By Cowen, R.
Science News , Vol. 157, No. 11
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.
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. …