The Meaning of Human Existence (9 page)

You may occasionally hear human societies described as superorganisms. This is a bit of a stretch. It is true that we form societies dependent on cooperation, labor specialization, and frequent acts of altruism. But where social insects are ruled almost entirely by instinct, we base labor division on transmission of culture. Also we, unlike social insects, are too selfish to behave like cells
in an organism. Almost all human beings seek their own destiny. They want to reproduce themselves, or at least enjoy some form of sexual practice adapted to that end. They will always revolt against slavery; they will not be treated like worker ants.

9

Why Microbes Rule the Galaxy

B
eyond the Solar System there is life of some kind. It exists, experts agree, on at least a small minority of Earthlike planets that circle stars as close as a hundred light-years to the Sun. Direct evidence of its presence, whether positive or negative, may come soon, perhaps within a decade or two. It will be obtained by spectrometry of light from mother stars that passes through the atmospheres of the planets. If the analysts detect “biosignature” gas molecules of a kind that can be generated only by organisms (or else are far more abundant than expected in a nonliving equilibrium of gases), the existence of alien life will pass from the well-reasoned hypothetical to the very probable.

As a student of biodiversity and, perhaps more importantly, at heart a congenital optimist, I believe I can add credibility to the search for extrasolar life from the history of Earth itself. Life arose here quickly when conditions
became favorable. Our planet was born about 4.54 billion years ago. Microbes appeared soon after the surface became even marginally habitable, within one hundred million to two hundred million years. The interval between habitable and inhabited may seem an eternity to the human mind, but it is scarcely a night and a day in the nearly fourteen-billion-year history of the Milky Way galaxy as a whole.

Granted that the origin of life on Earth is only one datum in a very big Universe. But astrobiologists, using an increasingly sophisticated technology focused on the search for alien life, believe that at least a few and probably a large number of planets in our sector of the galaxy have had similar biological geneses. The conditions they seek are that the planets have water and are in “Goldilocks” orbit—not close enough to the mother star to be furnace-blasted, yet not so far away that their water is forever locked in ice. It should also be kept in mind, however, that just because a planet is inhospitable now does not mean it always has been so. Further, on a seemingly otherwise barren surface there may exist small pockets of habitats—oases—that support organisms. Finally, life might have originated somewhere with molecular elements different from those in DNA and energy sources used by organisms on Earth.

One prediction seems unavoidable: whatever the condition
of alien life, and whether it flourishes on land and sea or barely hangs on in tiny oases, it will consist largely or entirely of microbes. On Earth these organisms, the vast majority too small to see with unaided vision, include most protists (such as amoebae and paramecia), microscopic fungi and algae, and, smallest of all, the bacteria, archaeans (similar in appearance to bacteria but genetically very different), picozoans (ultrasmall protists only recently distinguished by biologists), and viruses. To give you a sense of size, think of one of your own trillions of human cells, or a solitary amoeba or single-celled alga, as the size of a small city. Then a typical bacterium or archaean would be the size of a football field and a virus the size of a football.

Earth’s overall microbial fauna and flora are resilient in the extreme, occupying habitats that might at first seem death traps. An extraterrestrial astronomer scanning Earth would not see, for example, the bacteria that thrive in volcanic spumes of the deep sea above the temperature of boiling water, or other bacterial species in mine outflows with a pH close to that of sulfuric acid. The E.T. would not be able to detect the abundant microscopic organisms on the Mars-like surface of Antarctica’s McMurdo Dry Valleys, considered Earth’s most inhospitable land environment outside of the polar ice caps. E.T. would be unaware of
Deinococcus radiodurans
, an
Earth bacterium so resistant to lethal radiation that the plastic container in which it is cultured discolors and cracks before the last cell dies.

Might other planets of the Solar System harbor such extremophiles, as we Earth biologists call them? On Mars, life could have evolved in the early seas and survive today in deep aquifers of liquid water. Abundant parallels of such subterranean regression exist on Earth. Advanced cave ecosystems abound on all continents. They include at the least microbes, and in most parts of the world insects and spiders and even fish as well, all with anatomy and behavior specialized for life in totally dark, impoverished environments. Even more impressive are the SLIMEs (subterranean lithoautotrophic microbial ecosystems), distributed through soil and rock fissures from near the surface to a depth of up to 1.4 kilometers and comprising bacteria that live on energy drawn from the metabolism of rocks. Feeding on them are a recently discovered new species of deep subterranean nematodes, tiny worms of a general kind abundant everywhere on the surface of the planet.

There are places in the Solar System in addition to Mars to search for organisms, at least those with the biology of what we call extremophile on Earth. It makes sense to look for microbes in aquatic islets beneath or around the icy geysers of Enceladus, Saturn’s superactive
little moon. And as opportunity arises, we should (in my opinion) probe the vast aquatic oceans of Jupiter’s moons Callisto, Europa, and Ganymede, as well as Titan, a larger moon of Saturn. All are encased in thick shells of ice. Brutally cold and lifeless on the surface they may be, but underneath are depths warm enough to hold liquid organisms. We can eventually, if we wish, drill through the shells to reach that water—just as scientific explorers are now doing above Lake Vostok, sealed off by the Antarctic ice cap for a million years or more.

Someday, perhaps in this century, we, or much more likely our robots, will visit these places in search of life. We must go and we will go, I believe, because the collective human mind shrivels without frontiers. The longing for odysseys and faraway adventure is in our genes.

The ultimate destiny of the outward-bound astronomers and biologists is of course to reach still farther, very much farther, across almost incomprehensibly far distances into space, to the stars and potentially life-bearing planets around them. Because deep space is transparent to light, the detection of very remote alien life is very much a possible dream. Many potential targets will be found in the mass of data collected by the Kepler space telescope before it partly failed in 2013, together with other space telescopes planned and the most powerful ground-based telescopes. And soon. By mid-2013
almost 900 extrasolar planets had been detected, with thousands more believed likely to be found in the near future. One recent extrapolation (let me pause: extrapolation is admittedly a risky procedure in science) predicts that a fifth of stars are orbited by Earth-sized planets. In fact, the most common class of systems detected thus far include planets one to three times Earth’s size, thus with gravity similar to that of Earth. So, what does that tell us about the potential of life in outer space? First, consider the estimate that ten stars of various kinds exist within ten light-years of the Sun, about 15,000 within 100 light-years, and 260,000 within 250 light-years. Keeping in mind life’s early origin in the geological history of Earth as a clue, it is plausible that the total number of life-bearing planets as close as 100 light-years could be in the tens or hundreds.

To find even the simplest form of extraterrestrial life would be a quantal leap in human history. In self-image, it would confirm humanity’s place in the Universe as both infinitely humble in structure and infinitely majestic in achievement.

Scientists will want (desperately) to read the genetic code of the extraterrestrial microbes, providing such organisms can be located elsewhere in the Solar System and their molecular genetics studied. This step is feasible with robotic instruments, eliminating the need of bringing
the organisms to Earth. It would reveal which of two opposing conjectures about the code of life is correct. First, if the microbial E.T.s have a code different from that on Earth, their molecular biology would be different to a comparable degree. And if such proves to be the case, an entirely new biology might be instantly created. We would further be forced to conclude that the code used by life on Earth is probably only one of many possible in the galaxy, and that codes in other star systems have originated as adaptations to environments very different from those on Earth. If, on the other hand, the code of extraterrestrials is basically the same as that of native Earth organisms, it could suggest (but not prove, not yet) that life everywhere can only originate with one code, the same as in Earth’s biological genesis.

Alternatively, perhaps some organisms manage interplanetary travel by drifting through space, living in cryogenic dormancy for thousands or millions of years, protected somehow from galactic cosmic radiation and surges of solar energetic particles. Interplanetary or even interstellar travel by microbes, called pangenesis, sounds like science fiction. I wince a bit just bringing it up. But it should be considered as at least a remote possibility. We know too little about the vast array of bacteria, archaeans, and viruses on Earth to make any call about the extremes of evolutionary adaptation, here and elsewhere
in the Solar System. In fact, we now know that some Earth bacteria are poised to be space travelers, even if (perhaps) none has ever succeeded. A large number of living bacteria occur in the middle and upper atmosphere, at altitudes of six to ten kilometers. Composing an average of around 20 percent of the particles with diameters of 0.25 to 1 micron in diameter, they include species able to metabolize carbon compounds of kinds found all around them in the same strata. Whether some are also able to maintain reproducing populations, or on the contrary are just temporary voyagers lifted by air currents away from the land surface, remains to be learned.

Perhaps the time has come to seine-haul for microbes at varying distances beyond Earth’s atmosphere. The nets could be composed of ultrafine sheets towed by orbiting satellites through billions of cubic kilometers of space, then folded and returned for study. Such a foray conducted out into space might produce surprising results. Even new, anomalous species of Earth-born bacteria able to endure the most hostile conditions—or the absence of such organisms—would make the effort worthwhile. It would help answer two of the key questions of astrobiology: What are the extreme environmental conditions in which current members of Earth’s biosphere can exist? And might organisms originate in other worlds in conditions of comparable severity?

10

A Portrait of E.T.

W
hat I am about to tell you is speculation, but not pure speculation. It is that by examining the myriad animal species on Earth and their geological history, then extending this information to plausible equivalents on other planets, we can make a rough sketch of the appearance and behavior of intelligent extraterrestrial organisms. Please don’t leave me at this point. Refrain from dismissing this approach out of hand. Instead, call it a scientific game, with the rules changing to fit new evidence. The game is well worth playing. The payoff, even if the chance of contact with human-grade aliens or higher proves forever vanishingly small, is the building of a context within which a sharper image of our own species can be drawn.

Granted there is temptation to leave the subject to Hollywood, to the creation of the nightmarish monsters of
Star Wars
or the Americans-in-punk-makeup populating
Star Trek
. Learning about extraterrestrial microbes is one thing: it is not difficult to imagine in broad principles the self-assembly of primitive organisms at the level of Earth’s bacteria, archaeans, picozoans, and viruses; and scientists may soon find evidence of such microbial life on other planets. But it is an entirely different matter to picture the origin of extraterrestrial intelligence at the human grade or higher. This most complex level of evolution has occurred on Earth only once, and then only after more than six hundred million years of evolution within a vast diversity of animal life.

The final evolutionary steps prior to the human-level singularity, that is, altruistic division of labor at a protected nest site, has occurred on only twenty known occasions in the history of life. Three of the lines that reached this final preliminary level are mammals, namely two species of African mole rats and
Homo sapiens
—the latter a strange offshoot of African apes. Fourteen of the twenty high achievers in social organization are insects. Three are coral-dwelling marine shrimp. None of the nonhuman animals has a large enough body, and hence potential brain size, needed to evolve high intelligence.

That the prehuman line made it all the way to
Homo sapiens
was the result of our unique opportunity combined with extraordinarily good luck. The odds opposing
it were immense. Had any one of the populations directly on the path to the modern species suffered extinction during the past six million years since the human-chimpanzee split—always a dire possibility, since the average geological life span of a mammal species is about five hundred thousand years—another
hundred
million
years might have been required for a second human-level species to appear.

Because of all of the pieces that likely must also fall in place beyond the Solar System, intelligent E.T.s are also likely to be both improbable and rare. Given that, and assuming they exist at all, it is reasonable to ask how close to Earth might E.T.s at the human grade or higher be found. Allow me an educated guess. Consider first the many thousands of large terrestrial animal species that have flourished on Earth for the past four hundred million years, with none but our own making the ascent. Next, consider that while 20 percent or more star systems may be circled by Earthlike planets, only a small fraction may carry liquid water and also possess a Goldilocks orbit (to remind you, not so close to the mother star to be baked, not so distant to be kept permanently deep-frozen). These pieces of evidence are admittedly very slender, but they give reason to doubt that high intelligence has evolved in any of the 10 star systems within 10 light-years of the Sun. There
is a chance, slight but otherwise impossible to judge reliably, that the event has occurred within a distance of 100 light-years of the Sun, a radius encompassing 15,000 star systems. Within 250 light-years (260,000 star systems), the odds are dramatically increased. At this distance, if we work strictly off the experience of Earth, the uncertain and marginally possible changes to the probable.