Evolutionary history of plants
Plants have evolved through increasing levels of com-
plexity, from the earliest algal mats, through bry-
ophytes, lycopods, ferns and gymnosperms to the com-
plex angiosperms of today. While the simple plants con-
tinue to thrive, especially in the environments in which
they evolved, each new grade of organisation has even-
tually become more "successful" than its predecessors
by most measures. Further, most cladistic analyses,
where they agree, suggest that each "more complex"
group arose from the most complex group at the time.
Evidence suggests that an algal scum formed on the
land 1,200 million years ago, but it was not until the Or-
dovician period, around 500 million years ago, that land
plants appeared. These began to diversify in the late
Silurian period, around 420 million years ago, and the
fruits of their diversification are displayed in remarkable
detail in an early Devonian fossil assemblage known as
the Rhynie chert. This chert preserved early plants in
cellular detail, petrified in volcanic springs. By the
middle of the Devonian period most of the features re-
cognised in plants today are present, including roots,
leaves and seeds. By the late Devonian, plants had
reached a degree of sophistication that allowed them to
form forests of tall trees. Evolutionary innovation con-
tinued after the Devonian period. Most plant groups
were relatively unscathed by the Permo-Triassic extinc-
tion event, although the structures of communities
changed. This may have set the scene for the evolution
of flowering plants in the Triassic (~200 million years
ago), which exploded in the Cretaceous and Tertiary.
The latest major group of plants to evolve were the
grasses, which became important in the mid Tertiary,
from around 40 million years ago. The grasses, as well as
many other groups, evolved new mechanisms of meta-
bolism to survive the low CO2 and warm, dry conditions
of the tropics over the last 10 million years.
Colonisation of land
Land plants evolved from chlorophyte algae, p