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A brief history on the conceptual (r)evolution of the ‘Tree of life’

It was not Charles Darwin who introduced for the first time the ‘Tree of life’ into biological classification. But it was, of course, Darwin who revolutionized our understanding of the diversity and the evolution of life with his On the Origin of Species (1859). In fact, the only figure in the entire book was a treelike diagram representing the real genealogical history of species and not simply a vectorial subordination of groups toward a kind of ‘final cause’ such as the diagrams previously proposed by Aristotle, Linnaeus and others.
 
Opposite to the idea of a regular and independent gradation of beings, Darwin’s diagram shows that beings are organized in an irregularly branched tree, and at the same time that all these beings had the same origin. Ernst Haeckel depicted various versions of Darwin’s concept of evolution in many different works. One of his well-known figures was first seen in his work Monphyletischer Stammbaum der Organismen (1866). The key point of Haeckels’ works was that all beings had a monophyletic origin, including man. Not surprisingly, this new way of thinking was from a very early stage onwards transferred to the linguistic world. Indeed, the philologist Frantisek Celakovsky was probably the first linguist to present in 1850, even before Darwin, a treelike diagram representing the family of Slavic languages. However, the German linguistic August Schleicher, in his Darwinian Theory and the Science of Language (1863), was probably the one who popularized the use of a tree as a diagram the most to represent the history of languages.
 
According to some authors, Schleicher was incited by Haeckel to read the German edition of Darwin’s On the Origin of Species. In the 19th century, the young fields of historical linguistics and evolutionary biology explicitly established an analogy between language families and genera, languages and species, dialects and races/ethnies, idiolects and individual organisms. The emergence of language-as-species or language-as-organisms was crucial for the analogy works.
 
Up to this point in time, geneological tree models always depict speciation and branching, the branches of the tree don't cross. In 1909 the Russian biologist Constantin Merezkovsky revolutionized biological thought by the introduction of the concept of symbiogenesis. The key point of symbiogenesis is that all living organisms had a polyphyletic origin and that over time organisms in different evolutionary lines shared their genes horizontally and established temporary or permanent symbiotic connections. Crucial to his tree of life is that branches can cross.
 
Finally, recent advances in molecular biology and genetics promote the emergence of a new tree concept – a circular tree – where all the organisms are at the same evolutionary level on the perimeter of the circle, thereby eliminating the idea of progress in evolution.

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