Evolution__3rd_Edition

538 PART 5 / Macroevolution
Plants evolved adaptations for land
life...
Origin of
land plants
Origin of vascular plants
(1) vascularization
(2) roots
(3) leaves
Origin of seeds
change, or internal biological innovations, or some mix of the two. But there is no
consensus yet.
18.5 Evolution of land plants
The land was first colonized by microbes. Prokaryotic fossil cells have been found from
terrestrial environments over 1,000 million years ago. Terrestrial life existed only in
microbial form until the land was colonized by plants and fungi. Little is known about
early terrestrial fungi, but for plants we have widely accepted evidence of fossil spores
from 475 million years ago, and further evidence of fossil spores from maybe 550 million
years ago. Land plants are most closely related to a group of green algae called
charophyceans (Figure 18.7). Early fossil spores have a “tetrad” structure that is found
in some modern bryophytes, and most early plant fossils seem to be related to the
phylogenetic branches leading to modern bryophytes and pteridophytes.
The main events in the early evolution of land plants were the evolution of a resistant
spore stage, then the evolution of vascular tissue, followed by roots and then leaves.
Bryophyta such as moss have spores but lack vascular tissue. Pteridophyta such as ferns
have vascular tissue. Vascular tissue, and particularly vascular tissue that is built of
tracheid cells with lignin-containing cell walls, enables a plant to support itself on land.
The earliest fossils of whole plants, as distinct from spores, date back to around
430 million years ago. Clarksonia is one of the commonest fossils of this time. These
early land plants lacked roots and leaves, and simply had branching stems. We can infer
that they photosynthesized through their stems, because stomata are visible in the fossil
stems. Fossils with leaves appear 390–350 million years ago.
The evolution of leaves coincides with a dramatic fall of about 90% in atmospheric
carbon dioxide concentration. One hypothesis connects these events. The initial evolution
of land plants may have removed carbon dioxide from the atmosphere, not only by
their relatively minor photosynthetic activity through the stem but, more importantly,
Charophyceans
Bryophytes (e.g., moss)
Pteridophytes (e.g., ferns)
Gymnosperms (conifers)
Angiosperms (flowering plants)
Figure 18.7
Phylogeny of the main groups of plants. Land plants are most
closely related to a group of algae called charophyceans. The main
events in plant evolution are the evolution of land adaptations,
then seeds, and then flowers.
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