Geoscience Reference
In-Depth Information
Terrestrialization: the early emergence of the concept
PHILIPPE JANVIER
Mus´um National d'Histoire Naturelle, UMR 5143 du CNRS, CP 38, 47, rue Cuvier,
75231 Paris Cedex 05, France (e-mail: janvier@mnhn.fr)
Abstract: The transition fromwater to land is perhaps the most dramatic event in the history of life
after the rise of photosynthesis, sexuality and predation. During the late Neoproterozoic and the
early Palaeozoic, some green plants, fungi and animals happened to overcome the constraints
that linked them to the primeval aquatic environment of life, became progressively adapted to
the terrestrial, aerobic environment and finally contributed to its change through time. We do
not know how many taxa initially survived this trial, but we have some indication of the result
in extant life: a threefold world of pluricellular terrestrial organisms, all depending on each
other to various degrees and surrounded by a cryptic world of bacteria and unicellular eukaryotes.
Fossils provide us with acceptable information about the evolutionary history of only two of these
worlds: embryophytes and bilateralian animals. The molecules of their living representatives can
only begin to tell us how they gained the complexity that allowed them to achieve this remarkable
adaptation to life on land and in air.
Is terrestrialization a 'conquest', as it is often
referred to in popular works? Was it a conquest of
land, more space, more food, more light, more
oxygen or, conversely, more carbon dioxide?
Which selection pressures might have driven some
living beings away from the peaceful environment
of the oceans that prevailed before predation arose
in the late Neoproterozoic? How did - and do -
terrestrialized organisms change the physical enviro-
nment of Earth, triggering either global warming or
cooling? These questions are still being debated,
and are the subject of the present issue.
Terrestrialization entailed adaptation to gravity,
protection against deleterious ultraviolet light,
survival at high oxygen pressures (a gas that most
primitive aquatic animals only appreciate in small
doses; Corbari et al. 2004) and the constant need
for water, without which reproduction was initially
impossible and life impossible forever. Life on
land at the dawn of the Palaeozoic was as pleasant
for aquatic organisms as a weekend on the Moon
would be for us. In fact, terrestrialization is
perhaps better depicted as an ensemble of contin-
gent processes that started in the early Palaeozoic
but could well have ended quickly after with the
complete extinction of the major lineages that
shape the extant living terrestrial world. However,
as far as we know from fossil and living organisms,
none of the major eukaryotic taxa that became
adapted to life in air and on land (when adult)
have ever returned to an entirely aquatic and non-
aerobic life. Possible exceptions include, for
example, planorbid snails whose relationship to
other pulmonate snails remains obscure.
The elucidation of the evolutionary transition
fromwater to land is a long story that was intuitively
foreshadowed in Greek Antiquity. The idea that
aquatic life has preceded terrestrial life was envi-
saged by Greek philosophers such as Anaximander
of Milletus or Empedocles, and Aristotle's 'Scala
Naturae' also reflects this hierarchy. This is gener-
ally regarded as a more elaborate version of
obscure, pre-existing mythologies or intuitions that
were rooted in the deep past of earlier Eurasian
human populations, from which the Biblical narra-
tive of Genesis may also have been derived. The
latter was the dominant narrative in the Christian
world until the 18th century and the rise of new
scenarios that
involved transformations of
the
living beings through time.
An example of such reputedly heretical views is
Maillet's (1755) extravagant theory of a species-
to-species correspondence between marine and
terrestrial animals or plants. Sea anemones were
precursors to flowers and parrot fishes precursors
to parrots. However, the first argumented theories
of historical relationships between primitively
aquatic and terrestrial animals appear in Lamarck's
(1809) Philosophie Zoologique, in which inheri-
tance of characters acquired by need is the only
explanatory process. Although still strongly influ-
enced by the Aristotelian Great Chain of Beings,
Lamarck's views were essentially based on the
observation of characters and behaviours which
suggested to him that marine organisms were,
somehow, ancestral to land organisms. This was
already foreshadowed in his symmetrical classifi-
cation of plant and animals (Lamarck 1785).
The pre-Darwinian 19th century also produced a
number of comparative anatomy-based theories
about the transition from aquatic to land organisms,
notably for vertebrates, but most of these theories
