OPINION Open Access

From the scala naturae to the symbiogenetic and dynamic tree of life All living beings on Earth, from bacteria to humans, are connected through descent from common ancestors and represent the summation of their corresponding, ca. 3500 million year long evolutionary history. However, the evolution of phenotypic features is not predictable, and biologists no longer use terms such as “primitive ” or “perfect organisms”. Despite these insights, the Bible-based concept of the so-called “ladder of life ” or Scala Naturae, i.e., the idea that all living beings can be viewed as representing various degrees of “perfection”, with humans at the very top of this biological hierarchy, was popular among naturalists until ca. 1850 (Charles Bonnet, Jean Lamarck and others). Charles Darwin is usually credited with the establishment of a branched evolutionary “Tree of Life”. This insight of 1859 was based on his now firmly corroborated proposals of common ancestry and natural selection. In this article I argue that Darwin was still influenced by “ladder thinking”, a theological view that prevailed throughout the 19th century and is also part of Ernst Haeckel’s famous Oak tree (of Life) of 1866, which is, like Darwin’s scheme, static. In 1910, Constantin Mereschkowsky proposed an alternative, “anti-selectionist” concept of biological evolution, which became known as the symbiogenesis-theory. According to th