The Panspermia Hypothesis holds that life on our planet traces its origins to space—say, a microbe-laden meteorite landing on primordial Earth. The theory, conceived before humans even went into space, was actually born out of criticism of Charles Darwin and his perceived failure to explain how life began.

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Without an explanation for the origin of life, some 19th-century scientists argued, the theory of evolution was incomplete and lacked credibility. German biologist Ernst Haeckel wrote:

"The chief defect of the Darwinian theory is that it throws no light on the origin of the primitive organism—probably a simple cell—from which all the others have descended. When Darwin assumes a special creative act for this first species, he is not consistent, and, I think, not quite sincere."

Darwin bristled at such criticism. His research sought to explain how life had adapted and evolved, not how life had begun. The origin question, though important, had no bearing on his thesis. "It is no valid objection that science as yet throws no light on the far higher problem of the essence or origin of life," he wrote in the 1861 edition of The Origin of Species. Darwin observed that nobody had dismissed Newton's laws of motion because he had failed to explain "the essence of the attraction of gravity." Darwin own views of life's origins changed over time—in one private letter, he famously raised the idea of life emerging from chemical reactions in a "warm little pond." Still, he believed that science had not yet reached the point where it could offer anything beyond pure speculation.

"Life is eternal"

Other scientists took it upon themselves to "complete" Darwin's theory. Six years after The Origin of Species was published, a German physician named Hermann Richter presented his theory of panspermia. Our common ancestors, consisting of microscopic life, he argued, had arrived on Earth aboard a meteorite. Richter envisioned a universe where life was deposited from planet to planet, like a bee fertilizing flowers. At some point, he said, a meteor would pass close to our atmosphere and scoop up microbes for delivery to another world, where they would be the seeds for life that would evolve and adapt to a new environment beneath the light of a distant, alien sun.

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Richter had been inspired by the writings of the famous French astronomer Camille Flammarion, whose best-selling books helped popularize the theory of extraterrestrial life. In the ensuing decades, several prominent scientists—including Lord Kelvin and the Nobel Prize-winning physicist Svante Arrhenius—would champion the theory of panspermia. The obvious flaw in this theory, of course, was that while it explained how life had come to Earth, it still didn't explain how that life had come to exist in the first place.

But, the advocates of panspermia countered that life, like matter and energy, had always been an integral part of the universe, and the universe itself was eternal, without a beginning or an end. "Man used to speculate on the origin of matter, but gave that up when experience taught him that matter is indestructible and can only be transformed," Arrhenius wrote in his best-selling 1908 book, Worlds in the Making: The Evolution of the Universe. "For similar reasons we never inquire into the origin of the energy of motion. And we may become accustomed to the idea that life is eternal, and hence that it is useless to inquire into its origin."

For Arrhenius and others, their conviction in the theory of panspermia stemmed in large part from their belief that the alternative was too ridiculous to contemplate: the idea that life had emerged from non-living matter.

Traditionally, the term for this was "spontaneous generation," and it had been around in one form or another for millennia—stories about toads forming out of mud, or maggots mysteriously appearing on rotten beef. Although the scientific revolution had kicked most of that nonsense to the curb, as of the mid-19th century there were scientists who believed that they had found evidence for spontaneous generation at the microscopic level. That theory, however, was disproven once and for all through a series of brilliant experiments conducted by Louis Pasteur in 1871.

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Unfortunately, Pasteur's work gave rise to a misconception that the problem of the origin of life could not be approached by scientific methods. "Dead matter cannot become living without coming under the influence of matter previously alive. This seems to me as sure a teaching of science as the law of gravitation," Lord Kelvin declared. And, those who tried to prove otherwise in laboratories were deemed little better than alchemists. "Almost every year the statement is repeated in biological literature that we have at last succeeded in producing life from matter," Arrhenius complained. "Scientific criticism has, however, relegated these statements to the realm of fairy tales."

Enter the Chemists

But slowly, over the decades, real progress was being made in the emerging field of organic chemistry. In 1828, German chemist Friedrich Wohler synthesized urea—a chemical that his contemporaries had insisted could only be created by a living organism. By 1912, E. A. Schäfer, the founder of endocrinology, could say, with justified conviction, that the study of evolution "is passing every day more out of the hands of the biologist and into those of the pure chemist."

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Schäfer was a staunch critic of the Panspermia Hypothesis, arguing that, "It merely serves to banish the investigation of the question to some conveniently inaccessible corner of the universe." He and his colleagues were determined once and for all to do away with the label spontaneous generation: "Far from expecting a sudden leap from an entirely inanimate substance to a completely animate state of being," he wrote, "should we not rather expect a gradual procession of changes from inorganic to organic matter, through stages of gradually increasing complexity until material which can be termed living is attained?"

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The real turning point came when Alexander Oparin published the 1938 English-language translation of his opus, The Origin of Life. Oparin, a Russian biochemist, is routinely described as the "20th-century Darwin." Cyril Ponnamperuma, who was an internationally recognized theoretician on the origins of life, recalled that:

"The moment this book reached the English speaking audience it was devoured as no other book in the scientific world had been. Nobel prize-winning scientist George Wald described how, as graduate students, they had stopped their lab work when this book arrived. They read it privately in groups to absorb the new thinking and understanding of one of the most difficult problems of all science."

Oparin extended the Darwinian theory of evolution even further backward in time. Relying on geological evidence, he suggested that Earth's early atmosphere differed markedly from our modern atmosphere, and consisted of methane, ammonia, water vapor and hydrogen. As these chemicals mixed together over time, they formed more complex chemicals— including organic compounds and "pre-biotic" molecules that became the foundation for the gradual emergence of life forms. Oparin suggested this was a type of "natural selection"—various forms of simple life might emerge, but the most complex and efficient would survive and become the dominant form of life from which all other life on Earth evolved.

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Oparin's theories, and the research he inspired, shut down the theory that life could only have arrived on Earth from space. But, notable scientists including Stephen Hawking don't rule out the possibility, and, in recent years several experiments have demonstrated that comets and meteorites could have delivered the key components necessary for chemical life to emerge.