Geoscience Reference
In-Depth Information
bivalve or echinoderm (the open water vascular
system of the latter is probably the reason). Terres-
trial jellyfishes and octopuses remain only in the
realm of science fiction!
Adaptation is the long-term result of natural
selection in a particular environmental framework.
How adaptations became durably retained in a
number of related species and rapidly generalized
to larger clades is at the origin of the notion of adap-
tive radiation (e.g. amniotes or angiosperms). Biol-
ogists are currently trying to find the factors that
regulate such macro-evolutionary patterns that is,
whether such adaptive radiations could not be trig-
gered by factors which increase the diversity and
complexity of phenotypes, and on which natural
selection could act.
The expansion of taxa into an entirely new
environment that entails a profound reconstruction
of morphology and physiology may not be too
different in the case of terrestrial and abyssal
environments. Molecular biologists once put much
weight on massive genome duplications in ver-
tebrates as a possible explanation of their 'bursts'
of organismal complexity, but it is now clear that
this is due to a bias induced by massive extinctions
of members of grades (Donoghue & Purnell 2005).
Biologists are now discovering possible regulators
of multiple gene expressions; that is, the highly
conserved miRNAs (micro ribonucleic acids).
The distribution of miRNAs is increasingly large
in the major taxa that have taken part in the tran-
sition from water to land (e.g. embryophytes,
arthropods and vertebrates), thanks to their phenoty-
pic complexity (Sempere et al. 2006; Butterfield
2007; Heimberg et al. 2008). Is this an important
clue or yet another illusion of molecular character
distributions that seem beyond the bounds of
chance?
At any rate, miRNAs seem to regulate such a
large number of gene expressions at the cellular
level (thus structures and functions) that they may,
to some extent, hamper character losses. In other
words, once an organism is fully terrestrialized, it
is forever. Return to water (as has occurred in
angiosperms, arthropods or vertebrates) is generally
at the cost of additional anatomical and physiologi-
cal innovations and not mere 'degeneracy', as char-
acter loss was formerly referred to.
Is terrestrialization a one-way ticket? Probably,
and terrestrial organisms will continue to evolve
and disperse aerobic life wherever there is just
enough water, light and oxygen to survive, modify-
ing the physical environment of Earth in various
ways. Deciphering what has actually happened
on land since the dawn of the Phanerozoic is
enough for us palaeobiologists, but it may also
provide clues for predicting how our physical
environment may react
of biodiversity in the short term. Our data may
eventually be of interest to the supporters of 'terra-
forming', who dream of exporting phototrophic life
to other planets and thereby modifying the atmos-
phere in the same way as life did on Earth. But
that is another story.
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