By Mark Shwartz - Stanford University News
Daffodils,
tulips, roses and other flowers are so much a part of our daily lives that we
take them for granted. But to evolutionary scientists, the question of how and
when flowering plants appeared on Earth has gone unanswered for more than a century.
Mosses were the first plants to emerge on land some 425 million years
ago, followed by firs, ginkgoes, conifers and several other varieties.
According
to the fossil record, flowering plants abruptly appeared out of nowhere about
130 million years ago. Where did they come from, and how could they have evolved
so suddenly without any transitional fossils linking them to other ancient plant
species?
``An abominable mystery`` is how 19th-century naturalist Charles
Darwin referred to the origin of flowering plants, and the puzzle remains as controversial
today as ever.
Now a team of Stanford geochemists has entered the debate
with evidence that flowers may have evolved 250 million years ago - long before
the first pollen grain appeared in the fossil record.
``Our research indicates
that the descendants of flowering plants may have originated during the Permian
period, between 290 and 245 million years ago,`` says J. Michael Moldowan, research
professor of geological and environmental sciences.
``We based our findings
on an organic compound called oleanane, which we found in the fossil record,``
he adds.
Moldowan and his collaborators, research associate Jeremy Dahl
and graduate student David A. Zinniker, will present their findings at the annual
meeting of the American Chemical Society (ACS) in San Diego on April 2, during
a symposium titled ``Biogeochemistry of Terrestrial Organic Matter.``
Oleanane
Oleanane is produced by many common flowering plants as a defense against
insects, fungi and various microbial invaders. But the chemical is absent in other
seed plants, such as pines and gingkoes.
Using gas chromatography and
mass spectroscopy, Moldowan and his colleagues have been able to extract molecules
of oleanane trapped in oily rock deposits that are hundreds of millions of years
old.
``Our work has shown that oleanane is lacking from a wide range of
fossil plants,`` he notes, ``but the chemical is found in Permian sediments containing
extinct seed plants called gigantopterids.``
That makes gigantopterids
the oldest oleanane-producing seed plants on record - an indication that they
were among the earliest relatives of flowering plants, concludes biologist David
Winship Taylor of Indiana University Southeast, a co-author of the ACS study.
``This discovery is even more significant because we recently found gigantopterid
fossils in China with leaves and stems that are quite similar to modern flowering
plants,`` Taylor notes - further evidence that flowering plants and gigantopterids
evolved together roughly 250
million years ago.
Molecular fossils
Moldowan and his colleagues point out that the chemical fossil record can
be an important tool for studying the history of life on Earth. ``In our research
we use molecular fossils, or biomarkers, such as oleanane to provide evolutionary
and paleoenvironmental information from sediments and petroleum,`` he says.
Perhaps
one day this technique will help solve Darwin`s ``abominable mystery`` once and
for all.
Relevant Web URL: http://pangea.stanford.edu/GES/faculty/moldowan.html
Contact: Mark Shwartz, News Service
(650) 723-9296;
mshwartz@stanford.edu
Comment: J. Michael Moldowan, Department of Geological and Environmental Sciences
(650) 725-0913; moldowan@pangea.stanford.edu
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