to both charophycean green algae and bryophytes (resistant compounds in vegetative cell
walls and cytoplasmic desiccation resistance) as well as autapomorphic characters of the
bryophytes (histogenetic meristems, placental transfer tissues, and reproductive
propagules enclosed in a sporopollenin wall [spores]). These autapomorphies, in many
cases, can be identified as evolving from features already present in extant charophycean
green algae but used in a different context (i.e. pre-adaptive). For example, the
sporopollenin layer in the zygote wall of extant Coleochaete has been identified as pre-
adaptive with respect to the sporopollenin spore wall of land plants (Blackmore and Barnes
1987; Graham 1993; Wellman, in press). When considering mutations that enabled green
algal ancestors to survive in the subaerial environment as land plants, Knoll and Bambach
(2000, p. 8) note that ‘principal among these mutations must have been one involving a
simple change in the timing of gene expression for sporopollenin biosynthesis from just
after zygote formation to after meiosis and spore formation...such a mutation, arguably
lethal in water, would have provided protection against desiccation and harmful radiation
at a critically vulnerable phase of the lifecycle’.
Terrestrial life before the land plantsWhen considering the origin of land plants it is important to consider the terrestrial biota
(aquatic and subaerial) present before they originated. Prior to the origin of the
embryophytes the land surface was not barren as is often stated, but was almost certainly
inhabited with what is often, rather disparagingly, referred to as an algal scum. This biota
would have included photosynthesizing organisms (probably photolithotrophs such as
cyanobacteria and green algae—unicellular and multicellular) and possibly also parasites,
grazers, and decomposers (viruses, bacteria, archaea, and possibly also fungi) that
exploited the primary producers whilst either living or dead (Raven 1997; Boucot and
Gray 2001; Raven and Edwards 2001). Evidence for this early terrestrial biota occurs in
the form of fossil remains and geochemical signals (Keller and Wood 1993; Horodyski and
Knauth 1994; Rye and Holland 2000). This primitive terrestrial biota would have
contributed to the formation of biologically active soils (reviewed in Boucot and Gray
2001; Raven and Edwards 2001) and it is quite possible that it could have supported
multicellular animal life (possibly various arthropods, annelids etc.). Thus the earliest land
plants would have invaded a land-scape with rudimentary soils, but already inhabited by a
variety of other organisms.
Graham and Gray (2001) suggest that the algal ancestors of the embryophytes inhabited
shallow freshwater aquatic environments that were ephemeral and characterized by
unpredictable water availability. In addition to their propensity to dry out, other
unfavourable characteristics probably included variation in temperature and possibly also
salinity. They termed such harsh environments ‘environments of unpredictable
unfavourableness’. It should also be borne in mind that subaerial environmental
conditions would most likely have been very different from those today, in terms of
atmospheric composition and solar luminosity (see Graham 1993).
DATING THE ORIGIN OF LAND PLANTS 125