embryophyte lineage that may have possessed some, but not all, embryophyte characters,
and may even have lived in a predominantly subaerial habitat; (iii) potential ‘failed land
plant attempts’ that may have evolved from within the green algae independently, but
ultimately became extinct. Graham (1993) warned against equating land plants with
embryophytes, noting the possibilities that aquatic algae might have evolved an embryo
prior to invading the land or may have invaded the land before evolving an embryo. These
possibilities must be borne in mind, whilst recalling that all extant land plants belong with
the monophyletic group Embryophyta that evolved once and once only.
Despite the fact that there is overwhelming evidence that the embryophytes are
monophyletic, relationships among the major embryophyte groups are unresolved. It is
generally accepted that the bryophytes are basal and diverged earlier than the
tracheophytes (vascular plants). However, relationships among the extant bryophyte
groups (liverworts, hornworts, and mosses) are controversial. Most recent analyses
suggest that the bryophytes are paraphyletic with respect to the vascular plants, with a
moss/vascular plant sister group relationship, with either the liverworts or hornworts as
the earliest divergent group (Figure 7.1). However, this subject is contentious, and many
different phylogenetic schemes have been proposed (reviewed by Kenrick and Crane
1997; Kenrick 2000).
The evolutionary transformation from aquatic algae to
terrestrial land plantsBased on the knowledge that the embryophytes are monophyletic and evolved from a
charophycean green algal ancestor, it is possible to compare extant charophycean green
algae with purported early divergent embryophytes (liverworts or hornworts), and draw
some general conclus ions regarding the evolutionary innovations involved in the origin of
embryophytes and the likely nature of the most primitive forms. Recently, Graham and Gray
(2001) have undertaken just such an exercise. They suggest that the ancestors of
embryophytes were multicellular charophycean green algae that inhabited shallow
freshwater environments. They consider that, due to the unpredictability of such
environments (i.e. their propensity to dry out), these organisms would have evolved
characteristics that facilitated survival when water bodies dried out exposing them to the
harsh subaerial environment. By carefully considering characters present in extant
charophycean green algae and early divergent embryophytes, Graham and Gray were able
to hypothesize which features were required in order to make the transition from the
aqueous to subaerial environment, and identify which of these features were
plesiomorphic and which are autapomorphies of the embryophytes (presumably many of
which are essential acquisitions required for life in a subaerial environment).
Furthermore, they were able to identify potential pre-adaptive features among the
charophycean green algae.
Graham and Gray (2001) suggested that in order to survive in the subaerial
environment, the earliest embryophytes required characteristics that would: (i) prevent
decay during periods of dormancy, drought avoidance, and perhaps also short-term
desiccation; and (ii) enable reproduction in conditions of low moisture content. They
identified a number of such characteristics. These include plesiomorphic features common
124 CHARLES H.WELLMAN