Student Science Notes

Evolutionary Development and Classification of Life

(Currently in compilation (inc. graphics) )

The Fossil Record
Evolution from Sea to Land
The Evolution of Plants
The Evolution of Animals

The Fossil Record

A fossil can be the petrified remains of an organisms body, or a trace produced by an organism, such as a footprint. It favours hard body parts so there are very few fossils of worms or other soft bodied creatures unless they lie in favourable rock areas. Fossils are found in sedimentary rocks, espically those found in shallow seas, lakes and rivers. They are the major source of information about the course of evolution. Most, but not all fossils are similar enough to living species to be assigned to modern groups of organisms and for plants, they show the gradual evolution and dominance of angiospem plants. The fossil record of animals shows the major phyla appearing at about the same time. Some species have remained unchanged but many phyla have long since been extinct. Fossil records are incomplete, but the evidence points to all early animals being marine. The records of fossil indicates that early fish evolution took place in freshwater areas.

There are very few fossils that indicate links (of evolutionary diversity) between the major groups of plants and between intermediates between plant or animal groups.

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Evolution from Sea to Land

Fossil evidence for life appeared first in marine sediments. It shows a progreesion of life into freashwater areas and then to the land. Plants evolved a supporting structure (often wood) and root and stem systems. Leaves evolved to make more efficient use of sunlight and regulate water use. A seed system developed, eliminating the aquatic phase. Animals found places to shelter and hide in terrstrial environments and for both plants and animals the sometimes oxygen deficient (due to bacterial action) lakes and ponds forced the transition from sea to land. The earliest arthropods were marine and their cuticle and localized respiratory sufaces would have suited them to life on land. The crustaceans would have adapted the worst, depending on gills for respiration. Insects evolved tracheae to allow breathing in air and some can close the spiracle at the top of the trachae to control water loss. Their excretory systems conserved water and selectively excreted or retained nutrients. The adaption of spiders to leaving on land was so complete that there are no marine species left. Terrestrial verebrates are believed to have evolved from a relativly small group of fleshy finned bony fish, the Sarcopterygii (lungfish). These fish have lungs as well as gills and enable the fish to survive periods on lands. The development of the amniote egg allowed early vertebrates to complete their life cycle on land.

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The Evolution of Plants

The gradual process of evolution has resulted in the present day global distribution of over 250,000 species of plants. One of the most notable characteristics of this range of plants is their diversity in appearance, (with plant species having flowers of many different shades, colours and forms), in structure (such as the giant redwoods of California to the delicate daisy) and in habitat (from the Amazonian rain forest to alpine flowers in mountainous regions). Despite this variety, the large majority of these plants are able to complete their life cycle without dependence on standing water for gamete distribution and fertilization. It seems probable for the reasons listed below, that the seed based reproduction system provided the impetus for much of the diversity of contemporary plant life.

Although the particular and, as yet unknown, conditions for the first appearance of organic life may have appeared anywhere on the Earth's surface, it seems likely that the development from the initial stages occurred in the marine environment. Water is more effective than air at minimising temperature changes, at shielding the organism from a (at that time) reducing atmosphere and excessive solar radiation, at providing a supply of concentrated nutrients and removing waste products.

Fossil records, although incomplete, indicate that the earliest plants to develop were marine algae, followed by primitive ferns. Observations of their present day descendants have shown that these and similar plants have an aqueous phase in their life cycle. Although the sporophytes of these classes of plants may survive and develop in dry conditions, their gametophytes release flagellated sperm that require a film of external water for transport and to ensure fertilization. Such plants are therefore confined to geographical areas based around standing water. It seems reasonable therefore, to assume that similar early plants likewise required an aqueous phase in their life cycle. The fossil record also indicates that some of these early ferns were taller than present day large bushes. However, apart from these height variations (and the improvements in vascular tissue required to transport nutrients), relatively little evolutionary development appears to have occurred in this plant type.

Evolution occurs through the accumulation of random molecular and cellular changes. The effects of these changes may be beneficial to the plant (such as larger root systems that allow the plant more protection against uprooting by strong winds) or detrimental ( the development of a scent that attract herbivores). These random changes are mostly triggered by nutritional and environmental factors and plants exposed to such varying conditions would be expected to show a greater diversity than those plants confined to a relatively unchanging environment. It seems reasonable to suppose that the existence of different habitats, with varying soil conditions (from nutrient rich grasslands to sandy deserts), rainfall patterns (ranging from season to season as well as place to place), differing amounts of daylight and possibly insect and animal predidation patterns was, and is, the main factor underlying evolutionary diversity.

Those primitive fern like plants which, through evolution, acquired a seed based reproductive system increased their chances of having their gametes and zygotes dispersed further and exposed to a greater range of habitats than their spore based predecessors. The present day maple tree, for example has its winged fruit carried by the wind up 10 km from it's parent, while plants developing hooks and spines similar to those of clover and bur would have had seeds attach to the fur of passing animals and dispersed a relatively long distance. Insects and animals would be attracted to those plants which developed brightly coloured flowers (such as orchids and begonias, among many), scents, (such as musk and lavender), nectar (honeysuckle being one example) or fruit (for example, peaches and apples).

The development of an endosperm and a tough outer coat increased the survivability of the young plants in their new habitats. Such seeds would have been more likely to germinate under conditions favourable to their development and growth. In addition, on a global scale, the increase in geographical area would have allowed some plants to escape the Ice Ages and adverse regional weather conditions.

The diversity resulting from millions of years of evolution may be illustrated in terms of the structural developments and appearance of modern plants. In addition to the seed and flower forms mentioned above, other elements of plant structure are listed below.

1. The development of a steadily more efficient root system which gave some early plants access to a richer variety and supply of soil nutrients and helped anchor and support the plant. In addition, larger roots increase the area available for water and nutrient intake. Some plants, such as carrots and turnips are able to store food products in the roots.

2. The growth of a viable stem system incorporating a vascular system, would provide early plants with strong supports and nutritional transport for the leaf system. An efficient stem would allow each leaf to be exposed to the maximum amount of sunlight and plants possessing a tall stem would be less likely to be shaded out from sunlight by their neighbours. Some plants, for example the potato, have stems which can store food and enable the plant to survive adverse growth conditions.

3. Broad, thin leaves (such as those of the cucumber) allow some plants to maximise surface area for the absorption of carbon dioxide and the collection of solar energy. Leaves with waxy coatings and containing stomata minimise desiccation and these factors give some plants a competitive advantage.

Although early fern like plants may have developed and later lost some of these features, it seems likely, given the random nature of evolution, that habitat related factors such as those listed above would have played a large role in determining the appearance and structure of modern plants. In most of the cases described above, the appearance of a particular feature was in response to a characteristic of the habitat.

To explain the present global distribution of plants, it seems necessary to assume that some primitive plants, through the random nature of evolution, lost their dependence on an aqueous phase, and were therefore able to grow in a wider range of terrestrial habitats. This, in turn exposed them to conditions which increased the chances of further development. The appearance of a seed based reproductive system allowed the population of a greater variety of habitats, exposing plants to a greater variety of evolutionary forces and would seem therefore to have been the most important contribution to the evolution of terrrestrial plants.

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The Evolution of Animals

Biological evolution can be defined as change in the diversity and adaptation of populations of organisms. Many changes in structure, and the strategies based upon them, were required for the evolution of marine life to a terrestrial environment. Although the avoidance of desiccation played an important role, other structural changes were also necessary for the successful adaptation to life on land.

The land members of the kingdom Animalia can be divided between arthropods (mostly insects), a group of worm-like organisms (including annelida, nematoda and platyhelminthes) and chordates. Of these, the vertebrate group has the most complete fossil record of evolution from an aquatic medium.

Adaptations for water retention, in early land animals included the eyelids of the amphibians, along with the amniote egg and waterproof skin of the reptiles. Other important structures minimising water loss were efficient kidneys and the incomplete ventilation system, in which air entering the lungs is mixed with air already present to reduce loss of moisture during respiration. Desiccation in some of the earliest terrestrial arthropods may have been minimised by a flap covering the trachea and by localised respiratory surfaces.

As important as these structures were, the factors listed below also played a vital role in the spread of animals into a terrestrial environment.

Aquatic organisms have their weight supported by the water in which they live, this being one of the factors contributing to the position of the blue whales as the largest creature on the planet. Refinements and modifications to the relatively simple and soft boned fish skeleton were required to allow terrestrial animals to support their body weight. Vertebrate endoskeletons allowed a wide variety of animals to appear, ranging in size and ecological niche, in which strong bones, flexible connective tissue and efficient muscles were combined.

In the case of chordates, as well as supporting the animals weight, a strong bone structure within the limbs allowed flexible movements, such as those involved in running, jumping, climbing and manipulation of prey for feeding. A flexible neck, strong teeth and jaws gave the early land animals a greater likelihood of survival. Those early terrestrial arthropods which developed wings were able to take advantage of them to establish themselves in habitats containing flowering plants.

The development of tracheal systems and lungs allowed early animals greater energy to colonise the land. In mammals, birds and some reptiles a four chambered heart, a large improvement over the rudimentary circulation system in fish, ensured adequate blood pressure for both the pulmonary and systemic systems. The development and combination of efficient respiratory and circulatory systems allowed some early animals fast movement towards prey and away from danger.

Although not as restricted as plants by their habitats, animals are still greatly influenced by their surroundings and a greater variety of habitat related factors, such as food sources, soil conditions, weather patterns and the presence of other animals increased the rate of evolution of land animals relative to their marine ancestors. The above structural changes allowed animals access to differing habitats and exposed them to new evolutionary pressures. The appearance of endothermic mammals with a greater need for energy compared to that of reptiles provided a new force for evolution and increased the range of settled habitats, in turn further enhancing the rate at which new classes of animals emerged.

Behaviour patterns also altered as a consequence of structural changes. Fishes, amphibians and reptiles usually have little contact with their offspring after the gametes are dispersed or the eggs are laid. By contrast, the mammalian reproductive system of fertilisation and gestation within the body of the female (excluding monotremes), along with any additional parental attention for the juvenile animals, allowed many mammals to dominate their habitats. Patterns of behaviour such as hibernation allowed some mammals to survive adverse weather conditions while other groups of animals, primates for example, developed elaborate social systems, based around their limbs and larynx, which allowed, for instance, a measure of co-ordination in defence against predators. Some animals were able to take advantage of increased brain sizes, coupled with improvement in their senses, especially vision, to become efficient predators. Feeding habits and patterns based around more efficient teeth allowed a greater range of food intake relative to marine life.

It may be argued that the appearance of the amniote egg was the vital step in the colonisation of land by animals, allowing as it did reproduction on land. However, although this example of avoidance of desiccation was important, it would appear from the above that features such as a strong skeletal system to support body weight, a highly developed system of limbs to enable movement and manipulation, an improved cardiovascular system and the development of strong teeth and heat retaining body hair were all important contributors to the survival and development of early land animals. The use of these and other structures and the strategies based upon them greatly increased the rate of evolutionary development. The importance of these structures to present day animal evolution can be judged by comparing the relatively unchanged range of marine animal life with the much larger diversity of their terrestrial descendants.

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