From the scala naturae to the symbiogenetic and dynamic tree of life

Abstract

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 the symbiogenesis-scenario, eukaryotic cells evolved on a static Earth from archaic prokaryotes via the fusion and subsequent cooperation of certain microbes. In 1929, Alfred Wegener published his theory of continental drift, which was later corroborated, modified and extended. The resulting theory of plate tectonics is now the principal organizing concept of geology. Over millions of years, plate tectonics and hence the "dynamic Earth" has caused destructive volcanic eruptions and earthquakes. At the same time, it created mountain ranges, deep oceans, novel freshwater habitats, and deserts. As a result, these geologic processes destroyed numerous populations of organisms, and produced the environmental conditions for new species of animals, plants and microbes to adapt and evolve. In this article I propose a tree-like "symbiogenesis, natural selection, and dynamic Earth (synade)-model" of macroevolution that is based on these novel facts and data.

Figure 1

The "Moral Tree", published posthumously in 1505 in a book authored by Raymon Llull (1232-1315). This Spanish philosopher and Christian theologian mixed up natural phenomena with supernatural religious dogma and hence became the spiritual father of a medieval ideology called "Llullism" [part of a woodcut, adapted from ref. 8].

Figure 2

Upper part of the "Great Chain of Being" or Scala Naturae, as published in 1745 by Charles Bonnet (1720-1793) (left column). On the right side, the title page of the most influential book of Jean Lamarck (1744-1829) and his tree-like scheme of 1809 is shown [adapted from refs. 15 and 16].

Figure 3

Charles Darwin's early sketch of an evolutionary tree (or a coral), drawn in 1837 (A). Marine organism (Bossea arbignyana) collected by Darwin and classified by him as a coral ("family Corallinae"). Later it was discovered that this "coral-like" inhabitant of sea waters (B) is a red alga (family Corallinaceae, Phylum Rhodophyta). Red coral (Corallium rubrum), which grows on rocky sea bottom either in the depths or in dark caverns (C). This wide-spread species, which is found mainly in the Mediterranean Sea, was known to Darwin and possibly served as a model for his diagram (see Figure 4) [adapted from ref. 22].

Figure 4

Partial reproduction of the single illustration in Darwin's Origin of Species of 1859 (6. ed. 1872). This famous diagram may have been inspired by corals (or coral-like organisms) as depicted in Figure 3 B, C. Darwin's five theories are added to the figure (1. to 5.), which illustrate the transformation and diversification of species, which originate from a common ancestor (A) [adapted from ref. 7].

Figure 5

Adult specimen of "Darwin's coral" and ontogenesis of Haeckel's model system. Morphology of the coral Monoxenia darwinii (A), an organism discovered by Ernst Haeckel in 1873 in the Red Sea and later described by him as a new species, named in honour of Charles Darwin. The development of M. darwinii is shown in (B), from the fertilized egg (A/B) to the so-called "Becherlarve" (beaker larvae) or gastrula (K/I). Haeckel coined the term "Gastraea" to denote this phylogenetically conserved stage in animal development [adapted from ref. 27].

Figure 6

Reproduction of Ernst Haeckel's genealogical oak tree depicting the Kingdoms Plantae (plants), Protista (micro-organisms) and Animalia (animals). Note that the author explicitly pointed out that this general Tree of Life is monophyletic [adapted from ref. 28].

Figure 7

Symbiogenesis, natural selection, and the dynamic Earth as key processes that caused biological evolution. The Last Universal Common Ancestor (LUCA) evolved into the earliest self-replicating proto-cells (ancient microbes) ca. 4000 to 3500 million years ago. Over the subsequent eons, these archaic microbes evolved into numerous bacterial ecotypes that today inhabit every micro-niche where organic molecules (or light) are available. Moreover, these micro-organisms gave rise to larger, eukaryotic cells via symbiogenesis (primary endosymbiosis). These nucleated cells further evolved into multicellular organisms, such as algae, fungi, animals and plants.

Figure 8

Scheme depicting the idea of continental drift as envisioned by A. Snider-Pellegrini in 1858 (A). This concept was re-discovered and supported by empirical evidence by A. Wegener in 1929. Decades later, the theory of plate tectonics was deduced. Plate tectonics accounts for most of the planet's earthquakes, which may result in deep cracks in the Earth's surface (B) and the formation of mountains as a result of horizontal compression of the crust (C) [adapted from ref. 58 and from photographs of the US Geological Survey, 1938].

Figure 9

Model of the Earth's surface, which is broken into drifting fragments, the so-called tectonic plates. The "lubricant" of plate movements is liquid water. In this picture, the South American and African plates are highlighted. In addition, the moon, a solid satellite without water and plate movements, is shown in the upper left quarter. Red lines: Regions where volcanic eruptions occur frequently [adapted from ref. 75].

Figure 10

The massive 1872 eruption of the Vesuvius, the only active volcanoe in mainland Europe (Italy). Vesuvius is most famous for the 79 A. D. eruption that destroyed the Roman cities of Pompeii and Herculaneum. Plate tectonics is the major cause for these violent eruptions, which document ongoing magmatic processes driven by heat from the radioactive decay within the Earth [adapted from an anonymous painting, ca. 1880].

Figure 11

Volcanic eruptions can ignite wild fires that result in the destruction of the vegetation, soil micro-organisms, and less mobile animals. In this drawing, Zebras and other mammals are depicted that are just about to escape from a severe wild fire [adapted from a drawing of H. Harder, 1912].

Figure 12

The tree-like "Synade-model of macroevolution", taking into account all five Kingdoms of life on Earth. According to this theory, symbiogenesis (primary and secondary endosymbiotic events), (directional) natural selection, and the (internal heat-driven) dynamic Earth were and still are key drivers of macroevolution on our "planet of the bacteria" [adapted and modified from ref. 62].