Poster-Text (englisch)

Senckenberg-Poster TheEvolutionofAnimals Manfred Grasshoff and Michael Gudo with cooperation of S. Hilsberg, W. Oschmann and J. Scholz One of the most important aspects in biological sciences is the investigation of long-term changes in nature. It is commonly accepted that the universe, the planetary system, and all the organisms living on earth, are the result of a process of continuous modification and development. The architecture of recent organisms provides a key for the understanding and reconstruction of their history through the billions of years, of the process called evolution. Evolution is the gradual change of organisms through generations and time. We cannot directely observe evolution in nature. So we have to reconstruct how organisms could have changed and how their variety emerged under anatomical and structural-functional constraints. For any evolutionary transformation it is considered that the process has to be continuous. Organisms are not engines, which can be stopped for rebuilding. Organismic evolution can be likened to a gradual change in the running engine. Evolutionary research should not only consider obvious similiarities. The architectural elements by which an organism is designed are much more important. In particular, this means the mechanical properties of tissues, their arrangement and their functional connections. (In German this complex of structure and function is called ‘Konstruktion’ or ‘Bauplan’; the latter term is sometimes used as a German loan-

word in English scientific papers and text books). This approach provides new visions of biological research. Common knowledge of biological investigations can be integrated, but in addition, this approach provides answers for the question: is a gradual change of one bauplan into another possible? For example: can a dinosaur evolve into a bird? This question can be answered with a ‘yes’. We can show that the dinosaur bauplan supports the possibility of this evolutionary transformation. Another question: Can a dinosaur evolve into a mammal? This has to be answered with a ‘no’. We can show that the bauplan does not support this kind of transformation. This way of reasoning, its philosophical background, and the results on the reconstruction of evolutionary pathways, were developed by a group of scientists in the Senckenberg-Museum, Frankfurt, in cooperation with several colleagues. The new approach has been called "The Frankfurt Evolutionary Theory" and "Engineering Morphology" (in German: KonstruktionsMorphologie). The results are summarized in a graphic design. It is published as a poster, and presented in the museum as a wall-hanging in room 206 and in a modified form in room 104. The top part of the poster shows the formation of the earth and the development of the first organisms. The most ancestral animals are located in the center.

Evolution of Animals 1

From here the evolutionary pathways originate and branch off. Not all branchings of the animal kingdom can be presented in the design, we decided to include those leading to well-known animals or to some of specific zoological interest. On these branches you find the most important steps in the evolutionary transformation of organisms represented by technical drawings. At the end of each pathway a most recent or a known fossil representative is shown in a naturalistic form. These stand in contrast to the model-like, hypothetical and technical drawings. All extant animals are equally remote of their point of origin. Each animal by itself has attained its own stage of sophistication in the evolutionary process. We no longer share the traditional anthropocentric view of the world, in which man has taken the position of the top of a tree growing from so-called lower to so-called higher animals.

CONTENTS The origin of life 1 The explosion of bacterial life 2 The origin of eucaryotes 3 The diversity of Eukaryotes 4 The first of multicellular animals 5 Gelatinous fibrous body mass 7 The body-cavities develop 9 The worm-coelomates 9 The chordates 12 Peribranchial-chambers 13 Vertebral column and tooth 14 Jaws, skull and bones 14 Evolution in teching courses 16

Early Evolution of Earth and Life The origin of life

Red Planet

Archaean Atmospheres About 5 billion years Meteorites and Asteroids CO2 - Atmosphere ago accumulating and forming Planet Earth Brownish-red Planet fusing meteorites and Organisms produce O2 asteroids formed the and control the Greenhouse Effect hot red planet earth. Bacteria After an extensive about 4 billion years ago cooling the rocks crysBiofilms Protocells tallized and internal Archaea chemical reactions exabout 3,8 billion years ago haled various gases Motiloids (Proto-Eucarya) through volcanoes, Endosymbioses about 2 billion years ago forming the archean Eucaryotes: nuceli, fibres, atmospheres which mitochondria, cilia chloroplasts contained water but no free oxygen. The water condensed, and a rain lasting millions of years filled rivers, lakes, and for the bacteria. The stabilizing layer Just like soap-bubbles, they are able to the oceans. The world of water, called is the so called murein-layer. fuse, to repair small holes and to sehydrosphere was created. About 4 bilquester small bubbles. In the lipidlion years ago physical and chemical This also was the time when the gemembrane protein-molecules are inconditions in the water permitted the netic substance desoxyribonucleiccorporated, which were able to transformation of the so-called organic acid (DNA) originated. The DNA carport matter from the external medium chemical compounds. This fluid has ries the information for the distinct into the inner cavity and vice versa. been called ‘Ursuppe’, which means enzymes and molecules, which are imSuch membrane-enclosed structures ‘primeval soup’. These organic comportant during metabolism. In most were the result of chemical-evolution. pounds were formed at various sites of cases the genetic information is only a They indeed had some properties of the oceans and their shores, mainly insingle ribbon, ribonucleic-acid (RNA), living beings: metabolism, reproducvolving mineral surfaces, such as rock and it merges in some cases to a doution and merging. We call them proor clay. In such places solutions beble ribbon, DNA. Only the eucarya tocells. came concentrated and chemical reenclose these genetic information in actions took place in favourable condiThe selective permeability of the an additional membrane. They formed tions. It was the phase of chemicala nucleus which hold the genetic informembranes provided two options for evolution on earth. mation in a save place. metabolism. The first option was to sequester membrane-parts (vesicles) A decisive step in the chemical-evoluinto the inner cavity, in order to transtion, was the origin of lipids. Lipids are port substances from the outside into molecules which arrange theirselves in the inner space. This option allows to a flexible double-layer, forming memproduce organic substances in the inbranes. Such membranes are barriers ner of the cell. This is typical for the between solutions of different kinds. motiloids (proto-eucarya). The Such flexible membranes enclose second option was to produce a slime small reactive cavities and therefore and to deposit it on the outer memthey are an indispensible element of brane surface. These substances stabiliving beings. lized the outer shape. This is typical


The explosion of bacterial life A typical bacterium is not much larger than a thousandth of a millimeter. Bacteria do not have a nucleus and their small sizes result in simple shapes: rods, spheres or tubes. But it would be by no means adequate to understand these simple shapes as primitive organisms. From these simple shapes no inference can be made to their metabolism capacities. Bacteria live by the use of distinct matter and exotic elements, such as sulfur, sulfurhydrogen, copper, iron and even the radioactive uranium. Furthermore bacteria can live in extreme environments, such as the geysers in Yellowstone (USA), in New Zealand in 150 degree hot sulfuric-acid and they are also found in more than four kilometers depth in the earth‘s crust. Some of these microorganisms have been interpreted as living fossils and therefore they were called archaic bacteria, Archaebacteria or Archaea. Today it is well known that the diversity of the Archaea is as complex as that of the other bacteria. Accordingly life is divided into three domains: the Bacteria, the Archaea (both called procaryotes) and the Eucarya (Eukaryotes). More than 90 % of all procaryotes live on rock surfaces. Here their slimelayer develops a tendency to form the so-called biofilms. In such biofilms cycles of production and destruction of organic substances are established. When particles were added into this slime or when the cells actively produced carbonate matter, a layered sediment developed, a so-called stromatolite. Stromatolites exist in oceans, lakes and rivers. They are easily preseverd in the fossil record. The first stromatolites are more than 3.5 billion years old. They have been found in the West Australian desert – at a place which is called ‘North Pole’.

A certain group of bacteria was of particular importance for the evolution of the planet earth, the Cyanobacteria (in former times called the blue-green algae). They contain the enzyme chlorophyll which is capable of producing sugar out of water, carbon dioxide and energy from the sunlight. Hereby oxygen is released into the environment. 3.5 billion years ago this oxygen reacted directly with other elements. Iron was oxidized into iron oxid (Fe2O3). In water by help of bacteria, massive stromatolite-like sediments were accumulated, called the Banded Iron Formation (BIF). When finally all the iron was oxidized, free oxygen could accumulate in the hydrosphere and the atmosphere. Oxygen is a poison to cells, because it oxidizes organic substances. About 2 billion years ago, oxygen caused the largest mass extinction in earth‘s history. Even today many microorganisms are sensitive to oxygen, such as the Archaea and a number of anaerobic bacteria. Life was able to persist, since at the same time some bacteria were capable of using oxygen for energy supply. Finally an equilibrium of production and destruction of oxygen was attained. Planet earth is the only planet in our solar system, which has free oxygen in its atmosphere. The UV-rays induce the production of ozone (O3) in a high stratum of the atmosphere. In return the ozone layer reduced the amount of UV-rays reaching the earth‘s surface. The atmospheres of the planets Mars and Venus mainly consist of carbon dioxide. In contrast the atmosphere of the earth mainly consists of nitrogen, oxygen and methane and a small amount of carbon dioxide. On an inanimate planet earth methane would have been oxidized into carbon dioxide and accumulated in the atmosphere. This greenhouse-gas would


transform Earth into a planet similar to Venus, several hundreds of degrees hot. Only the metabolism of living beings continuously recycles both gases, oxygen and methane. Therefore the concentration of both these gases is contained in a steady-state equilibrium (homeostasis). Bacteria have modified the surface of the planet earth to a considerable extent, and they have provided an environment suitable for other forms of life. Bacteria control the greenhouse-effect and the overall impact of solar radiation through the mediation of the reflexion capacities (albedo) of earth‘s surface. The atmosphere, lithosphere, and hydrosphere are influenced and partly generated by organisms. All together constitute the biosphere, a realm of living matter. Up till now everything is based on the metabolic activity of bacteria.

The second endosymbiosis opened up a separate pathway, the evolution of plants. With the additional incorporation of cyanobacteria, which are able to conduct photosynthesis, these cells had created their own supply for energy and organic compound. The chloroplasts produce sugars out of water and carbon dioxide, which are the basis for production of distinct building materials.

The origin of eucaryotes The mobile protocells, which we call motiloids, lived in the shadow for long time, compared to the rich bacterial life. In these protocells a cell skeleton developed, made up of various protein molecules forming rigid rods (the microtubuli) and fibers. Fibers with tensile strength constructed cords and grids throughout the cell and under the surface of the membrane. Some of the molecules in the fibre grid are able to glide along each other; they can contract. Since they are anchored in the fiber grid they deform the entire cell and generate motion. Shifting and gliding consumes energy. Energy is supplied by special compounds, in which it is stored in a chemical form. This chemical energy is transformed into mechanical work, which creates heat. Fibers with tensile strength as well as contractile fibers play a major role in all organismic cells. The archaean motiloids were able to flow around bacteria and other particles, to incorporate them into their body, and to digest and use them as energy suppliers.

However, when bacteria were not immediately digested by the motiloid, but rather continued to live, the motiloid used the substances produced by these bacteria and a close interaction began: a symbiosis. As one partner lives inside the other, this relation is called an endosymbiosis. Such endosymbioses can be observed today in some fungi and algae. Two endosymbioses played a crucial role for further evolution. The first endosymbisis was one in which a kind of bacteria were incorporated that could gain their energy by combustion of organic substances with oxygen. In this process, energy was stored in special compounds, called ATP. This process is called respiration. Such bacteria transformed into permanent parts of the cell, the so-called mitochondria. Energy turnover increased dramatically in these cells and new options were opened for further evolution. However, the mitochondria cannot stop their activity, and they require a permanent supply of food and oxygen. Organisms with mitochondria have to eat and to breathe continuously.


The eucaryotes do not only have distinct cell organelles, but also nuclei, in which the genetic substance (the DNA) is located. The DNA became enveloped by a membrane and was in this way separated from the mechanical system of the cell. Cilia had already developed in this early phase of evolution. They are movable thread-like structures, whose beat enabled an unicellular organism to swim about in water. Cilia are preserved throughout the whole of evolutionary history. For example, ciliated cells are incorporated into the epithelia of many parts of recent organisms, such as: into lung, central nervous system, gut tissues, or coelom cavities. Cells with one or numerous nuclei, various fibers, one or several cilia, and cell organelles, are called eucaryotic cells. They are the cells of plants, animals and many other organisms, collectively called Eucaryotes. They inhabit our world today and they all live at the expense of bacteria.

The diversity of eukaryotes: Precambrian explosion of life A remarkable array of evolutionary developments roots in this early level of eukaryotic cells. We may speak of a Precambrian explosion of life which took place about 2 billion years ago. (For comparison, the so-called Cambrian explosion of life, about 600 million years ago, is in reality the beginning of the rich fossil record). The various types of eukaryotes differ from each other in the contents of their cells, the way in which they gain their food, and how they divide and grow. Plants, as we know them today, owe their existence to the second endosymbiosis shown in our drawing, in which cyanobacteria were additionally incorporated into eucaryotic cells. The cyanobacteria remained almost unchanged. They became the wellknown green chloroplasts of the plant cells. All of these eucaryotes with chloroplasts are called plants, although they comprise a variety of types. Photosynthesis with the aid of chlorophyll has never stopped, and today plants still produce most of the organic material. Plants in a stricter sense have a special kind of growth. The cell-nucleus divides into two, the cell enlarges, and produces internally a sieve-like plate with holes, so that on either side a nucleus is placed. The terrestrial plants and among the algae the Charophyceae are built according to this type; all of their cells are imperfectly divided. The sugar produced by assimilation is chemically transformed into cellulose which forms thick walls around the cells and enables the plant to construct large sized bodies. Fungi are, in contrast to common assumptions not plants at all. Their cell walls are made up of chitin. Fungi chemically attack dead or living organ-

isms, decompose them outside their body and feed on the degradation products, which are taken up in the form of small drops or as single large molecules. Protoctista is a collective term for quite a number of very different structural types of organisms. All of these divide their cells concentrically and completely by incision. In some types the cells remain in connection, glued together by tough gelatinous substances. Among such types are the large kelps of the sea, the red and the green algae. But many other Protoctista are unicellular. They show a large variety and represent a huge biomass as a whole. Many have chloroplasts and are called unicellular plants; others ingest particles, they are called Protozoa, called unicellular animals. Three of them are shown in our poster, a Paramaecium, a flagellate with chloroplasts, and a foraminiferan. The microscopically small protoctists, together with fungi and bacteria, are almost omnipresent in water and on land. On surfaces of various kinds, they live in thin layers, the so-called biofilms, in which they interact by chemical turnover and growth. Already in archaean times, biofilms played a major role in evolutionary processes. The land-plants with their incomplete cell-division are exceptional among all eucaryotes, (as well as the animals with their specifically structured body). The conspicuousness of plants and animals makes them so important for us, but we should not forget: the huge masses of unicellular organisms, fungi, and bacteria, including the bacteriaderived chloroplasts of plants, with their energy-turnover are the major driving force for the megamachine of life on earth.


The body was stabilised by the matrix of jelly and fibers, and could increase in size. The matrix expanded more and more, and filled the cytoplasm, until in each region of cytoplasm at least one nucleus and some endoplasmatic reticulum were retained. These parts of cytoplasm surrounded by the membrane of the expanded tube system are, by definition, cells, – or if they had several nuclei, cell complexes. The body was compartmentalized into fibrous matrix and cells. The surface remained covered by cytoplasm with cilia (later separate cells with cilia), as the gelatinous matrix may not be exposed to the open water, where it would swell and pour out.

Gallertoids: primitive connective tissues

The Evolution of multicellular animals For the understanding of the evolutionary history of animals we have to find answers for the question: How did the body construction which is typical for the animals (and only for them) evolve? In the center of the poster a series of four technical drawings shows these transformations. With these ancestral organisms the threshold to the animal kingdom was overcome. They are not drawn at the same scale. The first (in the back) stands for a small unicellular animal, less than a millimeter in size. The last (in front) for a larger animal, about some centimeters in size. The last stage represents the starting point for all other evolutionary transformations in the animal kingdom. During this process, the internal tubular system, the endoplasmatic reticulum, played a major role. It had evolved already in the archaic protocells, when the membrane from the surface turned inside, forming tubes in

the cell. As a consequence, there are two regions in the cell: the inner portion in strict sense, the so-called cytoplasm, and the inner portion of the tube system, which may be open to the surface. The tubes are flexible, they can be widened or narrowed, can merge or branch. Various molecules are incorporated into the membrane of the tubes. Here the chemical turnover takes place and various chemical compounds can be secreted into the tubes, stored or excreted at the surface. A typical example for the latter procedure are gland cells. The development towards multicellular animals (the Metazoa) started when compounds originating from sugar and proteins were retained in the endoplasmic reticulum. These deposits have gelatinous and fibrous properties. Some of the polypeptids are scleroproteins (chain-molecules), the basic substance of collagenous fibers. They have a remarkable tensile strength, and are the basic matter of connective tissue throughout all animals.


The result is a body construction in which many cells and cell-complexes are fused together with primitive connective tissues. This structural design, as it is typical for the animal body, had gradually slipped in. In the inner of these first ancestral animals, some cells specialized in contraction, later forming muscles. Because of chemical and mechanical constraints, contractile fibers could not extend themselves again. They have to be extended by outer forces. Therefore, they arranged automatically in a counteracting, so-called antagonistic way. This was only posssible in such a gelatinoid system. Pressure (by muscle contraction) on the one side tries to move the body mass to the counterside. The pressure is forwarded to the side where muscle fibers have to be extended. The final result are grids of connective tissue fibers and muscles, – a typical feature of the animal body. We call these archaic animals Gallertoids (this German word means: animals with a gelatinoid body design). The first evolutionary steps began in small unicellular animals, less than a millimeter in size. They increased as a consequence of the new mechanical properties, which expanded their

range of locomotion and to new food resources. The size of such a fibrousgelatinous body is almost unlimited; jelly fish are structured mainly on this basis, and some of them attain a size of more than a meter. The stability of the matrix permitted various body shapes, (not only the ellipsoids shown on the poster). But more decisive was the fact that folds and grooves at the surface could be formed that eventually closed to form canals. In the canals, the water stream was controlled by the beat of cilia, improving the ingestion of particles into the canal wall cells. Sexual reproduction and vegetative multiplication, both of which occur in unicellular organisms, must have continued during the development of multicellular animals. The Gallertoid was able to bud off parts of its body for vegetative multiplication, and cells could continue to exchange material in contact with other individuals. Eventually some cells specialized to form sperm and egg-cells. In many animals, budding and fission have been developed for specific modes of vegetative multiplication, apart from sexual reproduction. The structural elements and their mechanical properties, as outlined above, persist throughout the entire history of animal evolution. They are the invariant structures which provide the constraints for bauplans and evolution. In all following evolutionary transitions these existing structural elements and constraints are only modified. Tissues are rearranged, enlarged or reduced. Cells specialize in frame of their capacities. Thes develop into muscle cells, nerve cells, connective tissue cells, secretorial cells or gametes. The primitive connective tissues differentiate into the defined types of connective tissues with distinct mechanical properties. Some of them are stable against pure and simple shear or pressure. Others have elastic properties, some form two- or

three-dimensional grids. All in all more than twenty types of connective tissues are known for humans alone. Much more can be expected across the range of other animals. When skeletal elements developed – and this happened often in the animal kingdom – the body structure always as a whole determines where and how skeletal elements are constructed. Such hard parts consist of organic compounds like horn or chitin, and minerals like carbonate or siliceous matter. Such skeletons hold parts of the body in a definite shape without permanent energy supply. Shifting, enlarging, reduction of these elements and/or specialisation of the cells determine further evolution. The diversity of the animal kingdom, based on these relatively few structural elements, is impressive.

Animals which are close to the Gallertoids There are no recent or fossil animals known which completely correspond to the Gallertoids. However a number of recent and fossil animals stand relatively close to the ancestral Gallertoids, can be interpreted on the basis of this body structure or can be derived directly from ancestral stages. Trichoplax is about a millimeter in diameter, flat, creeping on hard substrate. This animal ingests particles into the cells of its underside. Trichoplax most likely evolved from an ancestral gallertoid which had not yet developed canals.


The Vendozoa are fossils, more precisely, they are casts of organisms, known only from Vendian strata, about 600 million years old. The rounded, often cushion-like forms are probably the relics of organisms that had the body structure of gallertoids.

A part of the Protozoa, the unicellular organisms, can probably be derived from early gallertoids, without folds or grooves, which remained small and continued to move by the beat of cilia. This explanation might be considered especially for the Ciliata, to which the Paramaecium belongs, and a number of Flagellata.

Evolutionary pathways The various shapes of the ancestral Gallertoid determined all the evolutionary pathways of the animal kingdom. In all pathways the structuralfunctional constraints are preserved. Evolution is only possible on the basis of what already exists. Evolution is characterized by diversification and, sometimes at certain levels of evolution, explosive like radiations can be observed in the fossil record.

Organisms with gelatinous fibrous body mass

Sessile way of life The larger the body, the less efficient is the beat of cilia for locomotion. Larger gallertoids began a sessile life style, this means, they settled permanently on the ground. This was possible as food collection in canals worked independently of locomotion. For sessile gallertoids, two further evolutionary directions were possible: increasing the number of canals and collecting fine particles, or enlarging a few canals and collecting larger particles.

Stromatopora

Gallertoids of a compact body shape, stabilized by the fiber-grid, continued to swim by the beat of cilia. The following evolutionary lineages descended from such forms.

The fossil Stromatopora had no internal skeleton, but they produced a calcareous substrate. Their soft body attained a flat shape by reducing their gelatinous tissues down to a thin layer with few canals.

Ctenophores The ctenophores (comb jellies) are basically similar to gallertoids, as their body consists largely of gelatinous fibrous tissues. However, their cilia are no more evenly distributed, but arranged to form an efficient ciliary propulsion system. They are united on plates, each plate consisting of long partly fused cilia, resembling a comb, and the plates stand in eight rows. A canal runs under each comb row. Two tentacles, which developed from protrusions of the body, catch particles for food.

Sponges In gallertoids that stayed with collecting fine particles, the canals multiplied and arranged in such a way that openings for ingestion and egestion resulted. The efficiency of driving the water stream was improved, when in canal wall cells the cilium grew to a long flagellum. The tiny protrusions on these cells, (present in all surface cells) became longer and arranged around the flagellum in the form of a collaret of threads. The finest particles, even bacteria, are caught by it. These cells are called choanocytes, they are placed in a central part of the canals, the choanocyte chambers. A skeleton developed, made up of collageneous fibers and calcareous or siliceous sclerites, preventing the sponge body from collapsing, when the body was hollowed out by more and more canals.


Corals and their relatives Sessile gallertoids with wider canals could catch larger particles. Gastric cavities for digestion developed behind the openings of such canals. Folds in the canal wall transported and managed the particles, the folds enlarged, forming deep pouches into the body mass. The tissue sheets between the pouches are called mesenteries. The flexible tissue of the foremost part of the canal formed a gullet, the pharynx, and tentacles developed in front of it. The system, stabilized by its inner water pressure and held in shape by the tensile strength of the

mesenteries, could be protruded over the body surface. Such is a typical coral polyp. The former gallertoid had been transformed into a coral, the ancestor of all later corals. The octocorals correspond to this structural type. They developed additionally calcareous sclerites embedded in the gelatinous fiber grid. All other corals and sea-anemones, the fossil Rugosa, and the various Hexacorallia, evolved form solitary polyps that descended from the ancestral coral by reduction of the common tissues.

In a different evolutionary lineage polyps with four pouches in the gastric chamber, developed a peculiar method of vegetative multiplication. The upper parts of the polyps were budded off, swam for a while settled down, and grew into a new coral polyp. When the period of swimming was extended, eggs and sperm began to ripen, and new polyp developed out of fertilized eggs. The alternation of polyp and medusa was set on its way. Examples of large medusae are the jelly fish of Scyphozoa and Cubozoa. The Hydrozoa have mostly small medusae but may form large polyp colonies.

The body-cavities develop In ancestral gallertoids with elongated body shapes, swimming by the aid of cilia was more and more replaced by a propulsion through lateral bending of the entire body, called undulation. In general, undulation puts the internal parts of any body under stress: during bending, the lateral parts are compressed on the concave side and stretched on the convex side, and in the middle line the two forces neutralize each other. In ancestral gallertoids, it was the jelly/fiber mass (penetrated by canals) that was exposed to this stress. As a response to this stress the lateral canals widened their diameter. They were fluid-filled and hence more easier deformed than gelatinous body structures. Such animals had to expend less energy for moving than others, and widening of lateral canals was favoured. However, the slender elongated body shape had to be maintained. The continuous stress caused a regular widening of those canals that ran from the middle to the sides, one behind the other. The resulting fluidfilled chambers were placed one behind the other, in a so-called metameric arrangement. They are the so-called coelomic cavities. These animals are called Coelomates, and they make up the largest part of the animal kingdom.


The various internal functions of the coelom-chambers have far-reaching consequences. The internal water pressure stretches the tissues and stabilizes the body. The connections to the central canal were cut. The central canal became the intestinal tract, the gut, and the coelomic-cavities were separated from it. The transverse walls between them, made up of fibrous tissue and muscles, are a kind of tensile cords, holding the diameter of the body in a slender shape. In an engineering sense, such a body is a system of flexible membranes and a fluid-filling. We call it a coelomate body organization. This body organization consists of connective tissues, muscles, covering cell layers, and fluids in gut and coelom chambers. The fluids constitute a hydraulic system which works as a hydroskeleton for the protagonistic and antagonistic musles. A number of functional properties of the coelomic-cavities were decisive for the further evolution. In the coelomfluid the metabolic end-products of the surrounding tissues were collected, and conveyed outwards through small canals. Furtheron cells for reproduction, i.e. sperm and eggs, developed in the coelom walls and left the body through the same canals. There was plenty of space in the coeloms and increasing quantities of eggs could be produced.

The gut remained in the center, running through the transverse tissue sheets, which held the gut in its place. A tissue sheet remained over and under the entire gut, the so-called mesentery. Accodingly the gut may have moved independently from the locomotory movements of the body. Nutritive molecules and oxygen had to be transported beween muscles and connective tissues. Therefore parts of the connective tissue became liquid in the form of internal tracks, which eventually organized themselves to form real blood vessels. Blood is fluid connective tissue. The gut transported its contents only in one direction. As a consequence, a mouth and an anus, and a fore- and a rear-end of the body developed. In Coelomates, we can distinguish a back (the dorsal side) and a belly (the ventral side). The right and left sides are equal. The entire organism is bilaterally symmetrical. Comparing the evolutionary pathways outlined up to now, four different types of body stabilization can be distinguished: (1) The ancestral Gallertoids and the recent ctenophores are stabilized by gelatinous fibrous tissues, (2) the sponges by internal skeletal elements, (3) the coelenterates by mesenteries in a central gastric cavity and (4) the coelomates by a metameric arrangement of coelom cavities. In the coelomates and in the coelenterates the ancestral organization can be modified by skeletal elements. In subsequent evolutionary pathways of the coelomates the coelom cavities are modified in various ways. In some cases it is completely reduced. At an early stage of evolution two lineages were separated: the coelomates with a central body axis, the chordates, and the coelomates with a metameric hydroskeleton, which we call worm-coelomates.

Hydroskeletons: The wormcoelomates Various kinds of animals descended from the ancestral worm-coelomates. There are numerous animals, inaccurately summarized as ‘worms’, but also arthropods and molluscs. In most evolutionary pathways the coelom cavities enlarged and made the body highly mobile. As a consequence these animals were not only able to make undulating movements, but also peristaltic movements (long/thin and short/thick). The latter can easily be observed in the earth-worm. The thin body wall bulged out in the segments and additionally small bristles formed.

Polychaetes, bristle worms In the evolutionary course of the marine bristle-worms (Polychaeta) the bulges of the body segments with their bristles enlarged more and more. They formed the so called parapodia, appendages used for swimming or burrowing. Recent polychaetes show a remarkable variety of parapodia and bristles. Many polychaetes have an evertible pharynx armed with teeth. Additionally, some polychaetes developed tentacular crowns around their mouth region. These animals live in the sediment. Many have adopted a sessile life style and they protrude their tentacles into the water.

The ancestral worm-coelomates used their mobile mouth region to ingest nutrients. A strong gullet, the pharynx, developed behind the mouth. The pharynx is highly flexible and mobile. Surface cells of the body secreted a layer of collagenous fibers, the socalled cuticula. It is thin, flexible, and highly tearproof. It supports the circular muscles by holding the body shape. The result was a complete hydraulic system composed of thin tissue membranes: the cuticula, the connective tissues and muscles form a functional unit. It enclosed the coelom-cavities in which the gut made its peristaltic movements to transport nutrition inside. Animals with such a body construction can colonise different environments. Various evolutionary pathways were now opened.


PostScript-Fehl

Arthropods

Oligochaetes, earth worms

For polychaetes with large and flat bodies, who lived close to the ground, a quite different evolutionary pathway was opened. This pathway led to the largest diversity in the animal kingdom, the arthropods. The body was flattened and the parapodia were shifted to the ventral side. These animals scraped particles from the ground and transported them between the parapodia to the mouth. The parapodia grew longer and finally became articulated extremities, i.e., legs. These animals kept their body more or less stiff, and the body wall began to harden in parts. But hardening was possible nowhere else than in the cuticula, which became thicker and was impregnated with chitin. The evolutionary level of the arthropods was attained. The chitinous cuticula can not grow, so these animals have to mould from time to time. The cuticula is split off from the underlying tissues, the body is soft for a short while, it grows, and secretes a new cuticula, which is finally hardened. The diversity of arthropods encompasses three quarters of the animal kingdom. The Chelicerata, the spiders and their kin, have one pair of extremities in front of the mouth, the chelicerae. All others have one or two pairs of antennae in that position, i.e., the fossil trilobites, the crabs and their relatives, the centipedes, millipedes, and the insects.

In another evolutionary pathway, distinct from polychaetes and arthropods, the parapodia were reduced. In some of these organisms the body was long and almost round. They have few small bristles and continued with peristaltic movements. Examples are the Oligochaeta, among which the earth-worms are well known. In the evolution of the Hirudinea, the leeches, the coelom was largely filled by muscles and a cell mass, called parenchyma. In this way the gut was tightly connected to the body muscles and enabled to generate enough force for sucking.

smaller and by vegetative budding colonies were formed, encrusting hard substrates or growing upright like corals. In Brachiopoda, a very large tentacular crown developed which was encased by a pair of shells. The shape of these animals resembles that of mussels.

Nematodes, roundworms Other worms with an almost round body specialized in undulating movements. As a result, the cuticula thickened to stabilize the body. The muscle-fiber-grid, mainly the circular muscles, was reduced up to some longitudinal muscles, so that the original metameric coelom cavities were fused to one large cavity. This is the body structure we find in the nematodes, the roundworms.

Bryozoa and Brachiopoda, Pogonophores Some of the worms, living on the ground, protruded a body segment at their front end developed into a tentacular crown around the mouth. The segmentation of the body was reduced and the animals assumed a sessile life style. The Pogonophores have long and small tentacles and a long and extremely thin body, in which the gut was reduced. Nutrition in the form of microparticles and molecules is ingested through the outer epithelium. Body segmentation and bristles persisted only at the hind end. In the evolutionary course to the Bryozoa the individuals became


Platyhelminthes, flatworms Evolution proceeded quite differently in those worm-coelomates that used the flexibility of transversal walls to flatten their body. This enabled them to creep along hard surfaces, and even to hold on to such a surface. In addition, with the production of slime, they could wrap around a prey and overcome it. The flattening of the body was improved when the coelom cavities were reduced and eventually completely filled by muscle fibers and a cell mass, the parenchyma.

The dense tissue could form suckers at any place and the mouth shifted more to the ventral side of the body. This is the body structure of the Plathelminthes. The Turbellaria, the flatworms, have a sucking gut. The Cestodes, the tape-worms, reduced their gut completely. They live parasitically in the intestinal tracts of animals.

Mollusca

plants could be attacked. Further transformation resulted in the fossil and living molluscs: the dorsal plates of chitons are a secondary arrangement, not reflecting directly the old metameric design. The worm-molluscs are descendants of similarly structured forms, characterised by a loss of the shell. A high and spirally rolled up shell characterizes the Gastropoda, the snails.

The evolutionary development to the molluscs started in a quite similar way as that of the flatworms. However, a muscle-fiber-grid on the ventral side enlarged, forming a cushion, movable in itself and able to adhere to a hard substrate and to creep along it: the creeping foot of the molluscs. The coelomic cavities were shifted to the dorsal side, and above them hard skeletal plates of horn and calcareous deposits stabilized the back side. These animals could tightly press their mouth to the substrate and graze food. On the lower side of the mouth horn bosses and teeth developed as a response to the stress of being pressed to the ground: the rasping tongue, the radula, evolved. Almost no surface has ever been resistant against this strong working rasp. The kinds of nutrition available to these animals could be extended dramatically, and even

The chordates The evolution of chordates is an alternative development in comparison to that of the worm-coelomates described above. It started in those ancestral coelomates that kept their mouth region relatively stiff. The side canals formed slit-like openings in the gut, the gill-slits, finally forming a gillbasket, the so-called branchial gut. These animals conveyed a water stream into the gut by the beat of cilia. The water left the body through gillslits and food particles were caught by sticky slime. The mass of slime and particles was transported into the gut. For locomotion, undulating movements were sufficient. As a consequence, the foremost part of the body developed a rostral filter system (and finally a head), and the larger part behind it became a locomotory system. This divison of the body was decisive for all further evolution of chordates.

In the Bivalvia, the mussels, the shell is divided in two halves. Among the Cephalopoda, the fossil ammonites and the living Nautilus have a chambered coiled shell, and in the squids the shell is shifted internally into the shell cavity.


Lengthwise arranged (longitudinal) muscles, alternately contracting on the right and left sides, allowed the body to bend and undulate. Under the stress of these actions, connective tissue fibers and all muscles were arranged in an efficient system. In the mid-line a portion of the ancestral fibrous-gelatinous mass remained and became enveloped by sheets of connective tissues. This was in fact a tube that could be bent but not compressed and not shortened. This axial rod is called the notochord, from which the name chordates was derived. It holds the length of the body without energy input, and the originally present circular muscles, which had balanced the forces of the longitudinal ones, could be reduced. These became arranged in a new way and formed a thick skin, including fibers and surface cells, as is typical for chordates.

Graptolites, Pterobranchia Further steps in evolution led to animals that wriggled themselves into the sediment and stayed in it permanently. They lived in the sediment and protruded their front end into the ambient water for food collection. The collar could extend like a funnel, and eventually split to form tentacles. The small Pterobranchia, still equipped with two gill slits, and the fossil Graptolites, constructed stiff tubes of collagen.

The longitudinal muscles enlarged more and more. The coelomic cavities lost their original stabilizing function. They were shifted to the ventral side, became smaller and merged to just a few cavities or only one cavity, in which the gut and finally other internal organs found their place. The body was slightly flattened and a fold in the skin made up a kind of fin in the midline of the body. This shape improved undulatory locomotion. In further evolutionary steps, hard skeletal parts could develop in the skin and in the inner connective tissues. This specific chordate bauplan stands in remarkable contrast to that of worm-coelomates, in which most of the body-mass is arranged around the large body cavity, and in which skeletal plates could develop only at the outer surface. The branchial gut region developed in quite different directions. Some evolved jaws and skull, some extended the branchial gut enormously, and others became borrowing and creeping worms and finally developed tentacles.

Enteropneusts, acorn worms The evolution of worm-shaped chordates and their descendants began in those ancestral chordates that used to live near the bottom, where food particles accumulate which have sunk down in the water column. The foremost part of the body, above the mouth, was made up of flexible tissues, and could be used for wriggling and digging in the sediment, stirring up particles. This part enlarged and finally developed a trunk-like digging organ, the so-called proboscis. The rear part of the body lost its locomotory function and the notochord and the muscles were reduced. A transitional part remained between the two portions. Each of them had one coelomic cavity. The enteropneusts, the acornworms, show this body structure: they have a short stiffened structure in the collar, which can be interpreted as the remnants of the old notochord.


Echinoderms The evolution of echinoderms started in similar sessile animals with three coelomic cavities. Tissues near the body surface were stiffened by calcareous material in the form of plates. The fossil mitrates were an early sidebranch of this develoment. They had a row of elongated openings, interpreted as gill slits. The first true echinoderms root in sessile animals with five tentacles. The base of such a tentacle crown is cup-shaped, and it is a movable soft hydraulic system. With increasing body size, this tentacular cup could be supported on its back side, when the next body region extended around it. This part could be stiffened by plates and extended more and more. Finally the tentacles appeared as sunk into a plate-stiffened surrounding. This is the coelom-incoelom encapsuled body structure we find in living echinoderms: In a body coelom is a small coelom (rest of the reduced proboscis) and a large ringlike coelom with extensions into the tentacles. The sea-lilies still have a tentacular crown on a stalk; the starfish (sea-stars), the brittle-stars, the sea-urchins, and the sea-cucumbers, descended from upper body parts that were budded off and became independent of their stalk.

Peribranchial-chambers The gill slits cause a mechanical problem: the larger and more numerous they are, the more they weaken the body wall. Therefore gills arches are stabilized by stiffer tissue, mostly of cartilage. An additional stabilization developed when the body grew wider and around the branchial gut, so that a peribranchial chamber resulted. Wide flat body shapes developed, stiffened by plates of bone, as in the fossil jawless fishes, some examples of which are the Heterostraci or Osteostraci.

Chordates with a vertebral column and tooth elements

The living lancelet Branchiostoma has a slender shape, a large branchial gut and has a notochord over the whole length of the body. At its mouth small tentacles developed, used for food uptake.

Tunicates A large branchial gut continued to collect food without locomotion; such animals could assume a sessile life style. This development caused far reaching changes, by which the tunicates evolved. The system for locomotion, notochord and muscles, were reduced, however, it is still present in the free swimming larvae. The branchial gut extended to an enormous size, hanging in the thick tough peribranchial chamber, the wall of which, the so-called tunica, contains horn and chitin. The sea-squirts are permanently sessile; the salps have a thin-wall and have returned to free swimming.


Some chordates with elongated body shapes developed a vertebral column around the notochord and special tooth-like elements in the mouth and gullet. Examples of such organisms are the fossil conodonts which had complicated arrangements of various bars or tooth elemets. The function of these conodont apparatuses is not yet well understood, but in their microstructure these elements are quite similar to the teeth of vertebrates. The conodonts do not have any recent relatives, but in regard to recent findings, their position close to the vertebrates is most likely.

toral girdle and pelvic girdle) developed into the shoulder girdle and the pelvic girdle. They both provided joints for the legs and surfaces for the attachment of the leg-muscles. Together with the vertebral column a carrying system was constructed, which allowed the complete body to be lifted over the ground. With this evolutionary step the stage of tetrapods was attained.

Chordates with jaws, skull and vertebral column Ancestral chordates with vertebral columns additionally developed jaws and skull. The branchial basket was alternately expanded and compressed. These movements drove a water current through the gill slits, so that breathing and feeding was improved. Even larger particles were swallowed by the pharynx. Some of the rostral branchial arches were enlarged and arranged as a cartilaginous jaw. The result was an efficient apparatus for catching and holding large particles and even living prey. The movements of this primordial jaw-apparatus were oriented up and down – which means vertically –, while the movements of the body for swimming were oriented left and right – which means laterally. These different movements were mechanically separated from each other by the last gill arch (i.e. visceral arch). This skeletal element enlarged more and more, until the complete trunk musculature found its attachment at its surface. We call this last visceral arch the shoulder girdle. Since it separated the head from the trunk, it played a crucial role for further evolution of fishes and tetrapods. Around the head a massive cartilaginous capsule developed and the notochord became surrounded by various cartilage elements, suppressing the range of lateral bending.

Fishes In the evolutionary course of fishes bony scales developed in the skin tissues. These scales were arranged in such a way, that they limited the range of lateral bending. The head was stabilized by large bony plates, and the cartilaginous capsule, enclosing the brain and the sensory organs was replaced by bony material. Finally a bony skull was formed. The lateral, dorsal and ventral fin folds of the ancestors differentiated into certain fin-types. In the fossil record the first fishes that were representatives of these evolutionary stages are the placoderms. Their head was reinforced by large bony plates and their body shape was stabilized by a cartilaginous vertebral column and by large scales in the skin. The cartilaginous fishes, another evolutionary pathway, not figured in the poster, have only small tooth-like scales all over their entire body. A bony skeleton developed only in the pathway to the bony fishes.

Tetrapods Most fishes developed lungs from a bulge of the foregut and finally swimbladders. These fishes were able to breathe air. Some of them had a body organization which allowed them to transform their fins into legs and to carry the complete body on land. The two bony girdles (pec-


Today we can distinguish different tetrapods. The amphibians have to lay their eggs in the water, where their larvae develop. The reptiles are able to live on land permanently. Even those who went back into the water have to come onto land for laying their eggs. As an example for the reptiles a dinosaur is shown in the poster (Triceratops is the logo of Senckenberg-Museum). The dinosaurs are a certain group of reptiles which have their closest relatives in the recent (and fossil) birds. Some reptiles, the so-called therapsids are the ancestors of the mammals. From these early mammal-like reptiles a huge number of different organisms evolved. Man also belongs into the group of the mammals. Man evolved from schimpanzee-like ancestors and developed an upright walking position. As a consequence the brain enlarged enormously. This brain allows thinking, combining and teaching the following generations. Such a brain is unique in the entire animal kingdom, and on the basis of its intellectual capacities, man constructs his own view of the world.

Scientific theory, Darwinism Baumunk, B.-M. & Riess, J. (Hrsg.) (1994): Darwin und Darwinismus: eine Ausstellung zur Kultur- und Naturgeschichte. Katalog; eine Publikation des Deutschen Hygiene-Museums Dresden; 265 S. – (Akademie) Berlin. Janich, P. & Weingarten, M. 1999. Wissenschaftstheorie der Biologie. 315 S. – (Wilhelm Fink) München.

Further reading The evolution of animals is presented here in a new way, which is not like that of the standard textbooks. The engineering-like explanation of evolution and the reconstruction of evolutionary pathways contrasts sharply with morphometric and genetic phylogenetic trees. However, explanations are provided, which are missing in most traditional presentations. The results which are the basis of this poster have been developed since 1970 by the group „Kritische Evolutionstheorie“. Continous discussion with philosophers and testing improved the coherence of the methodology. The listed references should give an idea for further readings.

Frankfurt theory of Evolution and Engineering Morphology Scharf, K.-H. & Gutmann, W. F. (Hrsg.) (1993): Evolution von Organismen. – Praxis der Naturwissenschaften, 42: Biologie: 1-49. Gutmann, W. F. (1995): Die Evolution hydraulischer Konstruktionen: Organismische Wandlung statt altdarwinistischer Anpassung. Senckenberg-Buch 65, 220 S. – (Kramer) Frankfurt am Main. Edlinger, K. Gutmann, W. F. & Weingarten, M. (1991): Evolution ohne Anpassung. - Aufsätze und Reden der Senckenbergischen naturforschenden Gesellschaft, 37: 92 S. – Frankfurt am Main.

Weingarten, M. (1993): Organismen - Objekte oder Subjekte der Evolution? Philosophische Studien zum Paradigmawechsel in der Evolutionsbiologie; 314 S. – (Wissenschaftliche Buchgesellschaft) Darmstadt.

Chemo-Evolution and the origin of life on earth Kempe, S., Kazmierczak, J. & Degens, E. T. (1989): The soda ocean concept and its bearing on biotic evolution, 29-43. – In: Crick, R. E. (ed.): Origin, Evolution, and Modern Aspects of Biomineralization in Plants and Animals; 536 S. – (Plenum Press) New York. Wächtershäuser, G. 1997. The origin of life and its methodological challenge. – Journal of Theoretical Biology, 187: 483-94. Dullo, C., Mosbruger J. & Oschmann, W. (2000). Geobiologische und Paläobiologische Prozesse als Antrieb der Evolution des Systems Erde. – Kleine Senckenberg Reihe 36. – (Kramer) Frankfurt am Main.

Endosymbiosis and theory of the biosphere Margulis, L. (1981): Symbiosis in Cell Evolution. – (Freeman Co.) San Francisco.

Certain evolutionary pathways, and texbooks on animal bauplan Gudo, M., Gutmann, M. & J. Scholz (Hrsg.) (2002, in Vorb.): Concepts of Engineeringand Functional Morphology: Biomechanical approaches to recent and fossil organisms. Senckenbergiana lethaea. – Stuttgart. Gutmann, W. F. (1994). Morphologie & Evolution. Symposien zum 175jährigen Jubiläum der Senckenbergischen Naturforschenden Gesellschaft. Evolutionssymposion. Senckenberg-Buch 70, 454 S. – (Kramer) Frankfurt am Main. Storch, V. & Welsch, U. 1991. Systematische Zoologie. – Gustav Fischer, Stuttgart, New York, 731 S. Westheide, W. & Rieger, R. 1996. Spezielle Zoologie – Erster Teil: Einzeller und Wirbellose Tiere. 909 S. – (Gustav Fischer) Stuttgart, Jena, New York.

Acknowledgements A number of colleagues did read the text and its english version and made valuable suggestions. Dr. Sven Baszio, Prof. Dr. Euan Clarkson, Doris von Eiff and Dr. Eberhard Gischler. To all of them we extend our thanks. For financal support we thank PASS IT Consulting GmbH.

Evolution in teaching courses

Mollenhauer, D. (1994): Endocytobiosen. Anlässe zu Revisionen in Taxonomie und Phylogenetik: 339-363. – In: Gutmann, W.F., Mollenhauer, D., Peters, D. S. (Hrsg.) (1994): Morphologie und Evolution. – Senckenberg-Buch 70; 454 S. – (Kramer) Frankfurt am Main.

The poster „The Evolution of Animals“ is available together with this text either in German or in English. Certain materials for teaching are also avaiable (overheads, slides) and guided tours on the evolution of animals through the Senckenberg Museum can be offered (Phone: 069/7542-357).

Krumbein, W. E. & Lapo, A. V. 1996. Vernadsky's Biosphere as a Basis of Geophysiology. 115–134. In: Lovelock, J., Margulis, L., Saunders, P., Whitfield, M., Goodwin, B. & Ho, M.-W. (Hrsg.): Gaia in Action-Science Of The Living Earth. – (Floris Books) Edinburgh.

Please sent your orders to: Schriftentausch der Senckenbergischen Naturforschenden Gesellschaft, Senckenberganlage 25, 60325 Frankfurt am Main, Phone: 069/7542-246, e-mail: [email protected].

Levit, G. S. 1999. Biogeochemistry-Biosphere-Noosphere: The Growth of the theoretical system of Vladimir Ivanovich Vernadsky. Studien zur Theorie der Biologie Band 4. – (VWB) Berlin.
