The oldest fossils of land plants are 420 million years old, but a recent
study showed that pond scum first made landfall approximately 100 million
years earlier.

Scientists have used plant genetic data at "molecular clocks" to estimate
how long ago various species split based upon their differences in DNA-to
figure out their evolutionary history. However, they were unable to identify
the lowest, or earliest, branches of the plant family tree. Vascular plants
such as trees, crops, and flowers have been known to come along sometime
after liverworts, hornworts, and mosses. However, the order in which of
these three appeared remained a history, making the molecular clock studies
with missing puzzle pieces.

In the study conducted by Philip Donoghue from University of Bristol and
other researchers, it was concluded that the exact configuration of the base
of the plant family tree doesn't matter to dating the first land plants. All
the analyses indicate that land plants first appeared about 500 million
years ago, during the Cambrian period, when the development of multicellular
animal species took off. The study showed that the first land plants arose
earlier than we thought, regardless of current uncertainties about which
land plants evolved first. This finding has important global implications,
because we know early plants cooled the climate and increased the oxygen
level in the Earth's atmosphere," conditions that supported the expansion of
terrestrial animal life, says Tim Lenton, an earth system scientist at the
University of Exeter in the United Kingdom who was not involved with the
work.

[www.sciencemag.org]
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