La Lettre de l'Académie des sciences n°37-38

37 38

La Lettre

HOMO

ARCHAEA

BACTERIA

EUKARYOTA

Archaea

Bacteria

Eukaryota

Thermococcales

Methanococcales

Archaeoglobales

Thermoplasmatales

Methanosarcinales

Halobacteriales

Euryarchaeota

Sulfolobales

Desulfurococcales

Thermo-

proteales

Crenarchaeota

Plancto-

mycetales

Low-GC,

Gram positive

Thermotogales

Chlamydiales

Deinococcales

High-GC,

Gram positive

Cyanobacteria

Proteobacteria

Aquificales

Spirochaetes

Mitochondria

Chloroplasts

Opisthokonta

Fungi

Choanoflagellates

Metazoans

Radiolaria

Cercozoa

Rhizaria

Alveolates

Stramenopiles

Haptophytes

Chromalveolata

Crypto-

phytes

Jakobids

Euglenoids

Diplomonads

Excavata

Land plants

Plantae

Amibozoa

Green algae

Red algae

Glaucophytes

Mycetozoans

Pelobionts

Entamoeba

Studying the genetic kinship of living species (based on ribosomal RNA) reveals a

bushy tree of life in which man appears on a lateral branch.

Step 2: cell biology and evolution

The living being is not merely a bag of enzymes: it has a shape and an organization. At the most elementary

level is the cell, which accumulates proteins and nucleic acids. With higher organisms, a nucleus is to be found

in each cell, inside a watery cytoplasm, circumscribed by a membrane made of lipids. “

Omnis cellula e cellula

”

(“

Every cell [stems from] another cell

”), suggested Virchow as early as 1855. During the development of the

embryo, not only do cells multiply, but they also differentiate into distinct types – muscular, hepatic and so on.

This microscopic diversity is in line with a broader diversification, the diversification of the general shapes of living

beings.

Starting with Aristotle, the description of species expanded in the century of the Enlightenments with the Swedish

Linné and, in our country, with Buffon. These species were classified within a hierarchy in a table illustrating some

“growing perfection” fromone to the

other.At

the summit was seated “man”, then the Creator above, responsible for

this harmonious “

scala naturae

”. The table is fixed and immutable. Until Lamarck, on 11 May 1800, in the opening

speech of his course at the Muséum d’Histoire Naturelle, abandoned this vision of the world to the benefit of a

revolutionary concept, the evolution of the species: species are not fixed, they “transform” themselves. Darwin,

59 years later, took over the transformism thesis but abandoned Lamarck’s inheritance of acquired characters to

the benefit of natural selection as the drive of evolution. Two centuries of biological research would validate this

Darwinian model. The study of fossils has shown that species, or even entire groups, appeared and later, in great

part, disappeared. Death has become the essential cog in the evolution of life. Unexpectedly, gene transfers

occur between sometimes

very distant species, and the

tree of life becomes uncertain

and irregular. It takes the

shape of a bush, where

Homo

sapiens’

ancestors appear

on a lateral branch: human

beings are not taking pride

of place at the summit of the

scale of beings any more.

The developments of a new

discipline, genetics, would

reveal unbreakable bonds

between

evolution

and

development. As early as

1866, Mendel acknowledged the existence of stable and transmissible hereditary characteristics – such as the

colour and form of the peas he cultivated in the garden of his monastery – which he ascribed to invisible “factors”

that have been called “genes” since then. Morgan, in the 1920s, demonstrated, with the fly, that these genes

are localized on the chromosomes, in each cellular nucleus. He considered they were likely to be modified by

mutations that were transmissible through heredity. Molecular biology has established that each protein of the

organism is encoded by a genetic sequence of DNA molecule and that mutations in such sequences are at the