marine carbonates may derive from a variety of sources of different age, either through
reworking of pre-existing^87 Sr-rich deposits or because the strontium derives from
dissolved calcite of metamorphic origin. Furthermore, increases in continental silicate
weathering may relate to factors other than development of a significant flora, perhaps
increasing during glacial intervals or periods of enhanced tectonic activity and related
uplift. Consequently estimates of increasing continental weathering (and relating this to
the origin of land plants) based on evidence from^87 Sr/^86 Sr ratios must be treated with
caution.
Molecular clock evidenceGraham (1993) considered the timing of the origin of land plants and noted that ‘in the
future it may be possible to use the ticking of molecular clocks (Zuckerkandl and Pauling,
1965) in the nucleic acids of modern organisms to obtain information on the time of
origin of one or more possible embryophyte clades’. However, Graham warned that ‘a
prerequisite would be information on relative rates of nucleotide substitution in green
algae and “lower” land plants having similar generation times, and many sequences would
be needed in order to reduce the chance of stochastic error in estimation of divergence
time (Li and Grauer, 1991)’. Recently, Muse (2000) reviewed current knowledge of
rates and patterns of nucleotide substitution in plants. His judgement is fairly gloomy
regarding prospects for a workable molecular clock for plants. Three of his main
conclusions are: (i) the three plant genomes vary extensively in both synonymous and non-
synonymous substitution rates; (ii) genes within each genome display a wide variety of
synonymous and non-synonymous rates; and (iii) there does not appear to be a time-
calibrated molecular clock, at least at higher taxonomic levels.
None the less, scientists have begun to experiment with the use of molecular clock
techniques to estimate divergence times among the plant kingdom (e.g. lycopsids:
Wikström and Kenrick 2001; angiosperms: Sanderson and Doyle 2001; Wikström et al.
2001) and there has even been an attempt to estimate the time of the origin of land plants.
Heckman et al. (2001) used molecular clock techniques to estimate when land plants first
appeared, and came up with a minimum estimate of approximately 700 Ma, some 225
million years earlier than suggested by the fossil record. Clearly this disparity with fossil-
based estimates requires attention.
Heckman et al. obtained divergence time estimates for a green alga (Chlamydomonas)
versus embryophytes, and bryophytes (the moss Physcomitrella) versus tracheophytes
(various angiosperms). They analysed non-chloroplast nuclear protein sequences which
permitted animal-based calibration. Calibration was undertaken using multiple external
calibrations from older divergences among animal phyla and kingdoms (plants, animals,
fungi) derived from an analysis of 75 nuclear proteins calibrated with the vertebrate fossil
record (Wang et al. 1999). Times were estimated using two techniques: the multigene
and average-distance approach. Their results suggest that the green algae/land plant
divergence was 1061 Ma±109 myr and the bryophyte/tracheophyte divergence was 703
Ma±45 myr. They suggest that the latter provides a minimum molecular clock estimate
for the colonization of land by plants.
138 CHARLES H.WELLMAN