When Did Land Plants Diversify?
Posted February 1, 1998
The earliest land plants consisted of a simple plant body with a few cells. Plants with more elaborate systems, including plants similar to today's liverworts, hornworts,
mosses, and vascular plants, had evolved by the end of the Devonian period, around 360 million years ago. Most research on the evolution of land plants has focused on the late Silurian
through the Devonian (408 to 360 million years ago) because the earliest fossils of plants were found in sites dated to these times. Scientists thought that these plant fossils clearly
showed two distinct lines of diversification of land plants: the clubmosses and their extinct relatives; and the living vascular plants, including the ferns, horsetails, and seed plants.
Recently, however, the discovery of fossil spores, new information about green algae, and new advances in molecular classification have pushed back the date of the
early diversification of land plants to the Ordovician period. Fossil spores in groups of four with decay-resistant walls have been discovered in mid-Ordovician sites, indicating that
land plants were already adapted to conditions on land as early as 476 million years ago. By the early Silurian (432 million years ago), fossils include individual spores rather than
groups of four, and species that are clearly recognizable as clubmosses are present. Other small plants (generally no larger than 10 cm tall) from this time period show tracheid-like
tubes and are clearly related to vascular plants. Fossil plants also show that by the early Devonian to the mid-Permian (398 to 256 million years ago), there was a substantial increase
in vascular plant diversity.
More recent phylogenetic studies indicate that land plants probably originated from green algae. Living green algae have several characteristics that are also found
in living land plants. Scientists now hypothesize that during this transition to land, new metabolic pathways developed from the primary metabolism of algae and early bryophytes. Scientists
also suggest that certain characteristics of land plants, such as gas exchange surfaces, did not evolve until plants were exposed to the different conditions found on land.
References
Engel, M.H. and S.A. Macko. "The Origin and Early Evolution of Plants on Land." Nature, September 4, 1997, Vol. 389, No. 6646, pp. 33-39.
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