|Life From Extraterrestrial Seeding? |
Dating of Moon Impacts Suggests Link to Flowering of Complex Species
| By a Washington Post Staff Writer |
Friday, March 10, 2000
A sudden proliferation of living things on Earth 500 million years ago coincided with an increase in meteor and comet impacts on the moon, providing new evidence that exotic organic compounds from space may have played a role in the evolution of life on our planet, a new study suggests.
A chemical analysis of lunar "soil" picked up by Apollo 14 astronauts indicates that impact rates from meteors, comets and interplanetary debris increased nearly four-fold beginning about 400 million years ago. That corresponds--very roughly--with the period of lush and rapid diversification of animal types on Earth known as the "Cambrian explosion."
The new report, reported in today's issue of the journal Science, adds new evidence supporting a theory that life may have originated here after Earth was "seeded" by extraterrestrial chemicals in a process called panspermia.
"You have to really think seriously about the introduction of new species of organic compounds that could have acted to catalyze, if not actually spawn, more diverse kinds of life," said Paul R. Renne, a geophysicist with the University of California at Berkeley and the director of the Berkeley Geochronology Center.
Although life on Earth seems to have originated nearly 4 billion years ago, single-celled organisms prevailed for most of that time. The forms of complex life were extremely limited until about 650 million years ago, when within a 150-million-year period scores of new body designs suddenly emerged. At least a dozen of the great categories of the animal kingdom arose in that period, including segmented worms, animals with hard outer skeletons and jointed limbs, creatures with rudimentary spinal cords and many more.
The exact cause of the explosion is unclear, but abrupt sea-level changes are thought to have formed a changing variety of ecological niches, each of which offers new opportunities for evolution. Impacts of meteors or comets might have aided that process, creating novel environments that could only be exploited by new kinds of creatures.
In addition, scientists now know that numerous organic compounds exist in deep space, and can be carried by itinerant objects. Interest in the idea that the Earth may have been "seeded" by extraterrestrial chemicals revived recently with preliminary indications that a Martian meteorite found in Antarctica may have contained evidence of primitive life forms. The notion of panspermia--whether directed or accidental--remains the subject of lively debate.
The new report means "you have to really take seriously the possibility of two things," Renne said. "One is that even small objects [crashing into Earth] would have had some impact on life. Not an extinction-level event, but enough to create environmental stress that results in adaptation and diversity." The other is that the basic building blocks of life arrived from outer space.
To understand how many objects might have whacked the Earth hundreds of millions of years ago, scientists have to study the moon. That's because Earth's weather quickly erodes or inundates signs of impact craters, and eventually the motion of the planet's tectonic plates sweeps everything on the planet's surface under the crustal rug. Our planet "is a terrible recorder of geological history," Renne said. But with no air, weather or tectonic motion, "the moon is a great preserver," said Renne.
He and colleagues used sophisticated techniques to estimate the age of 155 glassy beads or "spherules" from the lunar surface. The spherules, about one one-hundredth of an inch in diameter, are created when high-energy impacts create so much heat that rock is melted into glass. The tiny pellets fly into the air and then fall to the surface, leaving a permanent record of the impact history.
To determine the age of the specimens, the scientists sampled the amount of argon trapped in the spherules. The amount of argon compared with the amount of potassium tells you the age of the sample.
The analysis showed that impacts began to increase substantially about 400 million years ago, a figure consistent with other evidence drawn from visible cratering patterns on the lunar surface, the Berkeley team found. The finding contradicts traditional assumptions that the impact rate in our solar neighborhood has remained relatively constant for the past 3.5 billion years or so.
"It's fine analytical work, and the interpretations are questionable, but not totally out to lunch," said planetary geologist and moon expert Paul D. Spudis of the Lunar and Planetary Institute in Houston. "But I found the connection to the Cambrian explosion to be quite forced," he said.
The Cambrian connection is not what the group set out to find. Nine years ago, they began wondering how to get evidence of a hypothetical object called "Nemesis," which occupies the equivalent place in solar system astronomy that the Loch Ness Monster does in vertebrate zoology.
Berkeley physicist and study coauthor Richard A. Muller has long argued that the apparent pattern of mass extinctions that have occurred on Earth every 30 million years or so could be explained if the sun had a sort of evil twin "companion star." When Nemesis sweeps into the solar system in its long orbit around the sun, it would presumably drag a lot of comet and meteor material with it, prompting a rain of rocks and species extinctions.
As it turned out, the analytical technique could not identify time intervals as short as 30 million years. But it did indicate the impact increases over the past 400 million years.
There is no easy explanation for that phenomenon. "The best guess would be injection of new material into the solar system," Spudis said. "It could be a large-body, long-period comet. Or some collection of debris. Or something else. We just don't know."