Biology Reference
In-Depth Information
between cell lineages. His analysis demonstrated in a very detailed
way how this cell selection explains the transition between unicel-
lular and multicellular organisms. However, it does not break with
genetic determinism, as does ontophylogenesis.
The model of the heap of cells is very basic but illustrates a gen-
eral principle which explains the origin of multicellularity and the
context in which it develops. It is evident however that from this
origin the structure of organisms has become more and more com-
plex over the billion years that they have been evolving, and that
this increase in complexity has optimised their functions.
Consequently, in multicellular organisms with a blood circulation
system distributing nutrients to organs situated at some distance,
the elementary logic that we have described certainly does not suf-
fice, but the increase in the complexity of organisms during evolution
does not nevertheless invalidate ontophylogenesis. The appearance of
a circulation system only emphasises the importance for multicellu-
lar organisms of being able to distribute resources to all the parts of
the organism. It in no way implies that older mechanisms have been
eliminated.
Evolution does not work like an engineer who rationally
replaces the parts of a machine. It often leads rather to traits or
mechanisms that have appeared at different periods of the evolu-
tion of the organism being superimposed, rather than their being
purely and simply replaced. At the early stages of embryogenesis,
even in a complex organism, nourishing the tissues has to continue
in line with a model like the heap of cells, all the while the embryo
is a collection of cells in the process of differentiation with the
organs not yet in place. At the adult stage, the model of the heap
of cells is still relevant to explain tissue organisation within certain
organs irrigated by the blood system. The mammalian liver pro-
vides such an example. It is divided into three areas each with a dif-
ferent function corresponding to gradients of nutrients arising from
afferent blood vessels which supply blood rich in oxygen, nutrients
and hormones. Across these three areas, there are also gradients of
enzyme activity which correlate with the nutrient gradients and
enable the nutrients to be used optimally. Genetically, these gradients
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