FAR from evolving in splendid isolation, life on Earth may have been
repeatedly disrupted by bursts of gamma rays from space. The bursts would have
disturbed the Earth鈥檚 atmosphere, exposing organisms to high levels of mutagenic
radiation.
Gamma-ray bursts (GRBs) are the most violent explosions in the
Universe鈥攑robably the death cries of massive stars collapsing into black
holes. Astronomers usually assume GRBs happen too far away to have much
influence over events here on Earth.
But John Scalo and Craig Wheeler of the University of Texas at Austin now
believe that GRBs intense enough to affect life on Earth have occurred around
1000 times since life began here. To work out how often you might expect bursts
in our neighbourhood, the team put together the numbers of GRBs spotted in the
distant past with estimates of how the rate of star formation in the Universe
has changed over time. 鈥淲e came up with a figure of about one event every 5
million years or so,鈥� says Scalo. 鈥淪uch GRBs would be powerful enough to affect
the Earth鈥攐r any planet in the Galaxy鈥攏o matter where they went off
in the Milky Way.鈥�
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Because GRBs only last about 10 seconds, the rays probably wouldn鈥檛 directly
affect life on the surface. But the X-rays they degrade into strip electrons
from atoms in the atmosphere, eroding the ozone layer and exposing organisms to
UV radiation from the Sun. The researchers calculate that the radiation would be
enough to kill off sensitive creatures, while causing high levels of mutations
in others鈥攑ossibly even kick-starting their evolution.
鈥淚f sufficient to alter ozone layers in a major way, gamma-ray bursts would
have very significant effects on all kinds of life on Earth,鈥� agrees Mitch Sogin
of the Marine Biological Laboratory in Massachusetts. Ionisation of the
atmosphere could also affect the climate below, as well as changing the spectrum
of sunlight reaching the ground.
In a paper to be published in The Astrophysical Journal鈥攖he
first ever to deal with a biological subject鈥擲calo and Wheeler say the
atmospheric effects could persist for weeks or months, comparable to a 鈥渘uclear
winter鈥�, while the ecological effects might last even longer. Organisms that
reproduce quickly would be worst affected, since it takes many generations for
mutations to spread through a population. 鈥淲e鈥檙e principally talking about
bacteria, fungi and other organisms with short generation times,鈥� says
Scalo.
The researchers hope their results will encourage biologists to consider how
GRBs might have influenced the course of evolution. 鈥淭he impact of gamma-ray
bursts has not been widely discussed, nor do biologists, including myself, have
much information about their frequency or exposure levels,鈥� admits Sogin.