March 9, 2000
BERKELEY, CA ó Should evolution's "survival of the fittest" creed be
supplemented with an appendage that reads "survival of the
A team of Berkeley researchers, analyzing the history
of impact cratering on the moon, has reported a surprising increase
in the frequency of impacts over the past 400 million years that may
have played a central role in the evolution of life on Earth.
SCANNING ELECTRON MICROSCOPE PICTURE OF A GLASS SPHERULE
BACK FROM THE MOON BY APOLLO 11.
THE SPHERULE IS ABOUT 250 MICRONS
Photo by Tim Culler/UC Berkeley
In the March 10 issue of the journal Science, researchers with the
U.S. Department of Energy's Lawrence Berkeley National Laboratory
(Berkeley Lab) and the University of California at Berkeley (UC
Berkeley), and the Berkeley Geochronology Center (BGC) announced the
discovery that impact cratering on the moon (and, by inference, on
the earth) suddenly increased about 400 million years ago.
"From our point of view, thatís in the recent past," says
Culler, a UC Berkeley graduate student who is earning his Ph.D. on
Culler is one of the co-authors of the paper in
Science, along with Richard Muller, who holds joint appointments
with Berkeley Lab's Physics Division and UC Berkeley's Physics
Department; Paul Renne, a UC Berkeley geologist and director of the
BGC, an independent non-profit research institution; and Timothy
Becker, BGC laboratory manager and expert in radiometric
The data published by this team show that the impact cratering rate
had dropped steadily until the unexpected rise when the impact rate
returned to the same levels as 3.5 billion years ago.
increase coincides with the "Cambrian explosion," a period in which
life on Earth took off with a dramatic burst in the number and
diversity of species.
"Although most people assume that impacts cause death and
destruction, it is possible that the additional stress of the
impacts forced life to become more diverse and flexible," says
"Just as we stress trees, through pruning, to make them give
more fruit, the stress cause by catastrophic impacts may have forced
evolution into new directions."
Noting that the earliest records of life on earth date from the
period approximately 3.5 billion years ago, when their results show
the intensity of impacts was decreasing, Renne says:
others have speculated before, life began on Earth many times, but
the comets only stopped wiping it out about three or four billion
Muller originally suggested the dating of lunar spherules as a way
of getting evidence for or against the existence of comet showers,
brief periods when the number of comets in the sky increases,
raising the risk that several of them are likely to hit the Earth at
Dating the chronology of impact craters on earth is difficult
because of erosion, sedimentation, and plate tectonics. Although
lunar craters are preserved in a near pristine state, it was
presumed that a sample would have to be collected from each
individual crater in order to date it, requiring an expensive
re-visit to the Moon.
Muller contended that the required data could be found in any gram
of lunar soil, including those brought back by the Apollo missions.
The impact record would be preserved in spherules -- microscopic
glass beads formed when droplets of molten basalt splashed out of a
crater by the heat and force of an impact subsequently cooled and
The Berkeley team obtained from NASA a gram of lunar soil inside of
which they found 155 spherules. The age of each of these tiny
spherules was determined at the BGC with an ultrasensitive technique
based on the ratios between two argon isotopes. From the ratio of
argon-40 to argon-39 as measured by neutron irradiation followed by
laser-driven mass spectroscopy, the age of the spherules could be
These 155 lunar spherules ranged in size from less than 100 microns
to more than 250 microns and came from lunar soil picked up in 1971
by the Apollo-14 mission crew near Mare Imbrium (Sea of Rains), the
dark crater that dominates the moon's face.
Statistical and chemical
analyses showed that the spherules studied came from approximately
146 different craters.
"Even though we donít know which crater was the source of each
spherule, the distribution of the ages of the spherules from a
single lunar site should reflect the age distribution of craters on
the Moon," Muller said.
The results of this study not only carry implications for the
evolution of life on Earth but also for our understanding of the
"It is not difficult to understand the slow
decrease. It corresponds to a gradual cleansing of the solar system
by Jupiter, the Sun, and passing stars. But it is difficult to
conceive of a mechanism that could trigger an increase, particularly
one that lasted 400 million years."
Muller suggests that the sudden increase offers indirect evidence
for a companion star to the sun. When the existence of such a star
was postulated in 1984, Muller suggested that it be called Nemesis
after the Greek goddess of retribution.
"The increase in impacts could be due to a sudden change in the
orbit of Nemesis," Muller says. "If a passing star perturbed
into a more eccentric orbit, that would account for the increase in
Berkeley Lab is a U.S. Department of Energy National Laboratory
located in Berkeley, California. It conducts unclassified scientific
research and is managed by the University of California.
Contact information for science reporters:
Richard Muller has joint appointments with Berkeley Lab's Physics
Division and UC Berkeley's Physics Department. He can be reached at
(510) 486-7430 or via e-mail at:
Paul Renne is the Director of the Berkeley Geochronology Center, and
has a joint appointment with the UC Berkeley Department of Geology
and Geophysics. He can be reached at (510) 644-9200 or via email at:
Timothy Culler is a graduate student in Geology and Geophysics at UC
Berkeley. He can be reached at (415) 216-0142 or