Evolution of Nervous Systems and Brains 2

Evolution of Nervous Systems and Brains 2 Gerhard Roth and Ursula Dicke The modern theory of biological evolution, as estab- drift”) is incomplete; they point to a number of other lished by Charles Darwin and Alfred Russel Wallace and perhaps equally important mechanisms such as in the middle of the nineteenth century, is based on (i) neutral gene evolution without natural selection, three interrelated facts: (i) phylogeny – the common (ii) mass extinctions wiping out up to 90 % of existing history of organisms on earth stretching back over 3.5 species (such as the Cambrian, Devonian, Permian, and billion years, (ii) evolution in a narrow sense – Cretaceous-Tertiary mass extinctions) and (iii) genetic modi fi cations of organisms during phylogeny and and epigenetic-developmental (“ evo - devo ”) self-canal- underlying mechanisms, and (iii) speciation – the ization of evolutionary processes [ 2 ] . It remains uncer- process by which new species arise during phylogeny. tain as to which of these possible processes principally Regarding the phylogeny, it is now commonly accepted drive the evolution of nervous systems and brains. that all organisms on Earth are derived from a common ancestor or an ancestral gene pool, while contro- versies have remained since the time of Darwin and 2.1 Reconstruction of the Evolution Wallace about the major mechanisms underlying the of Nervous Systems and Brains observed modi fi cations during phylogeny (cf . [1 ] ). The prevalent view of neodarwinism (or better In most cases, the reconstruction of the evolution of “new” or “modern evolutionary synthesis”) is charac- nervous systems and brains cannot be based on fossil- terized by the assumption that evolutionary changes ized material, since their soft tissues decompose, but are caused by a combination of two major processes, has to make use of the distribution of neural traits in (i) heritable variation of individual genomes within a extant species. This is usually done by means of the population by mutation and recombination, and (ii) phylogenetic or cladistic method as originally devel- natural selection, i.e., selective environmental or oped by the German entomologist Willi Hennig [3 ] . genomic forces leading to better adaptation of those This method is based on whether a given character (or bearing the mutation and as a consequence to their trait) represents an ancestral, plesiomorphic , or a greater differential reproductive success. The modern derived, apomorphic state. The result of such an analy- synthesis holds that evolutionary changes are gradual sis is a “tree” called cladogram re fl ecting shared in the sense that larger, macro-evolutionary, changes derived characters called synapomorphies or homolo- are the sum of smaller, micro-evolutionary, changes. gies . Characters used for the construction of a clado- Other experts argue that such a gradualistic view of gram can be of any nature, but mostly are anatomical evolution (including “sexual selection” and “genetic or biochemical. The standard criterion between com- peting cladograms is the principle of parsimony stat- ing that the most likely evolutionary hypothesis is that G. Roth (*) • U. Dicke minimizing the number of independent (convergent or Brain Research Institute , University of Bremen , 28334 Bremen , Germany homoplastic ) steps of modi fi cation of the character e-mail: [email protected]; [email protected] under consideration. If different species of animals C.G. Galizia, P.-M. Lledo (eds.), Neurosciences - From Molecule to Behavior: A University Textbook, 19 DOI 10.1007/978-3-642-10769-6_2, © Springer-Verlag Berlin Heidelberg 2013 20 G. Roth and U. Dicke well as fi rst fi shlike animals and proto-amphibians about 500 mya, insects about 400 mya, amphibians 360 mya, reptiles 300 mya, mammals 200 mya, and birds 150 mya. First human-like animals (australopith- ecines) appeared 4 mya, and modern humans (Homo sapiens sapiens ) 200,000–150,000 years ago. 2.2 Organisms Without a Nervous System Fig. 2.1 “Tree of life”. Lifeforms are classi fi ed into prokary- Nervous systems and brains have a dual function, i.e., otes (unicellular, without a cell nucleus) and eukaryotes (cells the maintenance of inner “vital” functions of the organ- marked by * with a nucleus), the latter being either unicellular ( , ism and the control of behavior of that organism within only some examples shown) or multicellular (fungi, plants, and animals). The phylogeny of these groups remains an open fi eld a given environment [ 4 ] . Unicellular organisms exert of research the same functions and exhibit remarkably complex behaviors, although they do not possess, by de fi nition, coincide in the formation of a given character in detail a nervous system. (e.g., the structure of the inner ear or the genetic Bacteria sense nutritive substances (e.g., sugar) or sequence of a membrane channel), then it appears toxins (e.g., heavy metal) in their environments through more likely that these species are related and the coin- a large number of chemoreceptors as well as obstacles cidence of characters is due to common ancestry, i.e., through mechanoreceptors [5 ] . This diverse informa- that they are homologous . However, there is increasing tion is integrated and, through a chain of complex evidence that convergent-homoplastic evolution is chemical reactions, drives their fl agella for movement. much more common than previously believed, which Escherichia coli has six fl agella, each possessing a pro- weakens the principle of parsimony. ton-driven motor, that are combined to one single In the reconstruction of the evolution of nervous super fl agellum for forward propulsion (“run”) as soon systems and brains, however, the primary goal is as receptors detect an increase in nutritive or a decrease not the construction of new cladograms, but an answer in toxic substances. In the opposite case, the to the question whether within a group (taxon) of ani- super fl agellum disintegrates, and the single fl agella mals the presence or absence of a given character, e.g., move independently. As a consequence, E . coli starts the six-layered isocortex as found in all mammals, rep- “tumbling” and randomly changes its direction of resents an ancestral (plesiomorphic) or derived (apo- movement until the receptors sense a new gradient, and morphic) state. This requires the availability of a new “run” begins. The bacterium has a mini-memory, well-established cladograms, which are based on non- by which it can compare the incoming information with neural characters in order to avoid circular conclu- previous information, and this “knowledge” determines sions. However, such well-established cladograms do the behavior. Thus, even in these most primitive organ- not always exist, and therefore one often has to operate isms we fi nd the three basic components for adaptive with competitive cladograms. control of behavior, i.e., a sensorium, a motorium and in Figure 2.1 illustrates the present knowledge about between information storage and processing. Other the major groups of organisms – the “tree of life”. bacteria or archaea like Halobacterium possess light- According to most recent evidence, the earth was sensitive spots that make them swim toward sunlight. formed about 4.5 billion years ago (bya). First organ- Unicellular eukaryotes such as Paramecium or isms appeared about 3.5 bya as prokaryotic bacteria Euglena , despite their unicellular organization, exhibit and archaea (these are organisms without a cell a much more complex control of behavior than bacte- nucleus). Unicellular eukaryotes (which bear a cell ria or archaea [6 ] . They possess either fl agella com- nucleus) originated 2.7–1.6 bya, simple multicellular posed of microtubules and attached to their front, animals (sponges) about 1 bya, and coelenterates about which bend to perform a breakstroke, or cilia that 700 million years ago (mya). First deuterostomes and cover the entire body and are able to exert coordinated fi rst arthropods appeared 570 mya, cephalopods as movements, or pseudopodia. They gather information 2 Evolution of Nervous Systems and Brains 21 about their environment through voltage-gated, hyper- polarizing potassium and depolarizing calcium ion channels, while voltage-gated sodium channels are absent and found only from planarians, possibly cni- darians onwards. The calcium and potassium channels are used, among others, for the release of forward and backward movement. Many protozoans respond to chemical, tactile, temperature and visual stimuli. Euglena and Paramecium and other unicellular eukaryotes possess light-sensitive organelles for pho- totaxis. It is debated whether they already possess an intracellular system for central movement coordination. Figure 2.2 illustrates the phylogeny of the Metazoa, i.e., multicellular organisms, which are divided into Nonbilateria and Bilateria. Porifera (sponges, 8,000 Fig. 2.2 Phylogeny of metazoans, i.e., multicellular animals. Metazoans comprise non-bilaterally organized animals (such as species, all aquatic) are the simplest metazoans. They sponges and coelenterates) and bilaterally organized animals, possess “independent effectors” or myocytes , which which include those without a secondary body cavity (or coelom) have sensori-motor functions and directly respond to – the Acoelomorpha, and those with a coelom – the Coelomata; stimuli, but are not electrically excitable [ 7 ] . Sponges for further explanation see text can regulate the water stream through openings (ostia) inside their body by modifying the

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