Panspermia Revisited
by Edward Willett

Did life originate on Earth or did it arrive here from outer space?

Mainstream biologists will tell you the former. But over the years, the latter idea has always had a few supporters.

This theory that life came to Earth from elsewhere is usually called panspermia (which means "seeds everywhere.") It has a Greek name not just because scientists like Greek names, but because the first advocate of it that we know of was the Greek philosopher Anaxagoras. Aristotle, however, thought life originated through spontaneous generation from non-living ingredients, and his idea ruled for the next couple of thousand years.

But in 1864, Louis Pasteur announced he had proved that spontaneous generation didn't work--at least, not in the way it was thought to work. He showed that no microorganisms would grow in boiled meat broth kept out of contact with the air. If spontaneous generation didn't happen, scientists wondered, then where did life on Earth come from? One possible answer: bacterial spores from outer space.

Spontaneous generation wasn't dead, though. It soon came back in a different form, with the theory that in the primitive atmosphere of Earth (very different from the current atmosphere--more like the methane-laden atmosphere of Saturn's moon Titan), amino acids could form spontaneously and, given eons, organize themselves into self-replicating molecules that eventually evolved into us and every other living thing.

But panspermia wasn't dead, either. In the 1970s, British astronomers Fred Hoyle and Chandra Wickramasinghe discovered that interstellar dust contain organic compounds. They suggested that comets could carry bacterial life clear across galaxies, delivering it to any planets they happened to hit.

Today, astrophysicists find that the building blocks of life can be found just about everywhere in the universe. Specifically, they see (in the spectra of dust clouds and comets and other celestial objects) a group of chemicals called nitrogenated aromatics. Molecularly, these consist of a carbon skeleton in a ring--a square, hexagon or other shape--with alternating double and single bonds. (They're called aromatics because, well, they're aromatic: in various combinations, they might smell like almonds, or bananas...or something less pleasant.)

These carbon rings, with various additions, make up chemicals called purines and pyrimidines, among others. Half of RNA and DNA molecules consist of purines, and the other half of pyrimidines. These chemicals also make up chlorophyll, oxygen-storing pigments in animals, and enzymes that produce energy. 

This discovery has led to a new theory of panspermia, sometimes called pseudo-panspermia, that suggests that these basic building-blocks of life, along with water, might have been delivered to Earth by comet impacts, providing the seed material for life.

The full-fledged version of panspermia--the idea that full-blown microorganisms might have been delivered to Earth from space--is still out of the mainstream. But there's recently been a tantalizing hint of supporting evidence. At the Instruments, Methods, and Missions for Astrobiology VIII conference in Denver in early August, Richard B. Hoover, a NASA astrobiologist, showed scanning electron micrographs of the interior of a meteorite that appeared to show fossilized cyanobacteria.

Hoover was also part of the NASA team that made the controversial announcement a few years ago of the discovery of microfossils in a Martian meteorite found in the Antarctic ice. These photos, however, are far more striking, showing not just what might or might not be individual cells, but what appear to be fossilized colonies of bacteria--masses of them.

The meteorite in question fell near the French town of Orgueil on May 14, 1864. About 20 pieces, totaling about 12 kilograms, were recovered. The meteorite was extraordinarily soft and pieces softened with a knife would mark paper like a pencil. Chemists at the time confirmed that it contained organic materials, and it prompted a contemporary debate about whether life could arise in space.

Louis Pasteur himself, who had just announced his disproval of spontaneous generation, inoculated an organic medium with material from the interior of the meteorite to see if it contained any viable microorganisms: nothing grew.

Pasteur, however, didn't have a scanning electron microscope. Hoover's micrographs were only taken in July of last year, so it'll be a while before the evidence is assembled and submitted to a peer-reviewed scientific journal, at which time we'll probably hear a lot more about it.

If the discovery holds up, it doesn't prove life on Earth originated elsewhere--but it would be strong evidence that there is life elsewhere, and that alone would be an extraordinarily important scientific breakthrough.

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Here's an article on the meteoric microfossils, with pictures:
"Evidence for Indigenous Microfossils in a Carbonaceous Meteorite"