transformation from an aquatic green alga to a subaerial embryophyte relatively slow or
rapid? Were the characteristics that define embryophytes, and permit subaerial existence,
acquired more-or-less simultaneously or sequentially? Such thorny questions are at the
very root of the long standing debate on the tempo and mode of evolution and are not
easily addressed. There is a major disjunction between putative charophycean green algal
ancestors and embryophytes (based on characters of their extant relatives), and the
evolutionary transformation must have been considerable. However, from a theoretical
perspective at least, this evolutionary transformation could have been relatively rapid.
The ‘environments of unpredictable unfavourableness’ of Graham and Gray (2001)
would have exerted immense evolutionary pressures on multicellular green algae
inhabiting them. The major problem would have been periods of desiccation associated
with the water bodies drying out. In order to survive, the algae must have: (i) evolved
characters (morphological and biochemical) that permitted desiccation tolerance; and/or
(ii) produced desiccation resistant resting cysts or spores that either awaited the water
body to refill or were transported (subaerially) into another suitable aquatic environment.
Whether one, or most likely both, of these strategies was adopted, the immense
evolutionary pressures would quite possibly have promoted rapid acquisition of favourable
characteristics permitting subaerial survival. It is difficult to know if the plants evolved
these characters sequentially (in which case the organisms may have been facultatively
subaerial initially, with subaerial existence later becoming more permanent), or whether
these characters were acquired more-or-less simultaneously. I consider the latter more
likely, because evolutionary pressures conferring advantage upon organisms that could
survive subaerial conditions for longer periods of time would have been immense,
promoting rapid evolution of characters enabling subaerial exposure.
It should be noted that a prerequisite for reproduction in, and hence successful
colonization of, the subaerial environment is the production of abundant spores that are
easily dispersed subaerially (i.e. small) and well protected (i.e. with a resistant
sporopollenin wall). As soon as such spore-producing plants evolved, their
palaeogeographical spread would have been rapid because spores are ideally suited to long
distance subaerial dispersal by wind and most likely had the ability to self-fertilize, therefore
readily establishing palaeogeographically widespread founder populations (reviewed in
Gray 1985).
Fossil evidenceIntroductionTraditionally, fossil evidence has been the principle tool utilized in research into the
timing of the origin of land plants. Initially such research was based exclusively on land
plant megafossils. These are relatively complete remains, and early land plant megafossils
have been studied since the 1850s. More recently, since the late 1950s, attention has been
paid to the early land plant dispersed microfossil record. This consists of spores
(subaerially dispersed by plants during life) and phytodebris (fragmentary cuticles and
tubular remains produced on disarticulation of a plant).
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