The question of life’s origin has engaged the minds of humans since they first contemplated our place on Earth and in the Universe. The subject often elicits emotion—first because it involves ourselves, and second because biochemists don’t yet have a comprehensive account of the specific steps that led to life on our planet.

A Non-scientific Idea Many people have been raised to accept unquestioningly certain principles, one of which is that life originated by means of a God or gods. The theological or philosophical idea that life resulted from such a supernatural process is a belief. Admittedly, it might be a perfectly good belief, but it remains just that—a belief—for no unambiguous information, acceptable in a laboratory of science or a court of law, confirms the creation of life by a supernatural being or beings. Scientists have no clear data whatsoever supporting the idea that someone or something deposited already-made life on planet Earth long ago. Furthermore, we have no known way to test experimentally the idea that divine intervention created life.

Science is agnostic when it comes to God—not atheistic, as some people prefer to read that laden word wrongly—just agnostic. Aside from personal feelings or cultural persuasions, most professional scientists just don’t know what to make of a God or gods. We simply have no bone fide data on which to base a judgment.

The belief that life suddenly arose by means of some vitalistic process is outside the realm of modern science. Today’s scientific method, which is a means of inquiry based on reasoned logic bolstered by experimental and observational tests, cannot be used to study supernatural ideas for the origin of life. Accordingly, such ideas, unprovable even in principle, seem destined to remain beliefs forever, hence beyond the subject of science.

Three Scientific Proposals Several alternative theories for the origin of life do not require the help of supernatural beings. Each of these theories relies on natural principles and each can be tested experimentally. These theories are thus based on science rather than on theology, and only one of them has thus far survived the test of time, criticism, and debate.

First, life might have originated on Earth by means of panspermia, meaning “germs everywhere.” This idea, also called exogenesis, maintains that microscopic living organisms came to our planet from outer space. An asteroid or comet, perhaps containing primitive cells or simple bacteria, could have fallen to Earth at some time in the past, after which they evolved over billions of years into the more advanced forms of life now spread across our planet. That said, no meteorites—the landed debris of asteroids and comets—have ever been shown to harbor bona fide life.

The basic tenet of panspermia is that primitive life, which originated someplace else, was deposited on Earth’s surface by means of a collision with some other object that already harbored life. However, most space scientists argue that unprotected simple life wouldn’t likely survive the harsh environment of outer space or the fiery plunge into our atmosphere. High-energy radiation and high-speed particles in interplanetary and interstellar space, as well as violent friction and intense heat while moving through air, would almost surely destroy any form of life riding on the backs of small celestial bodies. On the other hand, microscopic spores might survive such alien conditions, provided they’re deeply embedded within the incoming rocks. If biologists have learned anything new about life recently, it’s that life is very hardy and often capable of surviving in extreme environments.

(Outlandish versions of the panspermia idea abound, perhaps the strangest of them being that life on Earth arose from the garbage dumped here eons ago by extraterrestrial voyagers! Likewise, extraterrestrials might have deliberately seeded our planet, if only because of missionary zeal. These and other bizarre variants of the panspermia theory have fueled science-fiction writers for decades, but working scientists are content with regarding them as truly “garbage theories.”)

A related aspect of panspermia has recently become popular—some call it “weak panspermia”—whereby only the ingredients for life, but not life itself, are delivered to Earth from space. With the rash of discoveries of organic molecules in interstellar space during the past few decades, as noted in the earlier STELLAR EPOCH, some researchers have proposed that not necessarily life itself, but the basic chemicals needed for life, might have arrived on Earth embedded in comets or asteroids. These molecules could have then acted as seeds that gradually spawned life by natural chemical means—endogenesis, as explained below. It is true that some meteorites, particularly the carbonaceous chondrites known to contain much carbon and to derive from the ancient asteroids, house an array of chemicals including life’s building blocks that apparently did survive the joy ride through Earth’s atmosphere.

The Murchison meteorite, which fell near Murchison, Australia, in 1969, is the foremost example of this kind of bolide containing raw materials capable of kick-starting life on Earth several billion years ago. Other meteorites have been shown to contain bubble-like organic globules similar to those produced in laboratory simulations of life’s origin described later in this CHEMICAL EPOCH, the most recent one having landed in Canada’s Yukon Territory just days into the new millennium. What’s more, simple organics have been clearly detected in some well-studied comets, such as Halley, Hale-Bopp, and Hyakutake that recently graced our skies while visiting the inner Solar System. At the very least, these findings show that such molecules needed for life can conceivably form in an interplanetary or interstellar environment, and that they might have reached Earth’s surface unscathed after their fiery descent.

On the other hand, many biochemists argue that organic chemicals could have formed just as easily (and perhaps more so) indigenously on Earth, without looking to outer space for answers to terrestrial puzzles. Even if the notion of panspermia someday becomes a more promising idea for the origin of Earth’s life, it doesn’t qualify as a valid theory for the origin of life itself. “Strong panspermia” (whereby intact life falls to Earth like manna from heaven) merely defers the question of life’s origin, transferring it to some other, unknown locale in the Universe.

Another theory of life’s origin—one that directly addresses the ultimate origin of life itself—goes by the name spontaneous generation. Here, life is thought to have emerged rather suddenly and fully developed from peculiar arrangements of non-life. This idea was popular as recently as a century ago, yet only because people were misguided by their senses. For example, small worms often appear on decaying trash and mice sometime seem to squirm spontaneously out of dirty linen. Such phenomena were once claimed as evidence for the spontaneous generation of new life from the decayed remains of old life. However, although the observations were correct, the interpretations of those observations were not. Hardly a century ago, most naturalists just didn’t realize that flies often lay eggs on garbage, after which the eggs hatch to become worms. Similarly, mice don’t originate in soiled sheets, though that may be indeed where they like to hide.

The theory of spontaneous generation was proved incorrect when scientists began carefully monitoring laboratory experiments. The 19th-century French chemist Louis Pasteur, in particular, was one of the first researchers to conduct experiments under sterilized conditions. By using specially designed equipment, he was able to show that any parcel of air contains microorganisms among other unseen contaminants. Without special precautions and close inspection, living matter often comes into contact with nonliving matter, giving the illusion that life originates suddenly in places where no life had existed before. However, by heating the air and thus destroying the microorganisms, Pasteur thoroughly disproved the idea of the spontaneous generation of life. Once sterilized and isolated, air remains free of life, even microscopic life, indefinitely.

A third theory of life’s origin is known as chemical evolution. In this idea, pre-biological changes slowly transform simple atoms and molecules into the more complex chemicals needed to produce life. Favored by most scientists today, the central premise of chemical evolution stipulates that life arose naturally from nonlife. In this sense, the theories of chemical evolution and spontaneous generation are similar, but the timescales differ. Chemical evolution doesn’t occur suddenly; instead, it proceeds more gradually, eventually building complex structures from simpler ones. This modern theory then suggests that life originated on Earth by means of a rather slow evolution of nonliving matter. How slowly and when precisely we are unsure.

Estimates of the timescale over which chemical evolution occurred can be inferred by studying fossils—the hardened remains of dead organisms whose skeletal outlines or bony features are preserved in ancient rocks. For example, Figure 5.2 shows how sedimentary rock, when magnified many times, yields clear evidence for the fossilized imprints of ancient individual cells—the simplest known form of life. Radioactive testing proves that the age of the rock is typically 2-4 billion years. This is taken to be the duration of time that the fossils have been buried, presumably having been trapped in the rock while it was solidifying, thus making them some of the oldest fossils ever found.

FIGURE 5.2 FIGURE 5.2 — The photograph at left, taken through a microscope, shows fossilized cells found in Canadian rock radioactively dated to be ~3 billion years old. The remains of these primitive organisms display concentric spheres with semi-permeable membranes and smaller attached spheroids. The image at right shows a magnified view of one of these ancient cells more clearly. The fossil’s inner wall is ~10-3 cm (or 10 microns) across. (E. Barghoorn)

Knowing that Earth originated ~4.5 billion years ago and that the oldest rocks crystallized from their early molten state ~4 billion years ago, we conclude that life likely originated roughly a billion years after Earth formed and <0.5 billion years after Earth’s crust cooled enough to support life. Since even older, as yet undiscovered, fossils probably lie buried somewhere in Earth’s rocks, we surmise that the most primitive forms of life may have taken hardly more than a few hundred million years to evolve chemically from non-life. Conceivably, they might have taken even less time, even as short as millennia or centuries. Clues to the history and tempo of life’s origin are likely written not only in their ancient structures (fossils) but also inside the cells and molecules (genes) of today's organisms.

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