Somewhere around four and a half billion years ago, planet Earth coalesced out of left over debris from the sun’s formation. For most of today’s inhabitants, early Earth would have been a hellish place: volcanoes erupting, comets and meteorites crashing down, oceans boiling and no oxygen whatsoever.
Yet within a billion years or so years, the planet was teeming with life. “According to their fossil traces,” says Christian de Duve, “the bacteria that lived 3.5 billion years ago were diverse and advanced,” adding that “No doubt, these early life forms were preceded by more rudimentary ones, themselves preceded by more rudimentary ones, themselves preceded by the common ancestor of all life.”
Scientists have been asking for years when this last universal common ancestor (LUCA, which might not have been a single organism but rather a collection of microbes sharing a common gene pool) arose and arguing about where it came from. The fight is far from over.
Darwin’s “warm little pond”
Charles Darwin wrote in 1871 that a “warm pond” containing “ammonia and phosphoric salts, light, heat, electricity” would chemically form a “proteine” compound which could undergo the still more complex changes necessary to produce a living organism.
But Richard Dawkins notes that self replication is necessary for evolution and this takes DNA or even RNA, neither of which had as yet been described and thus were unknown to Darwin. Because of this, Dawkins considers the spontaneous generation of protein as a key event in the origin of life “less promising than most of Darwin’s ideas.”
That does not mean that, given our current understanding of genetics, the idea of a warm little pond is not conceptually valid; just substitute DNA or RNA for protein.
Indeed, a goodly number of researchers are convinced that life began in a warm primordial ocean full of organic molecules, a concept that grew legs when a young graduate student called Stanley L. Miller working in Harold Urey’s laboratory at the University of Chicago decided to see what lightning could do in the hydrogen-rich mixture mimicking Earth’s early atmosphere.
Miller simulated primitive thunderstorms with repeated electrical discharges blasted into a witch’s brew of methane, ammonia and hydrogen, an experiment he did with the “reluctant consent” of Urey who considered the project too “iffy” for a doctoral thesis.
But the results – the generation of key amino acids and other organic molecules – made Miller an instant celebrity, says de Duve and Miller’s paper in the May 15, 1953 magazine Science launched modern research on the origins of life.
More recent research has seriously questioned the conditions of Miller’s experiments, suggesting that early Earth may have contained more carbon dioxide and nitrogen than methane and ammonia; the former are far less reactive than the latter.
However, since most of the first half billion years of our rock record has recycled back into the planet’s core, there is a decided lack of information about the nature of Earth’s prebiotic atmosphere. But it is possible, and even probable, that local syntheses were happening all over early Earth’s surface making global atmosphere not all that relevant.
For example, deep-sea hydrothermal vents are considered a good birthing place for life as are A̳n̳c̳i̳e̳n̳t̳ volcanoes and the crust at the ocean’s bottom. But the scientific community has as yet to reach a scientific consensus on where and how this was happening.
“In the meantime,” says de Duve, “unexpected support for the validity of Miller’s findings, if not his experimental conditions, has come from outer space.” Cosmic spaces are permeated by interstellar dust, microscopic particles that contain highly reactive combinations of carbon, hydrogen, nitrogen, oxygen as well as sulfur and silicon.
These molecules could be synthesized into biologically significant compounds in deep space and delivered to earth in meteorites. Carbon-rich meteorites collected on earth have yielded amino acids and other biologically important organic molecules.
The cosmic chemistry of life
The concept that life may have come from the universe has a long history. At the turn of the century the Swedish physicist Svante Arrhenius developed a theory for the origin of life on earth and called it “panspermia,” postulating that life in the form of bacterial spores was transported to earth by light pressure.
By “panspermia,” Arrhenius meant, says de Duve that “the seeds of life exist everywhere in space and are showered continually on earth.”
Because of the intense radiation in space, this was considered implausible by Francis Crick, who with James Watson described the DNA helix in 1953 and Leslie Orgel, a pioneer in prebiotic chemistry.
They devised another proposal, called “directed panspermia,” suggesting that microbes were brought to Earth on a spaceship sent by a distant civilization somewhere in the universe. Not unexpectedly, few people seem to have believed them.
Enter astronomers Fred Hoyle and Chandra Wickramasinghe who “claimed that viruses and bacteria continually arise on tails of comets and fall on the Earth with particles of cometary dust,” says de Duve.
They even considered that some of these “germs” could be pathogenic and start disease. Hoyle and Wickramasinghe also argued that organic molecules are deposited on Earth during close encounters or impacts with comets, joining the gene pool and making evolution possible.
While there is no solid evidence that a spaceship or its inventors exist, “things are different for comets and other celestial objects, such as meteorites,” says de Duve.
Most recently, for example, NASA scientist Richard B. Hoover has convincingly shown that microbial remains, looking very much like cyanobacteria, are embedded in the Murchison meteorite. Knowing how adaptable, hardy and A̳n̳c̳i̳e̳n̳t̳ cyanobacteria are on earth, they are a very likely A̳n̳c̳i̳e̳n̳t̳ ancestor.
Searching for life elsewhere in the universe
Gustaf Arrhenius cautions that we need to “differentiate between claims for life in meteorites ascribed to panspermia and life on other planets, primarily Mars, or satellites – most focused on Titan- in our solar system.”
Richard Hoover, who deserves praise for his dedicated effort, he says, “has to fight the same battle as anybody else – the risk of contamination affecting everything that has touched Earth or earthlings.”
Mars, he says, is a different case entirely, “Here we deal with debate over inorganic mineral structures that have a striking wormlike shape and are therefore believed by believers to be fossilized organisms. The question of extinct or existing life on Mars is being brought closer to resolution by NASA’s intense mission programs.”
“The best and almost available material to convincingly show space-based life,” says Arrhenius, “would be protected, returned or in situ analyzed Martian soil or ice, or cometary particles captured by space probes.” The latter, he says, have been studied “but, alas, no bugs so far.” Thus, according to Svante’s grandson Gustaf, “panspermia remains a tantalizing but unproven concept, one of NASA’s targets.”
Ah stardust, the stuff that scientific dreams are made of.