European Space Agency Special Publication ESA SP 518, 337-340.

CAN EVOLUTIONARY CONVERGENCE BE TESTED ON EUROPA?

Julian Chela-Flores

The Abdus Salam International Center for Theoretical Physics,
Strada Costiera 11, PO Box 586; 34014 Trieste, Italy, and
Instituto de Estudios Avanzados, Apartado
17606 Parque Central, Caracas 1015A, Venezuela.
Phone +390-40-2240392; fax: +390-40-22-42 41; e-mail: chelaf@ictp.trieste.it;
URL: http://www.ictp.trieste.it/~chelaf/index.html

ABSTRACT

A major objective in solar system exploration has to be the insertion of appropriate biology-oriented experiments in future missions. We discuss various reasons for suggesting that this type of research be considered a high priority for feasibility studies and, subsequently, for technological development of appropriate melters and submersibles. With the assumption that Darwin's theory is valid for the evolution of life anywhere in the universe [1], various degrees of convergent phenomena argue in favor of the conjecture that universal evolution of intelligent behavior is just a matter of time and preservation of steady planetary conditions. A preliminary test of this conjecture is feasible with experiments involving evolutionary biosignatures on Europa.

1. Introduction

Although Darwin's theory of natural selection is recognized universally as the basis for the discussion of any branch of biology, its relevance to what we now call astrobiology was only clearly pointed out during the commemoration of the centenary of Darwin's death [1]. The points made on that occasion are still relevant: "[Darwinism] is probably the only theory that can adequately account for the phenomena that we associate with life". Dawkins' main concern, and ours, is with the question of whether there are principles that are fundamental to all forms of life that may evolve in the universe. His main criticism is that our writings have been rich in how extraterrestrial life might work, but poor in the discussion of how it might evolve.
Today, two decades later, when we are considering biology in general, and astrobiology in particular, we have the advantage that new and significant experiments are available; they give us some insight into the relative dominance of natural selection over historical contingency. We also have much clearer ideas regarding the distribution of the chemical components of life in interstellar space; finally, there is growing evidence of the universality of the formation of solar systems, and their planetary hydrospheres and atmospheres. These various degrees of convergence will be discussed in detail in the companion to this paper [2]. For these reasons astrobiology is forcing upon us the need to reconsider two fundamental questions:
(A) How far can we attempt to forecast evolutionary responses to perturbations in a natural terrestrial environment?, and
(B) How far can we attempt to forecast evolutionary responses to perturbations of environments that are analogous to those of the Earth?
We have discussed these questions earlier in detail [2], and are briefly reviewed in Sec. 3. We have suggested that the chemical precursors of terrestrial biomolecules may be firstly, ubiquitous in the universe, and secondly that these chemicals may also be precursors of life on planets (or satellites), whose biospheres are analogous to our own. This leads to the question of testing evolution of life during solar system exploration.

2. CONVERGENT EVOLUTION

2.1 How can we test convergent evolution?

Having met different degrees of convergence in the various levels that lead to solar system environments favorable to life, it is reasonable to approach the empirical question of how far we can test the earliest stages of biological evolution in our own solar system. In order to approach possible tests of the first stages of evolution, we may benefit from the results of the Galileo Mission to the Jovian system: For it has brought to our attention aspects of Europa that are favorable to the origin of life. This Galilean satellite rotates around Jupiter in an elliptic orbit, a motion which is constrained by the other three Galilean satellites. As a result, Europa has an intrinsic source of energy due to the resulting effect of 'tidal heating', which is produced by the vicinity of this satellite to its giant planet. Gravity measurements of the Europan moment of inertia from Galileo suggest that Europa has an inner core, a rocky mantle and a surface layer, mainly of liquid water. Impact craters also suggest that there is an ice-covered inner ocean, since they are more shallow than would be expected on a solid (silicate) surface, such as that of the Moon. A robot especially built to penetrate ice overlying a mass of liquid water has been considered for some years - the so called 'cryobot'; the question of melting probes continues to be discussed. A somewhat more remote possibility is to build a corresponding submersible robot ('hydrobot') capable of bearing some experiments in its interior. With both of these robotic tools we would be in a position to make significant advances in planetary exploration: In the Europan ocean we are presented with the problem of deciding whether biology experiments should be planned in due course, and which tests should be taken to the stage of feasibility studies. One class of experiments shall be discussed in Sec. 2.4 within the area of electrophysiology.

2.2 Prebiotic, chemical and biological experiments

There are several levels of experiments and clarifications to be considered in the future Europa campaign [3]:
o Firstly, at the prebiotic level, the content of the Europan ocean should be ascertained. A suite of measurements should be envisaged, in order to establish the subsurface context in which an autochthonous Europan ecosystem would presumably flourish. It would be advantageous to know the relative abundance of the main cations and anions present in the ocean, besides the volatiles such as oxygen, methane and carbon dioxide. (Certain knowledge of the ions present could be considered as a preparatory contribution for the electrophysiological test described in Sec. 2.4.)
o Secondly, the chemical evolution context should be probed. Indeed, searches for amino acids and nucleotides make sense, since such advanced stages of chemical evolution have been detected even in other Solar System bodies, such as meteorites.
However, it should be underlined that there are still gaps in our understanding of the likelihood of life emerging on Europa; for instance:
o The existence of sufficient free-energy sources is possibly the largest unknown factor [3].
o If the ongoing debate on the origin of life at depth (independent of sunlight) can be settled [4], then Europa may have a resident autochthonous ecosystem.

2.3 Adequate responses to extant microorganisms

In spite of these uncertainties and the evident need for further theoretical and experimental work, we will inevitably have to consider simultaneously with the above-mentioned work, how to respond adequately to the eventual encounter with extant or frozen extremophilic microorganisms. Such an event could indeed occur at a very early stage of the Europan surface exploration. From previous experience with Europa analogs on Earth, in the Dry Valley lakes of Antarctica, it cannot be excluded that reprocessing of the ice-crust could bring microorganisms close to the surface from the body of water below, if a Europan ecosystem has evolved [5]. This possibility suggests, in turn, that biology-oriented experiments should be included at the earliest stage of exploration of Europa. Together with prebiotic and chemical evolution experiments, biological evolution experiments should also be included in the total payload available for the first landing missions.
In order to appreciate the significance of the class of experiments that will be discussed in the following section, we raise two related questions:
(1) If convergent evolution has taken place elsewhere, have microorganisms gone beyond the stage of the simplest prokaryotes?
One example of prokaryotic cells are cyanobacteria, such as Oscillatoria, which lack an internal nucleus. On Earth prokaryotic cells have evolved in what we may call a "geologic instant". It is sufficient to consider the Archean microfossils, which are older than two and a half billion years. Evidence for prokaryogenesis may even be one billion years older. On these ancient rocks there are microfossils that have been interpreted as cyanobacteria. Therefore, even in the remote Archean eon, we already have unambiguous biosignatures, which suggest the presence of simple prokaryotic life. Irrespective of the actual date of the Archean microfossils (2,5 to 3,5 Gyr BP), the thesis that prokaryogenesis occurred in a 'geologic instant' is persuasive. We should recall firstly that the Earth itself dates back to approximately 4,6 Gyr BP; and, secondly, that our own star, the Sun, is expected to provide steady conditions for life for another 4 - 5 Gyrs. Prokaryotes, therefore, seem to be a necessary consequence of planetary evolution of terrestrial-like environments (in view of the events that we have summarized above as a 'geologic instant').
(2) Is it possible to recognize in a given prokaryote whether the first steps towards primitive systems for the conduction of ionic currents have already taken place? If simple responses to ionic environments are found to occur, for instance in microorganisms in the Europan ocean, then the first steps towards the generation of neuron-like cells would already have been taken. These evolutionary events are precursors of subsequent biological attributes of living organisms that have strong selective advantage, independent of any specific lineage, such as neurons, nervous systems, brains and eventually signs of intelligent behavior (i.e., communication).

2.4 A bridge between microbiology and astronomy

The two related questions raised in Sec. 2.3 would lead us into establishing a bridge between microbiology and bioastronomy. We can appreciate that in the simplest microorganisms, such as the archaean Haloferax volcanii there is already a 'conduction system', namely a physiological response. We know that in this archaean there are calcium channels linked to movement [6]. In the tree of life, already at the low evolutionary level of cnidarians, say in the medusa or sea anemone, there are primitive nervous systems. These systems of neurons are referred as 'nervous nets' in the specialized literature in neurophysiology. At low stages in the phylogenetic tree of life on Earth, we already find nervous systems made up of neurons in flatworms and nematodes, but it is remarkable that these organisms have also a primitive brain, more precisely a cerebral ganglion. There are many examples of cerebral ganglions in animals. For instance, primitive nervous systems with developed peptidergic neurotransmitters and cerebral ganglions are known to have evolved in the roundworm (a nematode) Ascaris lumbricoides [7]. Typically the ganglion receives inputs form sensory organs and delivers outputs to muscles, via nerve filaments.
From these examples we can infer that in multicellular organisms, almost as soon as some coordinated electrophysiological responses are possible, they have been demonstrated to exist. (In addition to our examples we refer the reader to detailed reviews [8, 9].) We have assumed universal Darwinism (cf., Sec. 1). Consequently, these responses, which are observed at an early stage in the evolution in our phylogenetic tree suggest the following (testable) conjecture: nervous systems will also evolve at an early evolutionary stage elsewhere in the cosmos. If we find microorganisms in our solar system, we can begin to test this conjecture with the following question: Have microorganisms elsewhere developed ionic channels?, or have exo-microorganisms, instead evolved only mechanosensitive channels, responsible for simpler tasks, such as the control of osmotic pressure? All eukaryotes may have inherited potassium, calcium, and later sodium channels (essential for the emergence of the first neuron) from an ancestral cation channel during the Archaean, over 2,400 million years ago [8]. On the Earth biota these proteins are macromolecular pores in the cell membrane. In particular, in prokaryotes several channels have been identified by electrophysiology and genomic analysis, and their evolutionary link with eukaryotic channels have been discussed in detail [10]. Ion channels of the cellular membrane represent a first step in the pathway toward the emergence of simple nerve nets and nervous systems, which are highly dependent on these proteins. But already at early stages in cellular evolution we can appreciate significant differences in ionic channel responses: in the giant cells of the fungus Neurospora, for instance, impulses are very slow - lasting over one minute. This is in sharp contrast with the more evolved responses of Paramecium, where calcium action potentials are typically of the order of 100 msec [9].
It would be a significant step forward in our understanding of life in the universe if, for instance, we were prepared to respond to a reprocessing of the Europan ice, which could be inducing microorganisms close to the surface, as it already happens in the frozen surfaces of the Antarctic lakes (including Lake Vostok). The physiological response (action potential) of the microorganism could be compared with the extreme cases of action potentials typical of Neurospora, on the longer time scale, or Paramecium, on the much shorter time scale (cf., the above-mentioned values of the action potentials). The implementation of the electro-physiological aspects of this experiment, which would require only the assistance of remote control and the use of robotic arms, is a challenge that deserves attention during the planning of experiments [11]:
o The introduction of a microorganism in a previously prepared solution of ions.
o The alteration of the concentration mechanically.
o Inspection of the changes in cell polarization.
These three stages are based on standard electrophysiology. The question of which ions to select has to be given some attention, but calcium, sodium and potassium ions are evident choices. What useful insight could we expect to infer form these experiments? We only need to recall, as we have done in the present paper, that cells with radically different environmental requirements can generate radically different impulses.
The proposed experiment can be carried out either in the ocean (inside a submersible), or on the iced surface (inside a melter probe). New challenges arise from these unusual and novel settings:
o The question of miniaturization. This is a problem which does not seem to be beyond present day technology, and
o In spite of the straightforwardness of the suggested test, difficulties typical of remote control have not been encountered previously in the area of electrophysiology. (We recall that signals would take about half an hour to reach the submersible in the Europan ocean.)
We should underline the relevance of testing the initial stages in the evolution of a neuron-like cell that has been presented in this section. For instance, a significant evolutionary stage would be exposed by the discovery of microorganisms with the capacity of propagating an electric impulse of short temporal duration (i.e., of the order of msec). The larger issue in astrobiology regarding the universal evolution of intelligent behavior is clearly not restricted exclusively to the suggested search for ionic channels at the early stages of the evolution of nervous systems, which we have discussed in this section: The suggestion of searching for traces of extraterrestrial intelligence was already made in the middle of last century (the SETI project). Rephrasing the astronomer's efforts: what is at stake is the search for evolution of intelligent behavior.

3. DISCUSSION

The sharp dichotomy between chance (contingency) and necessity (natural selection as the main driving force in evolution) is relevant for the new science of astrobiology. Independent of historical contingency, natural selection is powerful enough for organisms living in similar environments to be shaped to similar ends. For this reason, it is important to refer to the companion to this paper for details on the following examples, which suggest that, to a certain extent and in certain conditions, natural selection may be stronger than chance [2], and hence the Europan search for life should focus on the early stages of the evolutionary pathway that would be analogous to our own: Firstly, sticklebacks are small northern hemisphere fishes for which it has been demonstrated that natural selection has been the driving force in the evolution that has taken place in three Canadian lakes. Secondly, Black European fruit flies that were transported to California offer a compelling case in favor of the key role played by natural selection in evolution. Finally, anole lizards from Caribbean islands offer some evidence for repeated evolution of similar groups of species, suggesting that adaptation is responsible for the predictable evolutionary responses of these lizards. We can speak in this case of evolutionary history repeating itself.
With regards to the search for the later stages of the evolutionary pathways, we should keep in mind general physiological and ecological aspects of brain evolution that militate in favor of convergence:
o The remarkable expansion of the cetacean neocortex preserves its basic modular subdivisions. This research line has led to the conjecture that there is a limited set of underlying mechanisms that are accessed to building brains [12]. Thus in the course of evolutionary history repetition of similar structures may occur. This also suggests that module size is evolutionary stable across species. Indeed, we can even question whether in the course of brain evolution species differences are really so different [13]. To understand the evolution of the cortex and the increase in behavioral complexity, it may be necessary to assume that anatomical alterations are generated from similar mechanisms. While the future of a given lineage cannot be predicted with certainty, conjectured common mechanisms of cortex evolution give us some certainty of types of modification that are likely to occur in brain evolution.
o In addition, evolutionary convergence of behavior has been identified amongst animals with the largest brains in the oceans and on land, namely the toothed whales (aquatic carnivores, for instance the sperm whale), and the elephants (terrestrial herbivores). These animals, in spite of their radically different habitats, resemble each other more than they do other animals with whom they share similar ancestries, diets, or environments. One specific example of behavioral resemblance is social organization. Since these animals are not closely related through evolutionary history, their shared attributes constitute an example of convergent behavior, not a coincidence [14].
To sum up, we have suggested how to obtain preliminary insights into the question of the distribution of life in the universe. The main conjecture of this work is that universal evolution of intelligent behavior is just a matter of time and preservation of steady planetary conditions. In fact, a preliminary test of this conjecture is feasible with experiments involving evolutionary biosignatures on Europa for the early stages of the evolutionary pathway to intelligent behavior. This aspect of solar system exploration should be viewed as a complement to the astronomical approach for the search of evidence of the later stages of the evolutionary pathways towards intelligent behavior.

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