5. The Evolution of Metazoa

5.2 Land Invasion by Chordates

The appearance of representatives of the chordate phylum has long been assumed to occur about 475 million years ago, during the Ordovician period, i.e. late in evolutive times. Finds in China (1999) show that at least two species of 2 cm long vertebrates, more evolved than Amphioxus, existed 530 million years ago, which provides evidence that vertebrates started well before the Cambrian period, between 565 and 750 million years ago.

The potentialities of this phylum could only be fully realized by a passage from an aqueous to a terrestrial environment. Furthermore, the bony fishes that gave rise to the mammals and ultimately to Man evolved themselves in fresh waters. In the absence of such a biotope constituted of solid ground plus ponds and lakes, the maximal level of consciousness reached on this earth would have been attained by mollusks of the cephalopod class (squids, calmars, etc.) competing for survival with arthropods as lobsters and scorpions. Even the presence of chordate brutes as sharks and hagfishes is not certain since these evolved apparently also from precursors of bony fishes.

The successful passage from an aquatic to a terrestrial life presupposes the presence of various features that can be re-utilized and redirected towards this new purpose. The absence of only one element of adaptation renders this invasion futile.

5.2.1 Lungs and crawling fins

The first chordates were devoid of an articulated inferior jaw and deserve their name: agnates (from Greek: "a" means without and "gnathos" means jaw). The most primitive forms discovered are about 10 cm long and are completely naked such as is Jamoytius, an eupharenide of the Silurian period (fig. 5.11).

Figure 5.11. The discovery of the fossil agnate "Jamoytius" was an extraordinary event in the field of comparative anatomy of the chordates. The animal has no inferior jaw, is deprived of a cephalic shield and is provided by a chord only.

Soon, an exterior hard cephalic shield protected these agnates. The remaining part of their body was apparently naked and no interior skeleton could be detected (fig. 5.12).

Figure 5.12. The development of a cephalic shield by the agnates (here cephalaspis lyelli) was probably a response to the challenge represented by the terrible scorpions (Eurypterides) of the Devonian period. Some of these aquatic scorpions reached a length of 3 meters.

Although there still is no direct evidence of the fact, it is believed that the agnates gave "birth" to two different groups. The cyclostomes are, like the agnates, deprived of an inferior jaw. The lamprey is one of their representatives. The agnates also gave birth to the protychtians during the Silurian period. The diversification and radiation of vertebrates was impelled by developmental innovations, among which the elaboration of the brain, the skull and jaws. Large-scale gene duplications have been tied to these innovations. The expansion and expression of some genes (the Dlx genes) correlate with the elaboration of the jaws. The success of vertebrates was due in part to the acquisition and modification of jaws. The transition from the jawless agnaths to functional jaw structures in vertebrates opened the door to evolution of complex skull and ear structures as well as dentition structures. Early jawed vertebrates as the acanthod fishes displayed a remarkable symmetry between their upper and lower jaws as well as in their dentition. These symmetries disappeared later in evolutionary history. A complex series of cellular and molecular interactions underlies the assembly of the vertebrate face. During the early evolution of mammals, the major upper-jaw element became fragmented. Parts of it fused to the brain case or gave rise to elements of the mammalian pharynx. Other parts turned into the mammalian incus or became the third middle ear bone of the mammalian new hearing apparatus.

During the Silurian period, a continent emerged from the waters and the Caledonian mountains rose high above the sea. Their remains are found in Ireland, Wales, Scotland and Scandinavia. The bony fishes (Placodermus) in the fresh waters and lakes of the Devonian period and the elasmobranches (Cladoselache), which reverted to the seas, evolved from a protychtian. The elasmobranches are the origin of sharks, skates and rays. Initially, elasmobranches and bony fishes were indistinguishable. Both retained bony cephalic plates. It is only during later evolution that the elasmobranches relinquished any attempt to ossify their internal skeleton and adopted a curious type of scales as seen in the shark in its fearful renewable teeth. Modern sharks and hagfishes are chondrichtians. They have retained the potentiality to secrete bony material and this explains why these fishes still secrete the hormone calcitonin that commands this production.

The sharks are, as far as sensory organs are concerned, over-endowed. Their sight is pretty good, even in the dark, and they can also perceive movements and vibrations generated in the liquid medium, by special organs located all along their side. This allows the animal to accurately locate an obstacle and exactly know the size of an organism wandering in its vicinity, as well as its speed and movement direction. The sharks also produce an electrical field. They possess electro receptors capable of spotting differences of 0.5 microvolts per centimeter and this is exceedingly little. Preys that produce electrical micro fields are thus detected in this way. Finally, the olfactive system of sharks is so developed that it detects substances diluted 1 part to a million parts of water and the shark also hears perfectly well. The shark is however no match to sea-dwelling mammals such as dolphins.

The elasmobranches did not pursue the development of a device resembling a lung and their physiology does not allow them- as a rule – to leave saline waters. They are forever restricted to the oceans, to which they are beautifully adapted, although some of them have adapted to life in big lakes (e.g. the sharks living in the Atitlan Lake of Guatemala) and rivers. Whereas primitive bony fishes developed a rudimentary lung that could be – and was in teleosteans – transformed into a buoyancy device, the elasmobranches never developed such a buoyancy device. As a result, modern sharks are generally obliged to keep swimming all the time, under penalty of sinking. In fact, some species store air in their stomach, while others use the oil of their liver as ballast.

The first group of elasmobranches, i.e. the xenacanths, appeared about 400 million years ago, during the Devonian, in saline waters. They adapted to fresh water, which allowed them to survive the first great mass extinction, at the beginning of the Trias. They disappeared 40 million years later, at the end of the Trias. The hybodonts appeared 365 million years ago, also in saline waters, and also adapted to fresh water, which allowed them to survive the permio-triassic crisis and become the dominant fishes in the fresh and saline waters of the Trias. They declined in the oceans during the Jurassic, due to the competition of modern sharks. These neoselaceans are strong and swift animals able to chase and catch their prey instead of ambushing them. Only those species of hybodonts that fed on ammonites survived in saline waters. They continued to do well in fresh waters, despite the competition of the neoselaceans, also because their teeth adapted to crunch hard shells.

The hybodonts disappeared together with the dinosaurs, 65 million years ago, after having thrived a record 300 million years. The neoselaceans again colonize fresh waters, i.e. the Mississippi, the Amazon, the Zambezi, the Ganges and the Brisbane in Australia. The elasmobranches, as a group, detain the record longevity and diversity among the vertebrates. They represent an unequaled evolutive success.

The bony fishes, subdivided initially into the dipneusts and the crossopterygians, maintained the ossification of the head and developed an internal bony skeleton. Their strong cephalic shield is thought to have been sufficient to adequately protect them against the attacks of huge primitive scorpions (the eurypterids) measuring 1 to 2 meters. More importantly, the crossopterygians had the first show of lungs. A present day representative of the crossopterygians is the coelacanth "Latimeria" that readapted to marine life and was first caught in 1939 in the Indian Ocean (fig. 5.13).

Figure 5.13. Latimeria is the sole known representative of the Crossopterygians still living. It haunts the deep cold waters of the Indian Ocean. The animal has paired fins that may serve as crawling devices on the sea floor and thereafter on solid ground. The articulation of a paired fin is shown upper-right.

Dipneusts are also still living today and also have a lung. They have, for about 375 million years, remained the haunters of brackish ponds in Africa, where they can still be observed. They have thus remained perfectly faithful to their original environment.

Among the bony fishes, the crossopterygians had a predisposition for the passage from an aqueous environment to a terrestrial one, of which the dipneusts were deprived. The crossopterygians were endowed with paired fins. The fins of the crossopterygians could be transformed into limbs able to carry them over land. Actually, the injection of a thyroid hormone into a bony fish (Periophtalmus variabilis) has the effect of transforming its fins into rudiments of pentadactil members. The ovovivipariness of crossopterygians must have further helped them in their endeavor to carry the species safely from one pond to another.

The crossopterygians gave rise not only to the tetrapodes, initiating with the amphibians, but also to the actinopterygians. Not many of the actinopterygians are left. The sturgeon living in the Caspian Sea is a representative. From the actinopterygians stem the teleosteans, which are now the immense majority. These teleosteans have maintained the presence of a rudimentary lung, which serves now as a buoyancy device.

The gastropods (e.g. snails), like the crossopterygians, have gills and a kind of lung, even in some forms exclusively adapted to sea life. They were thus preadapted to terrestrial life. The arthropods (e.g. crabs, lobsters) have gills, which can transform into lungs. The acquisition of a device enabling the utilization of atmospheric oxygen was thus apparently not an insurmountable problem. Fishes have evolved devices other than lungs in order to breathe atmospheric oxygen. The South-American electric eel has transformed its mouth into a lung, while its gills are completely degenerated. Some fishes are able to use their gills for breathing air. In this case, the gills are so organized that they do not collapse under the force of gravity, as gills usually do out of water. The South-American fresh water eel-like fish Symbranchus marmoratus has developed such a device. Other common South American fresh water fishes are breathing air through the gut. Hoplosternum littorale has reached the point that it cannot survive anymore without breathing air. The fish has about the totality of its internal cavity filled with guts and these guts are almost completely filled with gas.

Air breathing supposes the existence of oxygen in the atmosphere, in quantities large enough to make it economical for animals to try to live on it. Plants produce atmospheric oxygen. Plants must thus have largely preceded animals on solid ground and in liquid media. At the beginning of life, the shallow seas must have been choked with bacteria, using up the majority of the oxygen, so that a positive trend must have existed towards an environment -the air – where there would be an escape from them. One might likewise admit that the production of antibacterial agents by molds, such as penicillin, aureomycin, etc., was an early defense mechanism against bacteria, indispensable for survival purposes.

5.2.2 Oxygen transport

The weight of swimming animals is supported by the surrounding medium. This is not anymore the case for running or flying animals. These, in turn, have the advantage of moving in a medium of low viscosity and low density, i.e. the air. In all three cases, the cost in energy necessary to move one gram of animal over a given distance increases, as a general rule, the smaller the animal is. An exception to this rule is flying insects where cases are found where the cost of flying is smaller for smaller animals.

Also, flying is about five times cheaper to move to a distant point, than is running. Migratory birds are a proof of this: one can hardly imagine a human moving 1,000 kilometers non-stop without taking food or rest. The energy cost of swimming, again, is lower than that of flying by a factor of about two.

These considerations show firstly that animals which are moving have an inherent advantage in building up to a large size: it costs less to move; secondly, that the step out of the water onto land is very expensive and thirdly, that it would not be astonishing to see land-traveling animals either conquer the air or return to the water, since both media are cheaper to wander in. To leave water for land is very expensive and this step requires the availability of an adequate system of energy transport and consumption.

Some mollusks, such as the snail and the octopuses, as well as arthropods such as the primitive horseshoe crab (figure 5.14) and the scorpion, have blue blood.

Figure 5.14 Horseshoe crab. The primitive horseshoe crab lives on the shores of the Atlantic, along the Florida gulf coast. It is with us since about 550 million years (Cambrian period).

This blue blood is due to a copper-rich protein that serves as an oxygen carrier for these animal species. Although copper is not very abundantly available in nature, the preferred blood pigment was originally the copper pigment as it establishes strong chemical bindings with proteins. The efficiency of oxygen transport by this system is, however, low.

There is no easy way to determine how many chordates were present in the streams and ponds of the early ages. However, the newly appeared animal form, moving freely in the water medium, no longer on the bottom, needed fuel. The presence of chordates reduced in some way the existing primitive animal population, including the zooplankton produced as offspring by the earlier coelenterates, echinoderms and protostomes. This ruthless predation may have eliminated the scorpions and permitted an extraordinary development of algae, which increased the amount of oxygen. At that epoch, the earth had substantially cooled down and the amount of oxygen dissolved in water was greater than previously. Animals were evolved that made use of this increased supply of oxidant by utilizing as an oxygen carrier the red hemoglobin, which contains iron. Iron could be used, despite the less efficient oxygen binding of hemoglobin, because more oxygen was available and because a mechanism developed that could achieve a much higher concentration of the hemoglobin in the blood, through its storage in the red blood cells. Without hemoglobin, a liter of arterial blood at a body temperature of 37°C transports about 3 milliliters of oxygen. With hemoglobin, it transports 70 times more. The system devised is such that oxygen transport and delivery are both improved.

A single red blood cell contains about 280 million molecules of hemoglobin. Each hemoglobin molecule contains 4 atoms of iron and each iron atom lies at the center of a group of atoms that form a pigment called heme, which gives blood its red color. Every hemoglobin molecule contains 4 hemes (figure 5.15).


Figure 5.15. A hemoglobin molecule is composed of four proteinic subunits, of which two are identical alpha-subunits and two are identical beta-subunits. The four red-pigmented hemes enclosed in a hemoglobin molecule are shown schematically here. Each heme carries an iron atom and fixes an oxygen molecule.

The hemoglobin of agnates such as lampreys, and of hagfishes, points strongly out to the possibility that the ancestral hemoglobin was monomeric, i.e. composed of only one heme instead of four. In the fish "Latimeria" i.e. the sole crossopterygian fish still existing, hemoglobin is, like mammalian hemoglobin, tetrameric. It shows however a stronger affinity towards oxygen. This is probably a primitive characteristic of early appearing vertebrates that lived in still poorly oxygenated environments.

The iron atoms present in the hemes bind oxygen (O2) only when the hemes are in association with globin. Hemes alone will not do. But in combination with globin, the taking up of oxygen is accelerated for the last iron when the other three have already taken up their oxygen. The freeing up of oxygen by the first of the other 3 irons also facilitates the release of oxygen by the last iron. Thus, when there is plenty of oxygen present, as for example in the lung, the taking up of oxygen is fast and easy. And conversely, in the body tissues where oxygen is scarce, release of it by the hemoglobin is also easy. Thus, with this system, oxygen transport and delivery are both improved.

Hemoglobins are found in jawless vertebrates and in diverse invertebrates ranging from flies (arthropods) to earthworms (annelids) to nematodes. The estimated time for the last common ancestral hemoglobin gene dates around 670 million years ago. Hemoglobins are also found in plants, the ancestral gene being present more than 1.5 billion years ago, before the divergence of plants and animals. The hemoglobin gene is truly ancient; it is found in Paramecium and Chlamydomonas and, in fact, precedes the divergence of prokaryotes and eukaryotes.

Mollusks and primitive arthropods, unable to perfect their oxygen-transport system, were forever condemned to a small size and/or a poor activity. Without adequate oxygen transport, large animals do not get enough oxygen to successfully continue. A final refinement of the newly developed system was the perfect separation of the venous from the arterial blood.

5.2.3 Water control

The gastropods, arthropods and chordates are the only animal phyla that have adequately resolved the problem of water control. An animal switching from an aquatic to a terrestrial life must be able to withstand large variations in the concentration of salts of its body. A terrestrial animal cannot continually drink salty water and urinate. Its body ought, in addition, to be watertight, at least if the animal wants to wander away from water supplies; otherwise it would soon dry up in sunny weather.

Such a prerequisite rules out the cephalopods, although they are mollusks. Calmars, squids and octopuses do not have watertight bodies and cannot control the concentration of salt within their bodies. These animal forms are bound to remain forever in an environment that furnishes a constant supply of water and salt, the ocean. In this environment, the cephalopods, which abandoned their shell in the course of evolution, did remarkably well. Despite the fact that their throat passes through their brain, their evolution was accompanied by a neat increase in consciousness and cephalopods are very intelligent animals having the capacity to learn and remember.

Another type of mollusk, the gastropod, i.e. snails, had representatives capable of retaining water in their bodies and able to withstand some variation in the body’s salt concentration. These mollusks were thus able to invade lakes and rivers and even leave the proximity of water. Their level of consciousness does not appear to be very high. Other protostomes adapted to land dwelling are the arthropods. Some tropical crabs have only very reduced gills. They suffocate in water. They live on solid ground and obtain their food from coconut trees. All of them from scorpions and spiders to butterflies, ants and bees, maintained a chitin’s external skeleton. The social insects have succeeded in the most amazing way to populate the earth. They are however unable to thrive in cold weather and cannot penetrate deeply into water. Also, they are bound to remain of a small size because they maintained an external chitinous skeleton. Out of the water, such a protective and sustaining system becomes too cumbersome if the animal grows beyond a certain size. A consequence of this is that there will never occur in this animal phylum a great concentration and accumulation of nervous material.

5.2.4 Muzzle

The only way the small chordates of the Devonian period could survive among the scorpions that appeared earlier in time was with the development and maintenance of a strong cephalic shield. Among the heritages received from the crossopterygians, we must mention this cephalic shield. Without it, no possibility would have existed for the development of the skull or the muzzle. The muzzle of amphibians is traced back to a skull pattern that is exclusively crossopterygian (Fig. 5.16).

Figure 5.16. The increasing obliquity of the arrow that links the maxillary (colored red) to the parietal bone, (colored black), demonstrates that the muzzle of a crossopterygian fish (Osteolepis) moves ahead when the tetrapodal stage is reached.

Comparative anatomy shows that the amphibians originate from crossopterygians. Ichthyostega is an extremely primitive amphibian (see fig. 5.17). Paleogyrinus is more evolved.

Yet, a muzzle is essential for carnivorous terrestrial life. Fishes can swallow their prey by aspiration but the amphibians need to catch them. In fishes, the nose is located at the fore-end of the mouth and is an olfactory organ only. In the tetrapods, the nose moves ahead with the muzzle and retains its olfactory function although it begins to be used for respiratory functions. During the evolvement of man, the muzzle regressed and disappeared. Yet, the nose had to remain in order to fulfill its respiratory function, namely the control of temperature and humidity of inflowing air. The flat nose of the Mongoloid and pigmented races are a final adaptation of the nose to cold, heat and dryness.

Conclusion: evolution is predetermined in that the heritage of a phylum cannot be ignored. The inability of the cephalopods to free themselves from a physiological dependency on constant salinity has doomed these evolutive forms despite remarkable achievements. The adoption of an external chitinous skeleton by the arthropods was advantageous for survival purposes in water and the scorpions of the Devonian period must have been terrible challengers to the protychtians. Among these, only the crossopterygians fulfilled the conditions for a successful access to land as well as to higher degrees of consciousness.

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