Jean-Sébastien Steyer has a PhD in paleontology, is a researcher at the CNRS and is attached to the Muséum national d’Histoire naturelle in Paris. He is one of the few specialists studying life on Earth before the dinosaurs. He has also been working on the reconstruction of extinct species for almost twenty years. The author of numerous scientific articles, this enthusiastic popularizer regularly publishes works that are rapidly becoming benchmarks, such as La Terre avant les dinosaures (2009), Exquise planète (2014), and Demain, les animaux du futur (2015). Between paleontological expeditions to the four corners of the globe, he is also a science columnist for the magazines Pour la science and Espèces.

If “nature is written in mathematical language”, it is also the result of species evolution, that-is-to-say transformation of an interfertile and viable gene pool into another. The sciences of evolution, or modern natural sciences, thus aim to connect species to each other. Yesterday, this networking took the form of a ladder, the Aristotle’s Scala naturae, with humns at the top. Today it draws a tree; the famous phylogenetic tree. The aim of phylogeny – or analysis of kinship relationships – is therefore to construct species trees. But the question of Who descends from whom now arises – since the entomologist Willi Hennig – that of Who is closest to whom. The connections are no longer scalar but bushy. They are woven by comparing genomes and/or morphologies with each other – as is the case in paleontology where the genetic material is often not fossilized. The skeleton is then carefully transcribed into a set of homologous morpho-anatomical characters. Thanks to comparative anatomy – ironically developed by Georges Cuvier in a fixist context – each character presents an ancestral state and at least one derived state. The phylogenetic tree then groups together the forms that share the most derived character states. Thus the “tree of life” is drawn, whose branches connect species that transform or die out. And we, sapiens, are only a fragile branch of this great bush. We share with other primates an opposable thumb, with other mammals a placenta, with other tetrapods limbs with fingers, with daisies cells, with bacteria DNA. Reading the tree becomes salutary to put us back in our place.