European Space Agency Special Report ESA SP 496, pp. 219-222.

SEARCH FOR MICROORGANISMS ON EUROPA AND MARS IN RELATION WITH THE EVOLUTION OF INTELLIGENT BEHAVIOR ON OTHER WORLDS

Julian Chela-Flores

The Abdus Salam International Centre for Theoretical Physics
P.O.Box 586; Strada Costiera 11;
34136 Trieste, Italy and
Instituto de Estudios Avanzados , Caracas 1015A, Venezuela.

tel. +390-40-2240392 / fax: +390-40-22 42 41;
e-mail chelaf@ictp.trieste.it / URL: http://www.ictp.trieste.it/~chelaf/index1.html

ABSTRACT

Within the context of how to search for life in the Solar System, we discuss the need to consider universal evolutionary biomarkers, in addition to those of biochemical nature that have already been selected for in the biology experiments of the old Viking and future Beagle-2 landers. For the wider problem of the evolution of intelligent behavior on other worlds (the SETI program), the type of experiments suggested below aim at establishing a direct connection between Solar System exploration and the first steps along the pathway toward the evolution of intelligent behavior. The two leading sites for the implementation of the proposed first whole-cell experiments would be, firstly, Europa after the Europa-Orbiter mission (Phillips and Chyba, 2001), either on the ice-crust (Greenberg et al., 2000), or in the ocean itself by means of a submersible (Horvath et al., 1997); secondly, such experiments could be implemented once isolated liquid water oases are identified in the Martian substratum.

EVOLUTIONARY BIOLOGY

Whether nature in an extraterrestrial context steers a predictable course, is clearly still an open question, but some hints from the basic laws of biology militate in favor of predictability to a certain extent:
These laws are natural selection and the existence of a common ancestor. All life on Earth can be interpreted, as evidence in favor of the fact that, to a certain extent, at a cosmic level evolution is predictable, and not contingent. This is subject to experimental tests.
We shall assume that natural selection seems to be powerful enough to shape terrestrial organisms to similar ends, independent of historical contingency. It could be argued that, in an extraterrestrial environment, the evolutionary steps that led to human beings would probably never repeat themselves; but, that is hardly the relevant point since the role of chance in evolution has little bearing on the emergence of a particular biological property, or more precisely a particular "biological objective".
The convergence toward such biological objectives is inevitable. This has been recognized by students of evolution for a long time. It often is referred to as convergent evolution. This may be illustrated with examples taken, for instance, from ornithology and neurophysiology:
o An example is provided by swallows, a group which is often confused with swifts, but are not related to them. In fact, the taxons to which these birds belong are orders, rather than families. Members of these two orders differ widely in anatomy; their similarities are the result of convergent evolution on different stocks that have become adapted to the same ways of living, in ecosystems that are similar for both species.
o Brain structures that look the same, but have arisen independently in different lineages, are considered to have arisen by convergent evolution. For instance, neurons can be classified as to whether they are driven by one eye or the other. This property is called 'ocular dominance' and has been observed in cats and monkeys. Hence, this property is likely to have arisen by convergent evolution, since cats and monkeys are very distantly related species, and other intervening lineages do not posses this feature.
To sum up. convergent evolution is a powerful concept that advocates for evolution leading to broadly similar biological properties

PRECURSORS OF EVOLUTION OF INTELLIGENT BEHAVIOR

We will consider first the rationale of why experiments on single cells may provide a test for the first steps in the pathway that leads to the evolution of intelligent behavior. It is reasonable to assume that cells first got together as a result of chance mutations, which favored the multicellular association. In turn they stayed together because they reproduced more successfully as a group, rather than as single cells.
Slime molds can be seen as a model of the first steps in multicellularity (De Duve, 1995). These uni cellular microorganisms segregate a chemical - cyclic adenosine monophosphate (cAMP), which leads to aggregation into a single macroorganism.
Each unicellular eukaryote on contact with cAMP expresses new surface molecules with 'lock and key' possibilities that, after they randomly come into contact, they remain locked to each other. This implies that in our solar system there may be indicators at the level of single cells of the first steps of the evolution of intelligent behavior. I will attempt to clarify this point in the following sections.

UNIVERSALITY OF SOLAR SYSTEM EXPLORATION

Our previous remarks are pertinent thanks to the surveys of the Vikings in the 1970s and Galileo in the 1990s.
Astrobiology has begun to take seriously the possibility of biological evolution in aquatic biotopes, other than terrestrial ones; essentially, this occurred after the initial shock of the first Voyager close-up views of the frozen surface of Europa. This discovery was followed up by the campaign of the Galileo Mission with better quality images. Experimental tests are feasible in the oceans of the Solar System, or in isolated water reservoirs on Mars. In other words, some of the new environments are nearby, where the first steps of evolution of intelligent behavior may emerge, right inside our own solar system: The leading candidates are Europa and Mars, in that order.
In fact, a question that we may raise is: Are there constraints on biological evolution in solar systems?
Indeed, this is a question that can be faced with experimental techniques: At the time of writing over 60 planets are now known to circle round almost the same number of nearby stars; in analogy with our Jupiter. The new Jupiter-like planets may have Europa-like satellites, or even Earth-like satellites.
In other words, the question of what happens in a given solar system, such as ours, is relevant not only for its own planetary system, but it is bound to have important astrobiological implications elsewhere in the galaxy as well. The crucial question to answer is which properties should we attempt to recognize in the first microorganisms that are currently being searched for inside our own solar system. This is the main topic of the present paper.

UNIVERSAL EVOLUTIONARY BIOMARKERS

ESA will attempt to search for microorganisms in just two years time with the Mars Express mission, using the Beagle 2 lander, which will search for noble gases and organics. It will also be provided with a corer mounted on a robotic arm.
Beagle-2 is a direct descendant of the Viking probes, one of which landed at the Utopia Plains, which is northeast of the landing site of the future lander of ESA. We have argued that some experiments should be discussed to test whether evolution of intelligent behavior is a logical consequence of the emergence of a eukaryotic cell, which is the minimum cellular plan that will allow a neuron to emerge.
Even in simple eukaryotes, electrophysiological research has demonstrated that ion channels are involved, for example, in the movements of the protozoan Paramecium (Villegas et al., 2000). Besides, in the simplest nervous systems of cnidarians ultra-structural studies have identified ion channels.
The emergence of ion channels is a basic stage in the evolution of intelligent behavior anywhere. This conjecture is based on convergent evolution.
In a Solar System setting, a very preliminary evolutionary experiment that could be discussed would be if, in a given biotope, the microorganisms are firstly eukaryotes (Chela-Flores, 1998a), and secondly whether they have already developed ion channels.
A sketch of the proposed experiment is, firstly:
o Introduce the microorganism in a solution of calcium ions.
o Then change the concentration mechanically.
o Proceed to inspect the changes in cell polarization. This is based on standard electrophysiology.
New challenges have to be taken into account:
o Miniaturization. This is a problem which is not beyond present day technology, and
o Remote control, since signals would take about half an hour to reach the submersible, in the specific case of Europa.

A NEW GENERATION OF BIOLOGY EXPERIMENTS IN THE SOLAR SYSTEM

The Europan ocean is about 100 km deep. The thickness of its ice-crust has been estimated to be 10 km (Pappalardo et al., 1999). However, it may be much thinner in some isolated spots (Greenberg et al., 2000).
These remarks support the feasibility of the type of experiments described at the end of the last section. They are compatible with Europa landing and penetrating missions in terms of a penetrator (cryobot) and a submersible (hydrobot). This type of mission began to be discussed towards the end of last century (Horvath et al., 1997).
Such technology, once the instrumentation and trials on Earth analogs are completed, presents a challenge since the scientific community has to decide on the limited number of possible experiments that could be inserted into the hydrobot.
We have argued in favor of searching for universal evolutionary biomarkers in the above outline of the design of the electrophysiology experiment, searching specifically for ion channel activity.
Since the presence of biochemical traces on the Europan surface is a possibility (McKay, 1998, 2001), it is relevant to pursue the analogous research in Lake Vostok, particularly concerning eukaryotes.
Diatoms, are some of the most interesting microorganisms to consider, given their ubiquity on Earth: In just one litre of sea water one may find as many as ten million diatoms (Hoover, 1979). We have emphasized the presence of diatoms because these organisms comprise the largest number of algae in the benthic mats of the singular biotopes in Antarctica in the Earth's analogs of the Europan ocean (the Dry Valley lakes). In these Antarctic biotopes, eukaryotes have demonstrated to thrive in Europa-like conditions (Chela-Flores, 1998b).
This suggests that if evolution of microorganisms have taken place in the Europan ocean, the type of whole-cell experiments that we have discussed are a possible choice that should be considered. This option is in sharp contrast the Viking Biology Experiment, whose main motivation was not to probe directly a given microorganism, but to probe its metabolism; in other words, the first Martian biology experiments indicated a measurable reactivity in the soil of the Utopia and Chryse Plains:
O2 and CO2 were released when a nutrient solution was added to the soil. This was taken to be evidence for life. The biology experiments we have discussed are a natural follow-up to the first generation of experiments with landers. So far, the Viking and Beagle-2 experiments have focussed, for instance, on metabolic reactions, the search for noble gases, and organic chemistry.

DISCUSSION AND CONCLUSIONS

To fully appreciate the implications of the new type of space missions suggested here, some considerations in the theory of evolution should be given due attention:

(a). Evolutionary convergence.

(b). The total absence of refuges for evolution for life on Earth.

(c). The implications of the universality of evolution of intelligent behavior, implicit in the SETI project.

Point (a) has been discussed above.
Point (b) follows from the exclusion of refuges against evolution (Little et al., 1997). This new insight is based on the remark that Cambrian fauna, such as lamp shells (inarticulate brachiopods) and primitive molluscs (Monoplacophora), were maintained during Silurian times by microorganisms that lived in hydrothermal vents. In the current Cenozoic Era these hot environments have seen the extinction of such fauna. Hence, this remark rules out the possibility that these deep-sea environments are refuges against evolutionary pressures. In other words, the evidence so far does not support the idea that there might be environments, where ecosystems might escape biological evolution, not even at the very bottom of deep oceans.
It is then reasonable to assume that any micro-organism, in whatever environment on Earth, or elsewhere (including Europa), would be inexorably subject to evolutionary pressures.
In view of its implications, point (c) probably requires more attention: The rationale for the search for extraterrestrial signals of evidence of evolution of intelligent behavior can be divided in three stages:
oCentral nervous systems are bound to evolve.
The reason is that simple animals do have nervous nets. This follows from electrophysiological research, extensively reviewed in the Iberoamerican School of Astrobiology (Villegas et al., 2000). In other words, in the phylogenetic tree, already at the low evolutionary level of cnidarians, say jellyfish, there are primitive nervous systems. From this example we can say that almost as soon as some coordinated electrophysiological responses are possible in multicellular organisms, they have been demonstrated to exist. These are some of the reasons for conjecturing that nervous systems will evolve elsewhere.
o Brains are bound to evolve.
In the phylogenetic tree, the first appearance of a cerebral ganglion occurs very early, for instance in annelids (ancestors of common worms). The primitive nervous system of Cnidaria has been studied, but this is a taxon essentially of diploblastic organisms (ie., during development they just have endoderm and ectoderm). It is in the next step in evolution that we should now discuss: the case of triploblastic animals. These are animals that have a well-developed mesoderm, like ourselves. It is at this level in which primitive brains are first seen to evolve. In flat worms, there is one example, the Notoplana acticola, which has a primitive brain (a cerebral ganglion). It receives inputs form sensory organs and delivers outputs to muscles, via nerve filaments (Villegas et al., 2000). The evolution of higher animals occurred explosively during the Cambrian, over 500 million years ago. Soon enough after the emergence of the simplest triploblastic body plans, multiple phyla appeared in the Cambrian. These phyla successfully persevered through subsequent geologic periods, right up to the present: we would like to underline particularly phyla in major groups of coelomates animals: Chordates, Mollusks and Arthropods. In all of them, we do find nervous systems with the capacity to support sensorial discrimination, learning, social behavior and communication. The property of communication through the spoken language had to await the advent of the vertebrates to reach the human-level type of behavior. It is traces of these activities that are now being searched elsewhere in the universe, by means of the SETI project. There are many reasons to believe that brains may evolve elsewhere.
o Finally, the first steps towards intelligent behavior and complex language are bound to take place.
This is the more difficult case to study. It is precisely

what the SETI project assumes to be actually occurring on other worlds. However, we already know that in humans the origin of language is probably a consequence of natural selection. There is a long debate on the role of natural selection; it seems reasonable to make a simple assumption: Natural selection may favor the appearance of language, once a sufficiently complex brain has appeared in a given phylogenetic tree.
To conclude, in his formulation of his theory of evolution of life on Earth, Charles Darwin had in mind the evolution of living organisms. Today we have more insights into the nature of evolution, which allow us to study both theoretically, as well as experimentally, the possibility of life evolving elsewhere. In view of the substantial challenges ahead in instrumentation and trials on Earth analogs, we must now begin to plan a second generation of feasible evolutionary experiments with whole microorganisms. In those new experiments universal evolutionary biomarkers are to be searched for. One example is the ion channels in cell membranes, even though this is only a small step in the evolution of intelligent behavior in other worlds.

ACKNOWLEDGEMENT

The author would like to thank the Abdus Salam International Centre for Theoretical Physics, its Director, Professor M.A. Virasoro and ESA for financial support to attend the First European Workshop on Exo/Astrobiology, ESRIN, Frascati, 21-23 May 2001.

GENERAL REFERENCES

Chela-Flores, J. (2001): The New Science of Astrobiology: From genesis of the living cell to evolution of intelligent behavior in the universe. Chapter 12, Kluwer Academic Publishers: Dordrecht, The Netherlands, pp. 149-156. (In press.)

Chela-Flores, J., Lemarchand, G.A. and Oro, J., (eds.), (2000): Astrobiology: Origins from the Big Bang to Civilisation, Kluwer Academic Publishers: Dordrecht, The Netherlands.

Drake, F. (2001): New Paradigms for SETI, in Chela-Flores, J., Owen, T. and Raulin, F. (eds.), The First Steps of Life in the Universe. Kluwer Academic Publishers: Dordrecht, The Netherlands, pp. 395-398. (In press.)

Seckbach, J., Westall, F. and Chela-Flores, J. (2000): Introduction to Astrobiology, in: Journey to Diverse Microbial Worlds: Adaptation to Exotic Environments, Seckbach, J. (ed.); Kluwer Academic Publishers, Dordrecht, The Netherlands. Chapter 25, pp. 367-375.

REFERENCES

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De Duve, C. (1995): Vital Dust, Basic Books, New York, pp. 171-172.

Greenberg, R., P. Geissler, B. Tufts, and G.V. Hoppa (2000): Habitability of Europa's crust: The role of tidal-tectonic processes. J. Geophys. Res. Vol. 105, 17551-1756.

Hoover, R.D. (1979): Those Marvelous Myriad Diatoms, National Geographic Magazine, June. pp. 870-878, 1979.

Horvath, J., Carsey, F., Cutts, J., Jones, J., Johnson, E., Landry, B., Lane, L., Lynch, G., Chela-Flores, J., Jeng T.-W., and Bradley, A. (1997): Searching for ice and ocean biogenic activity on Europa and Earth. In: Instruments, Methods and Missions for Investigation of Extraterrestrial Microorganisms, The International Society for Optical Engineering, Bellingham, Washington USA, (R.B.Hoover, ed.), Proc. SPIE, Vol. 3111, pp. 490-500.

Little, C.T.S. R.J. Herrington, V.V. Maslennikov, N.J. Morris and V.V. Zaykov (1997): Silurian hydrothermal-vent community from the southern Urals, Russia, Nature Vol. 385, pp. 146-148.

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McKay, C.P. (2001): The search for a second genesis in our solar system, in Chela-Flores, J., Owen, T. and Raulin, F., (eds.) Loc. Cit. pp. 269-277.

Pappalardo, R. T., et al. (1999): Does Europa have a subsurface ocean? Evaluation of the geological evidence. J. Geophys. Res., Vol. 104, 24,015-24,055.

Phillips, C. and Chyba C. (2001): Europa: Prospects for an Ocean and Life in Chela-Flores, J., Owen, T. and Raulin, F. (eds.) Loc. Cit., pp. 25-34.

Villegas, R, Castillo, C. and Villegas, G.M. (2000): The origin of the neuron, in: Chela-Flores, J., Lemarchand, G.A. and Oro (eds.) Loc. Cit., pp. 195-211.