5. The Evolution of Metazoa

5.1 The Phyletic Lines

 

Figure 5.1. Geologic time chart. Ice Ages occurred 600 million years ago, 280 million years ago and 1 to 2 million years ago. They define the 3 geological great eras. Each era is subdivided in periods, themselves divided in epochs. Each new epoch defines new physico-chemical conditions on the surface of the earth. Note that I placed the origin of the chordates and agnates in the Ordovician. Recent Chinese finds have shown this to be erroneous: their origin is likely to be in the Precambrian.

Eighty-five percent of the earth’s time had elapsed when, 600 million years ago, the Infra Cambrian Ice Age occurred (fig. 5.1). The only kind of multicellular life for which Pre-Cambrian strata provide clear evidence is plant life, chiefly lime-secreting algae. Traces of other organisms of an animal nature can be recognized also, but these are very primitive and undifferentiated.

A geographical event of the intensity of the Precambrian Ice Age was followed, after a short interval, by a biological event of striking character. The consequence of the Infra-Cambrian glaciation was that the pullulating of the primitive living forms was slowed down. The cooling of the earth during the Precambrian period increased the concentration of oxygen and other gases in the water. Systems making use of this increased supply of energy could thus be devised. The primitive living forms which had developed a calcium skeleton must have had greater difficulties to survive and multiply in cold water than those which had a siliceous skeleton or no skeleton at all of a mineral nature. In that latter case, a hard, organic material, which in plants is cellulose, may provide the skeleton and in animals such as the arthropods (insects, spiders, scorpions, lobsters, etc.) it is chitin.

The Cambrian strata that followed the Ice Age contain fossils of a remarkably varied array of multicellular organisms. They include representatives of nearly every major phylum possessing skeletal structures susceptible to fossilization. An all-important exception to this was the phylum of the chordates, which includes the vertebrates. Two species of vertebrates have been discovered in Southern China in strata dating back 530 million years but the expansion of the vertebrates was delayed until 475 million years ago, during the Ordovician.

5.1.1 The coelenterates

When the sponges adopt a mineral skeleton, this mineral is silicon. The glaciation opened the way for the proliferation of the sponges. Jellyfishes appeared also. These two living forms are composed of many cells and these cells have, within the organism, different functions. Some cells produce digestive juices, others evolve a skeleton, others play a defensive and protective role and others specialize in the reproductive function. The coelenterates (jellyfishes, corals and anemones) are more evolved than sponges. They are definitively animal in nature and superior to the unicellular or barely differentiated pluricellular organisms they supersede.

Once the Infracambrian Ice Age was over, the secretion of a calcareous mineral skeleton became possible by originally naked organisms, which survived the earlier age precisely because of the absence of such a skeleton. The medusoid free forms of jellyfishes and the fixed forms of corals and anemones represent the coelenterates. Many coelenterate species pass during their life cycle from one form to the other. They possess a diffuse and primitive nervous system, which is denser around the mouth and around the umbrella. From the coelenterates stem the totality of the subsequently appearing animal forms. It is possible that the sponges are at the origin of the graptolites but these are now extinct. Two evolutive lines sprouted from the coelenterate master plan. These are the echinoderms and the worms.

5.1.2 The mesenterian hypothesis

Stemming from the jellyfish form of the coelenterates are the echinoderms (starfishes, urchins, holothurians, and crinoids), which have maintained the diffuse type of nervous system inherited from the coelenterates (fig. 5.2).

Figure 5.2. The mesenterian hypothesis postulates a medusoid form as the origin of echinoderms (Cycloneurians). "n" stands for concentration of nervous material.

All other animal forms derive from a cerianthid fixed form (fig. 5.3).

Figure 5.3. Cerianthus is an anemone that may serve as a model of origin for higher living forms in the mesenterian hypothesis. Its body increases in size by addition of more and more septa, all originating from one point. Despite the appearances, the species has a bilateral symmetry. Again, nervous material is concentrated around the animal’s mouth and around the middle of the body, the latter corresponding to the umbrella of the coelenterates.

The existence of this type of circular nervous system impeded the coales­cence and the later concentration of nervous material in a head. These animal forms were unable to reach a high degree of consciousness. The modern theory of evolution, based on nucleic acid analyses, holds that echinoderms are at the root of the chordates. This contemporary popular theory is based on genetic programs and disregards proteins, developmental biology and the morphological evidence of the radial symmetry of echinoderms. I will, however, not swear by it: key morphological features as segmentation and coeloms, used to classify the animal kingdom, might not be as conservative as hitherto thought and our view of the evolution of body plans, and the relationships of various animal phyla, may be more complicated than usually assumed. Note that the classification of worms is also subject to controversy.

The concept of the formation of the primitive worm from a coelenterate is more difficult to grasp. Let us postulate a cerianthid (fig. 5.4), which is a kind of anemone, i.e. a fixed coelenterate form, that would have many tentacles surrounding a deeply slit mouth. If the upper part of such a cerianthid gets loose, which is a common event within the coelenterate group, we will obtain a worm with many appendages, provided the sides of the deeply slit mouth are joined together so as to create a digestive tube (fig. 5.4).

Figure 5.4. The left part of the figure illustrates how a cerianthid form could have given rise to deuterostomes and chordates: a neural condensation (n) closes the buccal orifice of the cerianthid, leaving only the anus (a). A secondary mouth opens underneath the chord. The right part shows schematically the evolution of protostomes. The cerianthid prototype must turn around, with the mouth facing the sea floor. The neural condensation (n) runs along the gut of the animal and surrounds the mouth (m).With imagination, it is possible to visualize how a jellyfish could have started to crawl on the seafloor and give rise to echinoderms (starfishes).

The septa that compartmentalized the inner environment of the cerianthid were maintained in the new living form. The particular line of evolution of worms was very simply to add more and more compartments at the end of the chain, as is done in the cerianthid (see fig. 5.3). We still have a memory of this compartmentalization, with structures such as ribs and the vertebrate column.

The jellyfishes are primitive radially symmetrical animals that have two types of cells separating the exterior from the interior: the ectoderm faces the outer environment and the endoderm delineates the inner environment. All bilateral animals have in addition also a mesoderm that intercalates between the ectoderm and endoderm. The most primitive flatworm that possesses an endoderm is the acoel, but the acoel flatworms have no true gut, i.e. they have no true body cavities, called a coelome, hence their name “acoels”. The acoels also do not go through a larval stage but immediately grow adult, from the egg. This indicates that the passage through a larval state is a late acquisition in evolution. The acoels are not platyhelminth flatworms, as is commonly assumed, but belong to their own phylum and represent probably the most primitive bilateral animal phylum, appearing perhaps before the Cambrian, still existing2. Worms are an abundant and diverse group of organisms. Nevertheless, the relationship of nematode worms to other animal groups has remained controversial. Initially, they were placed at a distance from animal groups with true body cavities, i.e. with a coelome. When nucleic acid analyses took preponderance, ribosomal gene sequences implied that they had closer links to arthropods. These are a coelomate group and, to accommodate the worms, the superphylum of molting animals or Ecdysozoa was created. In 2002, Blair et al. abandoned the nucleic acid and turned to proteins for classification. Comparing the sequences of more than 100 proteins, they conclude that nematodes are distant from both the arthropods and the vertebrates, which are closer to each other than nematodes are. However, planarians have the ability to regenerate their heads. By following the formation of the new brain and characterizing the genes involved, evidence is that the brain organization seen in vertebrates had its beginning in flatworms. Some planarians brain regions are organized like those of the embryonic vertebrate brain. Worms are commonly seen as burrowing animals. It is true that some of the first animals burrowed through marine sediments in search of food and shelter. However, colonization of sediments in freshwater rivers and lakes by burrowing animals proceeded much more slowly. It is the last habitat that was colonized, in the late Mesozoic, 100 million years ago, long after fresh water, land and air had been exploited.

Two other evolutive avenues were explored. The first of these lines is the utilization of numerous appendages for crawling purposes, which gave rise to the protostomes i.e. arthropods as bees, ants, lobsters, etc. now called the Ecdysozoa. Ecdyzon is a Greek word meaning stripping and ecdyson is the hormone that promotes the sloughing of insect larvae and of crustaceans. As said supra, with the nematodes removed from the superphylum, the name Ecdysozoa has no more reason to be used. The second line, the deuterostomes among which are all the vertebrates including Man, involved the abandonment of appendages for crawling purposes and the invention of something radically different.

5.1.2.1 The protostomes

In order to provide the crawling appendages with a minimum of efficacy, the appendages must be reinforced with a solid yet flexible armature to permit movements. This is the organic, hard material called chitin that was devised by the arthropods (crabs, lobsters, and insects). To be able to use these appendages, the animal form had to turn around. The mouth of the cerianthid must face the ground so that the appendages can be used. Since the mouth and anus of these evolved forms face towards the seafloor, the original mouth of the anemone could continue without changes in its function. All animal phyla originating from a cerianthid prototype, where the original mouth continues to serve its original function, are protostomes. Most animal phyla on this earth are protostomes, such as mollusks, arthropods, annelids, brachiopods, etc. Some of these animal forms are so obscure that only zoologists know of their existence3.

Figure 5.5. The brain of an octopus is shown in top view. Brachial nerves run along the animal’s arms. The two large optic lobes are related with stalks to the central brain that encloses the esophagus, here presented in black. Excessive development of the brain will occlude the esophagus.

The nervous material of the ceriantid anemone is located around the mouth and it is still located in protostomes around the mouth and along the digestive tube, on the ventral face of the animal. A nervous condensation runs along the belly, with nodules located at the height of every appendage. Some protostomes, like the mollusks, have abandoned the chitin’s skeleton and the appendages, but still have their buccal orifice surrounded with nervous material (fig. 5.5). This feature is found in bees, ants, spiders, calmars, squids, mussels, scorpions, lobsters, etc. The formation of a true head is impossible with all living forms belonging to the protostome master plan because it would squeeze out the mouth. The defect was built in the prototype.

5.1.2.2 The deuterostomes

If the cerianthid prototype is deprived of its tentacles, there is a naked worm. Without appendages, there is no need for an external chitin’s skeleton, which would now hinder locomotion. In addition, the animal does not need to invert its living position to move. Consequently, the neural condensation of the primitive anemone form remains in a dorsal position, along the gut, in a new evolved form.

In order to have some locomotion power relying on the movements of the body, the naked worm, even initially, had to have a certain length, which was no exigency for the protostomes. It also needed along its axis an internal flexible stick, on which muscles could bind and find a point of attachment. This flexible stick is the chord and the chord appears as a condensation right under the neural axis, in a dorsal position. However, the primitive mouth of the coelenterate prevented the full development of the chord in its dorsal position. To alleviate this, a secondary mouth developed in the ventral position. An animal prototype provided with a secondary mouth is a deuterostome4.

The neural condensation will develop into nerves, in protostomes as well as deuterostomes. These nerves transport electrical pulses at a speed of about a meter per second. This is fast enough for small animals to respond with efficacy to the challenges they meet. For animals endowed with a large body size, the electrical pulse propagating along, for reaching the end of long nerves, must be fast for escape maneuvers as well as for predation. The protostomic cephalopods have met the problem by tremendously increasing the diameter of the nerves, which can be as thick as a few millimeters but this improvement is still not good enough for quick responses. Vertebrates enrobed their nerves with a thin membrane sheath composed of myelin, which allows speeds of electrical pulses of 50 to 100 meters per second. The oldest vertebrates that are myelinated are the jawed cartilaginous fishes, sharks and rays. Lancelets, hagfishes and lampreys, which are jawless, are not myelinated.

This system of fast communication very soon diversified in various sense organs adapted at picking up special information. Smell and taste capture the presence of chemical substances, photons of light are perceived by visual organs, air vibrations by a hearing device, equilibrium perception is also located in the ears of humans, Brownian movement 5 which tells us if something is hot or cold, sonar systems, pressure systems, etc., all developed.

The regulation of the internal environment is effected in humans by unconscious mechanisms. There are plenty of them. Pressure sensors along our arteries measure blood pressure and heart beats, CO2 sensors increase or decrease respiration and also kidney and liver functions, peristaltic movements of the gut, kneading of food in the stomach, etc. are all systems working automatically. From an evolutionary point of view, the awareness of the functioning of these systems would be a hindrance rather than a survival edge.

The state of tension of the muscles is, however, information that, initially in the course of evolution, might have been useful to be aware of, especially since, at those levels, consciousness itself was very feeble. Such knowledge would have provided information on the position of limbs and thus the body. Tendon organs and innumerous contractile muscle spindles indeed tell us at all times by nerve fibers leaving them and going to the cerebral cortex, what the tension of our muscles is. Man is unaware of such a muscle tension. It is very difficult to know if lower animals are conscious of such an internal phenomenon as tension in their muscles. The state of muscular posture was in early times probably signaled to the brain and this must have been a strong adjuvant in the development of the central nervous system. At least in Man, such information became superfluous and the insensibility of the muscles is in this species almost complete, although nerves still bring the information from the muscle to the brain. Insentience of the internal environment in the human species allows now for easy surgical interventions, since most organs are insensitive: provided the outer membrane, i.e. the skin, is perforated painlessly, since this organ is endowed with many sensory organs giving warning of harmful conditions, even the brain can be manipulated without pain. This allowed for easy trepanation operations in Neolithic times provided superficial local anesthesia was practiced. Such insentience is not necessarily true in other animal species.

The ultimate consequence of the translocation of the mouth from the dorsal to the ventral face provided a free swimming and elongated animal with a better means of locomotion, and the advantage was that, in due time, there would be a place in the dorsal position for the formation of a head filled with nervous material. The phylum of the chordates is the only phylum that developed according to the deuterostome schema.

Figure 5.6. Ascidia. The upper part of the picture shows a free-dwelling larva of ascidia. The lower part of the figure shows a colony of adult ascidia.

Amphioxus and ascidia (see fig. 5.7) are primitive members of the phylum. Amphioxus, known as the lancelet, lacks eyes or fins or jaws or a skull with a brain and looks like a miniature anchovy fillet. It possesses only a primitive backbone called the notochord. These simple marine organisms, the tunicates, live in filtering plankton. In their adult stages, most tunicates appear to have little in common with vertebrates. Sea squirts, ascidia, have neither tail nor head and live attached to the sea floor. Their larvae, however, look a lot like tadpoles, with key features characteristic of chordates. These feature a hollow nerve cord along the upper back, a notochord-a flexible rod of cells that functions like a backbone- and gill slits. Ciona intestinalis embryo has just 2600 cells and its genome is composed of only 160 million bases composing 16,000 genes. The animal lives three months. Very soon, however, the chord was reinforced with cartilage and deposits of calcium carbonate (i.e. bones). To maintain its flexibility, the mineral deposits were partitioned in accordance with the original septa of the primitive anemone. These deposits are the vertebrae and animals possessing vertebrae are vertebrates. The most primitive vertebrate alive today is the jawless hagfish.

Figure 5.7. Ascidia (Ciona intestinalis) and Amphioxus (Branchiostoma lanceolatum) are primitive chordates. They herald the phylum that ultimately leads to Man.

Length, formation of a chord and cephalisation are some of the features that made the chordates a phylum with high evolutive powers. Yet, whereas everything in this form of life indicates its vocation to move ahead, inertia is already apparent at the most primitive level of the phylum. Amphioxus is a sea-dwelling free form but ascidia lives its adult life hooked on a self-made peduncle.

Figure 5.8. T. Audesirk and G. Audesirk (Life on earth, Upper Saddle River, NJ: Prentice Hall, 1999) propose the following classification scheme for the major phyla of animals. Some of the key innovations that occurred are indicated in the ovals. According to this hypothesis, the starfish and sea urchins are most closely related to the Chordates.

A view of life based on structural and functional analyses of organisms, in which life moved from simple to more complex and from smaller to larger, recognizes 5 kingdoms (see fig. 5.9).

Figure 5.9. K. Nealson (Searching for life in the universe, annals NYAS, 950, 2001) proposes the prokaryotes (monera) as the basis of life. These organisms of a small size (1-2 micrometers) and rigid cell wall evolved into unicellular organisms (protoCtista) of a larger cellular size (10-25 micrometer) with flexible cell walls, able to phagocyte and take particles or other organisms up. These evolved into the 3 pluricellular eukaryotic kingdoms (fungi, animals and plants).

Figure 5.10. The same author proposes a phylogeny based on sequence analyses of ribosomal RNA genes. Such an approach yielded a view of life in which the major kingdoms shown in fig. 5.9 are grouped into one kingdom (eucarya) and the prokaryotes are expanded into two kingdoms (Archaea and Bacteria).

References

2. I. Ruiz-Trillo et al. Acoel flatworms: earliest extant bilateral metazoans, not members of platyhelminthes. Science 283, 1999, pp. 1919-1923

3. T. Cavalier-Smith: A revised six-kingdom system of life. Biol. Rev. 1998: 73, 203-266

4. The words protostome and deuterostome derive from the Greek words "stoma" meaning mouth and "proto" and "deutero" meaning first and second.

5. Robert Brown, (died 1858), showed that molecules within a liquid solution collide among themselves with a speed that is proportional to the temperature of the liquid.

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