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350
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OF
SCIENCE
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of regulatory genes, called “homeotic” genes. Their mutations bring about spectacular changes, such as,
in the fly, the substitution of an antenna for a leg.
These genes play indeed a role in the biogenesis of the brain. The recent deciphering of the complete
genome, however, has created one more paradox. As a matter of fact indeed, if the genome of yeast
contains about 6 000 genes against 13 000 for the drosophila, there is however no variation in the number
of genes that code for proteins from mice to humans – about 25 000 – and, between chimpanzees and
humans, the total sequences of coding genes only differs by 1.2 %! The anatomic and functional complexity
of the brain thus increases much faster than the complexity of the genome. One first hypothesis is that this
evolution may result from mutations in some critical regulatory sequences that control the development of
the brain. Another lies in the process of postnatal development.
At birth, human cubs have brains that weigh five times less than the brain of an adult, and the period of
maturation after birth is exceptionally long compared to other species – it lasts more than fifteen years.
More than half of the synapses of the adult brain, of the order of one million billions, form after birth. During
this period, fundamental learning takes place, such as the acquisition of walking, language or social
interactions. Then phases of synaptic exuberance occur with maximal variability, followed by selection
phases with the stabilisation of some connexions and the elimination of the others. The activity of the
network, whether it be spontaneous or triggered by the environment, regulates this synaptic selection
process, which is “Darwinian” but not genetic. Some variability even occurs between genetically identical
individuals. As we have suggested with Courrège and Danchin, “
to learn is to eliminate
”. Due to this
intense synaptic plasticity, cultures develop, pass from one generation to the other and distinguish social
groups from one another. “Cultural pathways”, such as those taken by writing, reading, or even ethical
rules, remain inscribed, as it were, in the brain. The brain of each individual “internalizes”, according to
Vygotsky, the features of his/her physical, social and cultural environment. Thus develops the “human
person” with his/her “
habitus
” associated to each individual’s story.
Would this physicalist approach – first Cartesian and then Darwinian –
drivemankind to lose part of its humanity?This is far fromcertain.
Here is indeed what Günther Anders tells us: “
Extend your
representative capacity in order to know what you
are doing
” (In
Nous, fils d’Eichmann, Rivages Ed.,
1999
). The dispositions of our brains, that enable
us to progress in the knowledge of what we are,
leave us with a heavy ethical responsibility. It
is for us to invent, with our brains, a future
that will open access to, as Ricoeur put
it, a “
good life with and for others, in just
institutions
” (In
Soi-même comme un autre,
Seuil Ed., 1990
) and, I should add, in a
sustainable environment...