IntroductionThe origin of land plants was one of the most important events in the history of life on
Earth. The invasion of the land by plants initiated the development of complex and
substantial terrestrial ecosystems. This had profound effects on the environment, with the
newly evolved flora having direct impact on the composition of the atmosphere, rates of
weathering and erosion, and consequently the nature of soils and sedimentation patterns.
The actual timing of the origin of land plants has been a long standing scientific concern.
Initial investigations focused on the fossil record (which has in itself proved contentious).
More recently different lines of enquiry have been explored: clues from environmental
change associated with the invasion of the land and evidence from molecular clocks. This
contribution critically reviews evidence for the timing of the origin of land plants, and
attempts to explain divergent results from the different lines of enquiry.
What are land plants and what was the invasion of the land?Land plants (embryophytes) and their evolutionary relationships‘Land plants’ constitute the Kingdom Embryophyta and are the most conspicuous
component of modern terrestrial ecosystems. They are widespread over the land surface,
except in the driest deserts and coldest upland and polar regions. They represent a
phenomenal biomass, and are relatively diverse with at least 300 000 extant species. All
of these species are multicellular organisms that are photosynthetic (except for rare
parasitic forms that have lost their photosynthetic capabilities) and inhabit terrestrial
environments (except for rare secondarily aquatic forms). The embryophytes possess
numerous autapomorphies (see below), including the fact that they all develop from a
multicellular embryo enclosed in maternal tissue (with the embryo nutritionally and
developmentally dependent on the maternal tissue for at least some time during early
development). Clearly the embryophytes are one of the major kingdoms of eukaryote life
and constitute the vast bulk of terrestrial biomass.
There is very strong evidence that the embryophytes are monophyletic, and essentially
evolved as an adaptive response to the invasion of the land (i.e. their origin is intimately
related to their invasion of the terrestrial environment). Evidence that the embryophytes
are a monophyletic group is convincing and is supported by morphological/anatomical,
ultrastructural, biochemical, and molecular data (reviewed in Graham 1993; Kenrick and
Crane 1997; Graham and Gray 2001). It is generally accepted that the charophycean
green algae are the closest extant relatives to the embryophytes, and that the first
embryophytes evolved from ancestors that would be classified with modern charophycean
green algae (Graham 1993; Graham and Gray 2001; Karol et al. 2001).
It is the opinion of many workers that, when we consider the origin of land plants, we
are essentially concerning ourselves with the origin of the embryophytes. However,
equating the origin of land plants with that of embryophytes can be problematic with
certain grey areas (as is often the case when attempting to define the origin of major
clades). One must bear in mind that equating land plants with embryophytes excludes: (i)
terrestrial green algae; (ii) possible organisms on the charophycean green algae to
DATING THE ORIGIN OF LAND PLANTS 123