Meteoric wallop may have diversified life

Some 540 million years ago, life on Earth exploded in diversity. On the seafloor, some creatures acquired hard shells, and the direct ancestors of modern animals suddenly appeared. A new study suggests that this evolutionary Big Bang blossomed about the time that cosmic debris began pummeling our planet at an increasing rate.

Glass bead from a sample of lunar soil measures 250 micrometers across. Culler et al.

The timing may not be mere coincidence, argue the geologists who found evidence for the revved-up impact  rate. The researchers speculate that a spate of meteoroids battering Earth might have ferried in large amounts of the organic compounds vital to life, or the pummeling could have sparked diversity by forcing organisms to adapt to a new environment.

Timothy S. Culler of the University of California, Berkeley and the Lawrence Berkeley (Calif.) National Laboratory (LBNL) and his colleagues derived this scenario from their analysis of lunar samples brought back in 1971 by the Apollo 14 spacecraft.

Unlike Earth’s surface, which receives frequent facelifts from erosion and plate-tectonic activity, the pockmarked lunar surface provides a nearly pristine record of the inner solar system’s greatest hits. The researchers relied on radioactive dating to determine the ages of tiny glass beads in the soil samples. Such beads arise when bits of debris crash into the moon and heat rocks to their melting point. As the molten droplets fall back to the surface, they cool to form a glass. The geologists estimate that the 155 beads they analyzed came from 146 distinct impacts.

As previous studies have indicated, the ages of the beads suggest that the rate at which debris battered the moon and Earth began to dwindle about 3.5 billion years ago. At that time, the solar system was maturing and many stray objects—asteroids and comets—were swept up by the gravity of Jupiter and the sun.

To the team’s surprise, the study also revealed that after settling down to an all-time low some 500 to 600 million years ago, the impact rate rose nearly fourfold over the past 400 million years. The rate remains high today. “It’s not surprising that the impacts tapered off about 3 billion years ago,” notes study collaborator Richard A. Muller of UC Berkeley and LBNL. “What is surprising is the reversion from a benign to a violent solar system about 500 million years ago.” Culler, Muller, and their colleagues, including Paul R. Renne of UC Berkeley and the Berkeley Geochronology Center, describe the work in the March 10 Science.

In 1997, a team including Alfred S. McEwen, now at the University of Arizona in Tucson, analyzed craters on the moon’s far side and suggested that the impact rate had doubled over the past 300 million years. The team has since questioned its own findings, but the new report “is entirely consistent with our 1997 results,” McEwen says.

He cautions that the California researchers have analyzed just 1 gram of lunar soil. “The assumption that this is representative of the cratering-history rate for the entire moon may be a giant leap,” McEwen says.

Renne subscribes to the notion that the increased impact rate seeded Earth with water and other compounds essential to life during the past 500 million years. This could have sparked the Cambrian, the brief era in which a profusion of animal life emerged. “Life already here would suddenly have a new stimulus, a greater need to evolve quickly, and more raw material to do it,” says Renne.

Muller maintains that the study supports his theory that the sun has an undetected stellar companion that periodically flings comets into the inner solar system. “I think it is a bit premature to draw any strong conclusions about the cratering record of the Earth-moon system and therefore about any implication for life,” says Alan P. Boss of the Carnegie Institution of Washington (D.C.). Nonetheless, the team has demonstrated “an extremely powerful technique” for addressing such questions, he says.

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