Sometimes I think we're alone. Sometimes I think we're not.
In either case, the prospect is staggering!

Arthur C. Clarke [2]

 

Some may consider this hypothesis rather improbable, but for the purposes of investigating life on other worlds, it only needs to be possible. We should also bear in mind that much (if not most) of the Earth's biomass lives below the surface. (Ref.) Planets and moons that at first sight seem uninhabitable may still harbor life beneath their surfaces if the right conditions exist, as is widely hoped for on Europa - a moon of Jupiter - and perhaps even Mars.

On December 7^th 1876, the converted British corvette HMS Challenger set sail on what was to be a four year voyage to systematically survey the world's oceans. A central aim of the expedition was to search the sea bed for clues as to the origin of life on Earth. [3] Although unsuccessful in this regard, Sir John Murray, editor of the fifty volume Report of the Scientific Results of the Exploring Voyage of H.M.S. Challenger, nevertheless described the voyage as "the greatest advance in the knowledge of our planet since the celebrated discoveries of the fifteenth and sixteenth centuries." [4]

Despite the achievements of the geologists of the time—in particular James Hutton and Darwin's mentor Charles Lyell[5]—nineteenth century scientists were unsure of the age of the Earth. Many educated people still believed the seventeenth century assertion of Bishop Usher, that the world had been created in 4004 BC. Added to this, and notwithstanding Darwin's impact on his peers, much doubt remained as to whether Natural Selection could account for the variety and complexity of life observed even if the world was older than had been traditionally believed.

More than a century after the Challenger returned to Portsmouth, and in spite of subsequent advances in understanding—both of the formation of the early Earth and of how lineages evolve—the mystery of life's origin remains unsolved. The instincts of the Challenger's scientists, however, may have been sharper than we suppose. Recent discoveries have taken origin-of-life researchers back to the bottom of the oceans, and in particular to the geologic features found at plate boundaries. [6]

What has led researchers to these sites has been the discovery of organisms living in what were previously considered lethally toxic conditions around oceanic hydrothermal vents. This finding has given rise to speculation that earthly life may have originated at one of these sites. [7] This is an appealing idea, but these vents only remain open for short periods of time—too brief, it would seem, for life to both begin and evolve sufficiently to survive after the vent closes. [8] Defenders argue that these objections presume hydrothermal venting four billion years ago resembled that of today. This is unlikely to be the case, and in any event rift zones may remain active for hundreds of millions of years—easily enough time for substantial genetic drift. As we shall see, the early Earth could hardly have been more different from the temperate planet we call home, and Hadean crustal activity is now a hotly pursued avenue of research. [9]

Further support for the sub-surface abiogenesis hypothesis comes from another, more recent discovery of bacteria [10] living in, and feeding on, the "glassy" basaltic rocks of the sea bed. [11] These bacteria are found around the globe, wherever bores have been taken, at depths of up to four hundred meters beneath the sea floor (and up to four thousand meters beneath sea level!). These bacteria are from the family Chemosynthetic Lithoautotrophs. The first part of their name refers to their particular means of metabolism—chemosynthesis—the manufacture of carbohydrates from CO₂ and H₂O using energy obtained from chemical reactions between non-organic compounds such as Si, H₂, NH₃, NO₂, and H₂S. [12] As for lithoautotroph, 'litho' is from the Greek for stone, 'auto' is Greek for self, and finally 'troph' is from the Greek to nourish.  'Autotroph' thus refers to the fact that these organisms gather their 'own' carbon from CO₂ rather than from organic sources.

Preliminary results indicate that microorganisms of this kind have been eating away at the Earth's recycling crust for billions of years, and it is now clear that they are responsible for changes in rock chemistry previously thought to be caused by purely physical processes—an important consideration when looking at the geologic and atmospheric composition of other worlds. There is also widespread speculation that the Earth's carbon cycle may also be influenced by these organisms. In any case, their existence demonstrates that the Earth's biosphere extends even further than previously thought, and is another reminder that life endures in the most unlikely places.

Chemosynthetic Lithoautotrophs are members of a wider group of microbes known as the "extremophiles" [13]—usually defined as organisms that make their livings in what seem, to us, especially hostile environments—at the margins of what is supposedly habitable. Extreme environments include: hydrothermal vents, hot sulphurous springs, even lakes underneath glaciers; and bizarre as it may sound, one of these "hellish" primeval environments—although devoid of oxygen, bathed in all manner of seemingly toxic chemistry, and subject to temperatures and pressures that would thoroughly cook most living things—may have been the nursery of life on Earth, and for a time the only place where any kind of life existed. Indeed, most organisms that live in these conditions would find the low-pressure, oxygen-rich, and ultraviolet-intense conditions of the contemporary Earth's surface every bit as lethal as their environments would be to us. To them, we are the extremophiles.

As evolutionary theorists from Darwin onwards have pointed out, [14] there are simply given environmental conditions—of temperature, acidity, pressure, chemistry and so on—and every organism functions within its preferred range. An organism simply exists in a given environment at a given time, and depending on the range of conditions it can endure, and the resources available, it either survives or dies. If it lives, and multiplies, the differential survival of its descendants is the mark of successful adaptation.

The early earth was quite unlike the world we know, and would certainly have been far too hostile for any human to endure. The physicist and astrobiologist Paul Davies begins his 1999 book The Fifth Miracle (since retitled The Origin of Life) with the following scenario:

Imagine boarding a time machine and being transported back four billion years. What will await you when you step out? No green hills or sandy shores. No white cliffs or dense forests. The young planet bears little resemblance to its equable appearance today. Indeed, the name "Earth" seems a serious misnomer. "Ocean" would suit better, for the whole world is almost completely submerged beneath a deep layer of hot water. No continents divide the scalding seas. Here and there the peak of a mighty volcano thrusts above the surface of the water and belches forth immense clouds of noxious gas. The atmosphere is crushingly dense and completely unbreathable. The sky, when free of cloud, is lit by a sun as deadly as a nuclear reactor, drenching the planet in ultraviolet rays. At night, bright meteors flash across the heavens. Occasionally a large meteorite penetrates the atmosphere and plunges into the ocean, raising gigantic tsunamis, kilometers high, which crash around the globe. The seabed at the base of the global ocean is unlike the familiar rock of today. A Hadean furnace lies just beneath, still aglow with primeval heat. In places the thin crust ruptures, producing vast fissures from which molten lava erupts to invade the ocean depths. The seawater, prevented from boiling by the enormous pressure of the overlying layers, infuses the labyrinthine fumaroles, creating a tumultuous chemical imbroglio that reaches deep into the heaving crust. And somewhere in those torrid depths, in the dark recesses of the seabed, something extraordinary is happening, something that is destined to reshape the planet and, eventually perhaps, the universe. Life is being born. [15]

Thanks to tectonic activity, nothing remains of the Earth's surface from the Hadean era, so finding evidence that indicates what was going on requires ingenious investigation. Evidence of ancient life comes in two primary forms: fossils, and deposits of organic carbon—both of which are found in rocks. This is problematic because there are no surviving rocks—and therefore fossils—from four billion years ago. [16]

The oldest carbon-bearing rocks discovered to date have been unearthed in Greenland (3.7 to 3.8 billion years old [17]), but claims that they demonstrate evidence of life are contested. The oldest reliable fossils yet discovered are 3.465 billion years old (GA - giga-annum), [18] and it is highly unlikely that they represent the earliest life on Earth. The inevitable conclusion being that life began, as Davies suggests, as soon as the planet had cooled sufficiently for liquid water to form.

A recent additional resource available to researchers is DNA. The rate at which DNA mutates is becoming increasingly better known and researchers are able to use this data as a molecular clock. If any extant microorganisms live in conditions similar to those that prevailed 4 GA ago, then perhaps they carry some vestigial clues within their DNA as to their ancestors' physiology and thus their environment. Hopefully this technique will yield results soon.

Also, as the Moon testifies, the early Earth was subject to persistent and violent bombardment by cosmic debris. [19] This fact is at odds with the widely held (and seemingly reasonable) supposition that for life to begin, exactly the right conditions must exist at some locality, followed by a substantial period of time during which these benign conditions (whatever they may be) prevail. This period of "gestation" is necessary for fragile life to spread and genetic variation to evolve, for without any variety for natural selection to operate on, all life would be identical and therefore equally vulnerable to extinction. It would seem, therefore, that any nascent life form evolving on or near the surface of our planet (and perhaps any planet) during the first few million years or so of its existence would be in grave and continuous peril.

On Earth, frequent climactic, chemical, and perhaps even sterilizing events may have occurred during that time as a result of impacts, [20] and it might therefore be profitable to consider alternative, more protected sites where the presumably delicate and complicated chemistry necessary for the formation of life could have proceeded with less interruption.

If, on the other hand, life did begin on or near the Earth's surface (or perhaps even in the atmosphere) during this period of heavy bombardment, as some suggest, [21] then it may have been that life began not once, but several times—somehow reappearing after each Darwinian pond evaporated or sterilizing impact struck. [22] This would mean that the appearance of life was no lucky chance, in fact quite the contrary, it would suggest that life cannot be prevented if the conditions are right. If Earthly life, in all its fragility, originated at the surface, it somehow survived both the inevitable cosmic hazards that occasionally sterilized the surface, and the caustic levels of ultra-violet radiation present at the time—perhaps forty times greater than we experience today. [23]

Rather than endowing emergent life with a clutch of extraordinary properties that few evolved organisms possess, I prefer to reflect on the certainty that the Earth's first living things must have existed in a survivable and nourishing environment. Professor Davies' (and others) conjecture seems, therefore, quite appealing: life on Earth began not in "some warm little pond" as Darwin imagined, [24] nor at the boundary between water and land, as has been supposed, nor even in the oceans themselves, but deep within the Earth's crust. The attractions of this model are: constant temperature and pressure from geothermal activity; a steady supply of energy and reactive chemistry; and protection from the volatile and violent environmental changes occurring at the surface. In this scenario, life may only have needed to begin once, and could have gradually evolved into more varied and robust forms before finally "emerging" on the surface of the planet when sufficient environmental changes and/or genetic drift had occurred. This might also partially explain why the earliest known fossils—of photosynthesizing and surface dwelling cyanobacteria and the early stromatolites—are relatively complex structures: their subterranean ancestors had already been around for quite a while.

Introducing this paper is a quote from Fred Hoyle in which he seeks to show that life cannot simply spring into existence by chance, and therefore that a more elaborate—and perhaps intelligent—mechanism must have played a part. His argument seems unassailable if we take his metaphor of a 747 being serendipitously assembled by a hurricane as an accurate reflection of the problem. But he overstates the case. The Earth's first life was surely more like the Wright Brothers' Flyer than a Boeing 747, and the metaphor looks quite different if we restate the problem thus: various items are scattered across a junkyard. Is it possible that the forces of nature will eventually configure a number of them in such a way as to produce something that can fly? Or, translated into Darwinian language, given certain molecules, favorable conditions and sufficient time, will nature put something together that can minimally replicate?

A question every bit as perplexing as life's origin is explaining how life has survived. The Australian astronomer Ray Norris has noted that aside from asteroid impacts, the effects of nearby supernovae and gamma ray bursters need to be borne in mind, as their effects should blast Earth with deadly radiation every 200 million years or so. [25] Norris believes that some of these events should be more than adequate to effectively sterilize the planet. Yet four billion years after life began on Earth, and despite several partial extinction events, life is still flourishing, so it seems either his calculation is very wrong, or Earth has been unbelievably lucky. Of course, organisms protected by several kilometers of rock and water would be far less vulnerable to these cosmic dangers. But if Norris's threat is right, then we need to consider the possibility that most life in the universe is underground—as it is on Earth. [26] The processes of life may have begun on many planets throughout the galaxy, possibly deep beneath their surfaces, but because of unfavorable conditions, including Norris's scenario, it has never evolved beyond the microbial stage or made it to the surface.

Conclusion

Where there is at least occasional access to liquid water, a source of carbon (even if it is bound to oxygen) and some harnessable energy, life thrives—in every place we look, and it is possible that some life forms present on Earth today closely resemble some of the earliest replicating organisms. The leading contenders for this distinction are microbes that shun oxygen and sunlight, and prosper in conditions of intense heat, pressure, and the steady reactive chemistry found in rocks—what has come to be known as the Subsurface Lithoautotrophic Microbial Ecosystem or SLiME. That similar ecosystems might exist on other worlds, including Europa, or even Mars, is no longer a startling claim. The question is not whether such organisms could exist on these worlds, but rather, were the necessary conditions ever in place for these kinds of organisms to evolve?

RSP 2001

See also here and:
http://www.ast.cam.ac.uk/AAO/local/www/jab/astrobiology/early_life.html
The exploration of Lake Vostock in Antarctica also touches on many of these themes - see:
http://www.scar.org/treaty/atcmxxvii/ip100subglaciallakes.pdf


Notes

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