaments,sheathsand mats are clearly recognizableas associated with morphotypesof highlydistinctiveand well-known polarized anddifferentiatedbenthicfilamentouscyanobacteria.TheOrgueil meteoriteis immediatelydestroyed bycontact with liquidwater (CloeЁ z,1864, Hoover,2006b). It is impossible thatthese photosyntheticmicroorganisms (which requireliquidwater to grow and a water/substrateinterfacein orderto form mats) could have constructedthe observedmats and benthicecological consortia within thedry black Orgueil rocks subsequentto thearrivalof theOrgueil meteoriteon Earth.Furthermore,thereare dramaticdifferences between the C/N and C/S ratios of theOrgueil filamentsand sheaths thanthose foundin living cyanobacteriaand old dead forms encounteredin herbariummaterial.Thereforeit is concluded that theindigenousmicrofossils of cyanobacteriafoundin the CI andCMcarbonaceous meteoritesprovidestrong evidencethat thebiosphereis open and is not restrictedto theplanet Earth.