Asteroid impact puts heat on Snowball Earth theory of key evolutionary jump
Scientists studying rocks near an ancient asteroid impact
structure in South Australian have uncovered evidence that could
change current theories explaining how life on Earth rapidly
diversified about 580 million years ago.
Dr Kath Grey of the Western Australian Department of Industry and
Resources’ Geological survey and an ACA associate researcher, Prof
Malcolm Walter, Director of the ACA and Dr Clive Calver of the
Tasmanian Department of Mineral Resources challenge the idea that
‘Snowball Earth’ – an intense period of glaciation about 600
million years ago, triggered the evolution of simple life forms
into more complex and familiar species.
In the May edition of the international journal Geology, Dr Grey
and her team put forward an alternative radical idea that 580
million years ago an asteroid impact played a pivotal role in this
evolutionary jump. The impact, known as the Acraman event, smashed
a hole in South Australia about four times the size of Sydney.
Up until then, for the first three billion years of Earth’s 4.5
billion year history, bacteria and simple algae had dominated life
on Earth. “Then almost overnight geologically speaking, the
ancestors of modern day animals and plants appeared in the fossil
record about half a billion years ago,” Dr Grey said. “The big
question is what caused the rapid proliferation of life at that
time?”
Research by other scientists suggests the evolutionary burst of
life between 600 and 540 million years ago was the result of an
intense period of global glaciation. However, if the findings of
Dr Grey’s research prove correct, the cause could lie beyond our
planet.
Dr Grey, who has studied fossil plankton (single-celled green
algae) from drill holes across Australia, has found that, as
predicted by the Snowball Earth theory, bacterial mats and a few
simple spherical species of plankton were the only organisms that
managed to survive the intense ice age.
“As the sea level rose at the end of the ice age, these spherical
forms increased in number,” Dr Grey said. “But there is no sign of
a new species emerging at the end of the intense ice age to
support ideas of the rapid diversification of life at this time.”
Dr Grey believes it wasn’t until about 20 million years later more
than 50 new and highly complex species suddenly replaced the small
number of simple species in the fossil record.
“What is really interesting is that the more complex spiny fossils
appear just above a layer of rock in South Australia associated
with the Acraman impact,” Dr Grey said.
In a related study, Dr Calver found significant carbon isotope
changes mirrored Dr Grey’s observations. Prof Walter has also
noted that patterns associated with the Acraman impact were
similar to those of mass-extinction and recovery events, and that
a large asteroid impact could have produced conditions ideal for
evolutionary change.
“Later impacts, like the 65 million year old Chixulub collision in
Mexico wiped out a diverse range of species, including the
dinosaurs,” Dr Grey said. “But with the Acraman impact, there were
only a small number of species around and the time to cause a mass
extinction event.
“Most of the species that did survive were highly resilient, and
had the ability to remain dormant through the cosmic winter that
followed. When conditions improved, these species had an advantage
over their competitors and were able to proliferate and
diversify.”
Dr Grey and her team have reasoned that the ensuing plankton
diversification must have played a vital role in the subsequent
development of the animals dependent on plankton as a food source.
