Professor George F. Hart

 A SHORT OUTLINE OF THE EVOLUTION OF THE VERTEBRATES 

These lectures examines the mechanisms and processes involved in the Evolution of the continuous and diversifying lineage of the Vertebrates. The purpose is to illustrate the magnitude of changes that can occur in a long and single evolutionary line, as adaptations accumulate through time. Much smaller segments could be used for the same purpose.  For example, the evolution of the Horses in North America, but essentially the same ideas prevails at all levels i.e. evolutionary theory is scalable. I choose the evolution of the Vertebrata because Homo is a vertebrate and examining this group will outline our own ancestral lineage.

The early history of cellular life that led to the Vertebrates began with the clumping together of eukaryotic cells into cooperating bundles, which proved a useful adaptation for survival. With specialization of outer and inner cells, as a symbiotic colony, the beginning of a multi-cellular organism can be seen.  Once such a colony was established, the development of other specialized cells could provide a strong adaptive advantage leading to a true multi-cellular organism. Once a viable multicellular base was established other novel mutations could evolve that had adaptive advantages towards living under a variety of selection pressures. The evolution of novel survival adaptations involved the development of one or more bio-chemical sequences at a time and their incorporation into the genome of the species / gamodeme. This was a slow process because most novel bio-chemical reactions that appeared were not incorporated into the metabolic pathways of the organism. Because the fundamental process is the extension of the metabolic pathways by modification not innovation evolution links all living system together. The process goes back to the primordial cells and is a legacy system. 

The organization of some cells into highly specialized bundles called tissue seems to be a natural consequence of multicellular organization, because it occurred so many times during evolution of the Eukaryotes[i]. The development of tissue resulted in major survival advantages. In a real sense this cellular survival mechanism, dating back to early multi-cellular invertebrates, led to the eventual evolution of humankind.

One further thing is clear. It was the increasing availability of oxygen produced by the earliest evolution of photosynthetic organisms living submerged in water [both fresh and marine] that was the primary selection pressure, driving the evolution of those organisms that used oxidative reduction in their metabolic processes, and leading to the evolution of the Kingdom Animalia. Moreover, the evolution of the land vertebrates is directly linked to the evolution of the land plants which established the environmental triggers for vertebrate adaptations. Once the ancient plants began to move onto the terrestrial land masses, during the late Silurian and early Devonian periods, both the oceans and the continents became the sites of prodigious production of oxygen and the partial pressure of oxygen, in both the atmosphere and hydrosphere, gradually increased to its present level.  With the adaptation of plants to the land the oxygen levels would climb even higher and in addition represent an attractive food source to any animal that could adapt to the terrestrial environment. Within the Kingdom Animalia, the vertebrate phylogeny evolved many significant adaptations leading to the successful invasion of all terrestrial land areas.  Other animal groups, such as the insects, were even more successful: but is the lineage of the vertebrates that eventually led to the most significant adaptation: the evolution of human consciousness.

 THE LINEAGES OF FISH

At the base of the vertebrate phylogeny are a group of fossilized mobile vertebrates, informally referred to as the fish. In actuality, the ‘fish’ include four major classes of Vertebrata. These are called the Agnatha, the Placoderma, the Chondrichthyes and the Osteichthyes, but both living and fossil anatomy suggests that their joint soft bodied ancestor evolved for millions of years prior to the development of forms that could be fossilized. Carroll [1988] in a seminal work on vertebrate evolution suggested an ancestral soft bodied form something like that depicted in figure 16. These were mainly bottom dwellers and probably fresh water organisms.

Early in the lineage of the fishes eyes, ears, and a single nostril on the top of the head evolved.  These all point to the development of a central nervous system, which reacted to visual, auditory, olfactory and electrical inputs from biological sensors. This proved an enormously adaptive advantage when the Law of Combinatorial Outcome is applied to analyzing the results obtained from the sense organs [ii].

A geological time chart showing the generalized evolution of the Vertebrata is given in figure 17. The earliest fossilized fish occur in deposits from the Cambrian Period and belong to the Class Agnatha or jawless fishes. Threat, attack, escape, defense and perhaps appeasement are vertebrate traits that caused predatory and defensive behavior to emerge as adaptive advantages.  The Agnatha underwent an explosive evolutionary burst during the Devonian Period, at the end of which most forms became extinct. During this time, the development of a structure that could bite, grasp, and manipulate, that had emerged at the end of the Silurian Period, evolved into a whole new predatory way of life. This structure eventually emerged as the jawed fishes belonging to the Classes Placoderma [armored forms that are now extinct], Chondrichthyes [cartilaginous forms that include the sharks and rays] and Osteichthyes [bony forms are the majority of fish found in modern oceans]. Other developments were the emergence of true pectoral and pelvic fins that increased swimming ability, and a more evolved cranium.  It must be emphasized that it took some 100,000,000 years for the adaptations found in these Classes to evolve from their Agnatha ancestors.  However, with these developments the jawed fishes became browsers on algae and predators on other water dwelling animals. The cartilaginous forms that developed during the explosive burst of the Agnatha were the ancient sharks.  They underwent further evolutionary radiation at the end of the Triassic Period when they gave rise to the modern sharks and rays. The bony fish possibly arose from an early shark-like form in the early Paleozoic Era and dominated the oceans by the middle of the Paleozoic Era.  Their adaptive advantages were a platy exoskeleton and a strong ossified endoskeleton. They also had a swim bladder which increased their stability.  It is from a specialized group of bony fish [called the Rhipidisian fishes] that the early amphibians diverged during the Middle Paleozoic Era. Carroll notes that the Rhipidisians were commonly occurring freshwater predators throughout the Upper Paleozoic Era. Amongst other features, these fish had fleshy lobed fins and well developed dorsal fins. The muscular nature of the fins is possibly an adaptation to pushing themselves along the bottom and proved critical in the eventual evolution of the four limbed [tetrapod] vertebrates as they progressed onto land.    

 Text figure 17

The series of adaptations that led to existence on the land were just as formidable for the animals as they had been for the plants. In particular, adaptation to land had to overcome those problems associated with oxygen supply, dehydration and skeletal support. Under-water organisms are not only supported by the medium in which they live, the water also keeps them moist and supplies them with dissolved oxygen.  All three of these problems were eventually solved by the appearance and selection of new adaptive traits.

The initial adaptation to oxygen existing in the atmosphere was developed by several fish groups during the Devonian Period. Proto-lungs evolved probably from an extension of the gut as an adaptation to living in waters with low oxygen. Fishes with these rudimentary lungs could gulp in air and as the fish dived, air would bubble into the blood vessels in the proto-lungs. The combination of use of oxygen, muscular lobed fins for movement, and scales which helped to protect against dehydration were major adaptive traits that allowed access to the terrestrial environment.

Once vertebrates had developed the adaptations that allowed them to exist for a short while in the raw atmosphere, their further evolution was strongly directed by available food sources. The development of the terrestrial vertebrates is intimately linked with the migration of plants onto the land, not only at this early stage in their evolution but also in later stages involving the reptiles and the mammals.  A landscape already occupied by plants is a landscape with a lenient selection pressure from the viewpoint of availability of food and competition.

 THE LINEAGES OF AMPHIBIANS

Arising out of the group known as the Rhipidisian fish, the true amphibians retain some characteristics of fish, particularly in their young forms. In fact, the early ancestors of the amphibians were not adapted to lead an active life on land. They probably used their primitive lungs to allow them to move from one pool, where oxygen was becoming depleted, to another where the oxygen level was higher. Eventually a mechanism evolved that was essentially a pump which forced air into lungs: the development of a rib cage and attached muscles.  By the late Paleozoic Era, once the amphibians became active land dwellers, they underwent an explosive evolutionary burst, with numerous adaptations into the coastal, lacustrine [lake], paludal [swamp and marsh], and riverine [river and stream] environments. The important characteristics the Amphibia had to develop in order to become true land dwellers are given in Table 3.

 Initially the amphibians had no enemies, for they were the only large animals on the land and they were probably all herbivorous.  As they gradually evolved during their migration onto the land, the appendicular skeleton [limbs] strengthened to bear the weight of the body. This resulted in an elevation of the body that became an adaptive advantage because the organism could move with less friction and thus move faster. Eventually competition led to the cost-benefit adaptation in which carnivorous forms evolved.  Associated with the elevation of the body a dermal shoulder shield separated from the back of the skull, possibly signifying an adaptation to predatory behavior.  These early amphibians are called the Labyrinthodonta and are derived from the stem amphibians, called the Ichthyostegidae, from which all the other forms originated. The leniency of the selection pressure initiated an explosive evolutionary burst at the beginning of the Mississippian Period with a plethora of new taxonomic groups within the Labyrinthodonta, arising during a short period of time. The earliest forms were reptile-like, more so than today's amphibians, and some species grew to at least 10 feet long.  However, of the numerous groups of organisms that evolved during their heyday, only the salamanders and newts (Order Caudata), the frogs and toads (Order Salientia) and some legless amphibians (Order Apoda) survive today.

 TABLE 3

One fact that is frequently overlooked about this part of the Vertebrate phylogeny is that it took some fifty million [50,000,000] years of trial-and-error adaptation before the amphibians truly mastered the terrestrial environment.  Even then, they could not stray far from water.  Not only was dehydration a problem but the ozone layer was weak and ultraviolet radiation from the Sun could be lethal for organisms exposed to it for any length of time.  Fifty million years is a long time, and numerous living systems developed that could not pass the barriers placed by the Law of Instability.  This long time for experimentation is the reason that such exquisite adaptive strategies could evolve leading to the Reptilia. 

From out of the numerous amphibians that evolved during the latter part of the Paleozoic Era, important adaptations occurred that led to the rise of the reptilian body plan. Needless to say, the abundance and associated diversity of the amphibians declined rapidly with the advent of carnivorous reptilian forms.

 THE LINEAGES OF REPTILES

The Reptilia are the first truly terrestrial vertebrates in that they do not have a stage in their life cycle that requires a return to the aquatic environment. The major adaptive characteristics of the Reptilia are given in Table 4.

TABLE 4

These features are associated with an improved efficiency for dwelling on land. It is possible that the increasing level of atmospheric oxygen caused the build-up of the ozone layer and this global environmental change helped the Reptilia to dominate the land. The selection pressure for the early reptiles was again minimal.  They could migrate further onto land away from permanent water sources for they had developed the amniotic egg and a scaly skin as a major dehydration preventative.

Moreover, in the hinterland there was plenty of food because the plants were already established in the interior parts of continents and the amphibians had not been able to migrate that far away from a permanent water source. The key to success was the development of a major adaptation: the amniotic egg [figure 18]. Reptiles, birds and mammals all share the amniotic egg and are grouped together as the Amniota. The amniotic egg is a reproductive adaptation that frees the egg from the aqueous environment.  In the amniotic egg, the young develop in an aqueous media within the egg and leave the egg in an adult form (although it is not yet itself sexually mature). In addition, the egg is fertilized internally and either expelled as soon as the shell is formed or kept within the body until the young hatches (internal live-birth). Some modern amphibians, as an adaptive mechanism against predation, have evolved a live-birth strategy in more recent times.

               Text figure 18

As would be expected, the earliest reptiles e.g. Seymouria of the Carboniferous Period were amphibian-like. These are classified as the Cotylosaurs and, because they gave rise to all of the other reptiles, they are called the stem reptiles. From these stem reptiles’ two main groups arose: the Synapsida [mammal-like reptiles] and the Diapsida [dinosaur-like reptiles]. Examining the evolution of the early forms of the Cotylosaurs it is seen that they were present during the late Pennsylvanian Period as the group called the Pelycosaurians [e.g. Dimetrodon]. They gave rise to the Therapsids which show mammal-like characteristics including enlargement of the dental bone at the expense of the other lower-jaw bones; and, the development of firmly rooted teeth divided into incisors, canines, and cheek teeth. These teeth allow food to be sliced and chewed to small particles (and thus a greater total surface area for digestive processes to act upon). In addition, the limbs in the Therapsids evolved so that they were more or less directly under the body, and the toe bones were reduced to the characteristic mammalian formulae of 2-3-3-3-3. All Therapsids became extinct during the Triassic Period, due to competition from the early Dinosaurs, but not before they had passed along their important adaptations as they evolved into the small mammals. Although the mammal-like reptiles dominated the landscape during the Permian and Early Triassic periods additional adaptations in the Stem Reptiles were continuing to take place that would prove highly competitive.  These adaptations proved to be a set of extremely successful modifications that led to a group of vertebrates called the Thecodonta. These were the original dinosaurs which successfully occupied most of Earth’s terrestrial, aerial and shallow aquatic environments.

The Thecodonta were small, lightly constructed ancestral dinosaurs. They had a tendency to be bi-pedal and thus were probably quite agile. Bi-pedalism necessitated a strengthening and modification of the hind legs and the re-arrangement of the bones in the hip region, and this provided the means whereby dinosaurs are classified into two major groups: the Ornithischians and the Saurischians.  Their adaptive characteristics are outlined in Table 5.

TABLE 5

The Ornithischians were the bird-like dinosaurs but oddly enough it was the Saurian group that eventually gave rise to the birds. The Saurischians are what most people think of when they hear the name dinosaur. They showed a wondrous divergence during the late Mesozoic Era as they adapted to the land, the sea, and the air. It is not known for certain what initiated their extinction, but the Meteoritic Impact Hypothesis is gaining continued support as the ultimate cause.  The most likely proximate cause was a global climatic effect that was suddenly imposed. This placed an intense selection pressure on all dinosaur individuals.

In general, the changing abundances of the major taxa of the vertebrates can all be related to changes in a major selective pressure, especially the opening-up of new living space, new food resources, and the advent of new predators into an established environment. The advent of an extra-terrestrial source that removed the dinosaur lineage might have been an isolated event, yet it is an example on a grand scale of the effects of selection pressure.

Flight evolved in the reptiles, the birds and the mammals. This once more indicates the amazingly adaptive nature of living systems, utilizing a myriad of cellular processes to take advantage of a low selection pressure. The main adaptive characteristics of the birds are given in Table 6.

 TABLE 7

Table 8 outlines the major developments of the Cenozoic Era that affected the evolutionary tempo.

TABLE 8

A geological time chart of the Cenozoic Era showing the evolution of the primates is given in Figure 19.
 

TEXT FIGURE 19

The earliest primates evolved during the period of mammalian expansion after the demise of the dinosaurs.  The ancestral forms are found in the Lower Paleocene Epoch where they are represented by a group called the Plesiadapiformes which eventually gave rise to the two major groups of modern primates: the Prosimii and the Anthropoidea. Adaptations to the arboreal habitat that characterize these two groups, led to changes in the skeletal structure, particularly the development of the grasping inner digit, and stereoscopic vision. Associated with the development of the stereoscopic vision was a forward movement of the eye sockets and the flattening of the face. The grasping inner digit and the stereoscopic vision were the two adaptations that allowed the brain to develop such excellent coordination of hand and foot with vision.  It eventually led to Homo, the weapon maker and hunter.

The Prosimii [pre-monkeys] are tree dwelling.  The evidence of fossil finds in Eurasia and North America indicate that a grasping hand and stereoscopic vision developed in this group as early as the Eocene Epoch. Modern prosimians are well adapted to mild, moist climates and during the early part of Cenozoic Era they are found in what were the tropical and subtropical climatic regions of Earth. Fossil remains indicate that the prosimians were widespread in the Paleocene, Eocene and Oligocene epochs, but began to decrease drastically when the Anthropoidea evolved in the Oligocene Epoch. Today the most primitive surviving form of the Prosimians is the lemur, a form that does not have stereoscopic vision. The lemur is distinctly quadrupedal, has a long bushy tail, and a small brain behind a slender pointed muzzle. Its eyes are fairly far apart, and the animal resembles an insectivore. Today the lemur is found only on the island of Malagasy [Madagascar], where they survived apparently because the island separated from the rest of Africa during the early Cenozoic Era and only a few mammalian predators ever developed on that island. Two other groups of Prosimians have survived. The tarsiers occur in Borneo, Sumatra and the Philippines and look like monkeys. They have a shorter muzzle than the lemur and eyes that are closer together with stereoscopic vision. The second group is the Loris which today lives in Africa, India and Southeast Asia.

The second group of primates is the Anthropoidea and this also diverged into two groups referred to as the infra-orders Platyrrhini and Catarrhini.  The Infra-order Platyrrhini contains the Ceboidea, commonly called the New World Monkeys e.g., Vakari (cat sized animal from the Amazon), Marmoset, and the Squirrel Monkey. The Infra-order Catarrhini contains the Old World Monkeys and the Great Apes. 

The New World Monkeys were evolving at the same time as the Old World Monkeys during the Oligocene Epoch, both probably diverging from a tarsier-like ancestor. They have a prehensile tail and an extra premolar. Modifications are seen by their forward facing eyes, more complex molar teeth, larger brain case, improved hands, and a bony bar protecting the eye orbit. These two groups evolved in a parallel manner into similar environmental niches. The New World Monkeys evolved in the New World, although it is only found today in South America; the Old World Monkeys evolved in Africa and Asia.

Within the Catarrhini the Old World Monkeys are grouped under the Super Family Cercopithecoidea and include forms such as the snow monkey, Indian langur, mandrill, and Barbary ape.  The Great Apes are grouped under the Super Family Hominoidea which, along with Homo, includes the orangutan, Gorilla and Pan [chimpanzee and bonono], and the fossil genus Australopithecus. Most recent work that adds the evidence from mitochondrial DNA has shown that humans, chimpanzee and bonono form a close genetic group separated from Gorilla.  Furthermore the evidence indicates that they all came from a common African ancestor and somewhat isolated from the Asian great ape, the Orangutan.  This evidence has led to a new classification of the super-family Hominoidea in which the family Hominidae is divided into two sub-families: the Ponginae [Orangutan] and the Homininae [including the Hominini and African apes] [Hilton-Barber and Berger, 2002]. Australopithecus and Homo lie within the Hominini.

The Great Apes diverged at the same time as the two monkey groups [old world and new world] were evolving i. e., during the Oligocene Epoch. Already by the Miocene Epoch, fossil finds indicate differentiation between the Great Apes and monkeys, with the New World Monkey living in isolation but the Old World Monkey and Great Apes existing in similar locations.  The evolutionary closeness of the members of the Homininae is apparent when it is recognized that the chimpanzee and modern humankind share an estimated 94-96% of their genome [iv]. The characteristics that separate the African apes from the Hominini are given in Table 9.

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