What is life? It is that property or quality of plants and animals that distinguishes them from dead organisms and inanimate matter. Living entities are made up of organic* material and form at least one or more cells. All life forms are composed of molecules that are not themselves alive. A cell is the smallest structural unit of an organism that is capable of independent functioning, for example, bacteria are single celled. *organic. Meaning of chemical compounds based on carbon chains or rings and also containing hydrogen with or without oxygen, nitrogen, or other elements. Carbon, almost alone among the elements, is capable of uniting with itself many elements to form compounds. It has been estimated that there are at least one million organic compounds.
An organisms is considered to be alive if it exhibits the following phenomena: 1. It can grow by taking in energy from their surrounding. This can be the chemical energy from digested food or radiant energy from the sun that powers photosynthesis which produces carbohydrates from carbon dioxide and water. Note, nearly all types of photosynthesis free oxygen as a byproduct. 2. It can reproduce entities that are similar to themselves and pass on some of their own characteristics. 3. It can respond to its environment and often heal itself when harmed. 4. It can evolve from generation to generation on account of random mutations of their hereditary characteristics (genes). This allows its offspring to adapt to a changing environment or take better advantage of an existing one, for instance, surviving or eliminating the competition. 5. It can self-regulate to keep its complex system in a steady state (homeostasis). What life forms are there? All life forms are either plants or animals or plant-animal hybrids. And the plan or instructions for each life form are found in its genes. 1. Plants (autotrophs) make their own food or nutrients but lack the power to move from their location to another. They are organisms capable of processing simple inorganic substances, such as carbon dioxide and inorganic nitrogen, into organic nutrients. In green plants, the process is photosynthesis which is powered by sun or artificial light. In some bacteria, the process is chemosynthesis which is powered by energy derived from the oxidation of certain chemicals. 2. Animals (heterotrophs) are unable to process inorganic matter into nutrients. They obtain their food from the ingestion and digestion of organic matter derived from living organisms. 3. Plant-Animal Hybrids (euglena) are one-celled organisms that live mainly in waters and can switch their nutritional way of life. Botanists classify them as plants (algae) because they can carry on photosynthesis. Zoologists classify them as animals (protozoans) mainly because they have a gullet through which solid food particles may be taken into the body. How did life originate? 1. It appears that life was an unplanned and accidental byproduct of blind natural processes. When the Earth's crust had sufficiently cooled down, there was first a chemical evolution. Vigorous chemical action (interaction of atoms) easily combined inanimate basic matter (elements) into self-organizing and copying molecules. This process included the making of the molecular building blocks essential for life. Also, this development has been replicated in experiments. 2. However, it still took zillions of trial and error experiments in the oceans and over 100 million years or so before the first primitive cells appeared. They resembled prokaryotes cells, that is, cells without a true nucleus. Once life had originated, the mechanics of evolution, genetic transfer of building instructions with variations due to mutation of these genes and natural selection, took over. But it still took 3,500 million years for modern humans to emerge. How did the first self-replicating organism come about? We do not know and do not even have a theory for a highly plausible mechanism for the spontaneous rise of self-replicating (genetic) instructions in nature sufficient to give rise to life and consistent with the concepts of the empirical sciences. All we know is that there was such a process because it did happen. Is there evidence that life can spring from nonliving molecules? The strongest evidence we have so far are viruses which are halfway between "living" and "non-living" matter. They are an intermediate form between cells that are living and molecules that are not. They are simple ultramicroscopic parasites that consist of nucleic acid, either RNA or DNA, incased by a protein coat. In an alien, non-cell environment, the virus (RNA or DNA compound) is simply a chemical. But in a hospital, cell environment, it absorbs molecules and arranges them into its compound and replicates itself. Thus, it is only because they cannot reproduce without a host cell that they are not considered living organisms. What does evolutionary biology explain? It accounts for: 1. The un-stability and constantly changing of species. 2. The branching off of all species from a single origin, and why evolution must be gradual with no major breaks or discontinuities. 3. The fact that all groups of organisms, including humanity, consists of uniquely different individuals. For instance, no two of the six billion currently living humans are the same. The key observation is that groups of organisms or species gradually developed with the passage of time from their primordial or primitive stage to their present specialized state, and that this development of all plants and animals is for the most part driven by two processes: 1. Chance genetic mutations are random modifications of hereditary transmissions, causes descendants to differ somewhat from their ancestors, that is, morphologically, in form and structure, and physiologically, functions and vital processes. These changes in the genetic material at the DNA level are mostly neutral or harmful in their effects. Improving changes are very rare. Harmful changes are eliminated by natural selection, but many are dormant (latent). They can continue to exist because the normal dominant gene prevents them from being activated. 2. Natural selection necessarily favors those variations that can best take advantage of an existing or changing environment at the expense and often extinction of organism that are inferior or cannot adjust. As the biologist Francois Jacob (1920) phrased it: It is natural selection that gives direction to changes, orients chance, and slowly, progressively produces more complex structures, new organs, and new species. Novelties come from previously unseen association of old material. To create is to recombine. In sum: Evolution responds to an organism's needs not by optimal design but by tinkering--, trial and error--that is, by slowly modifying existing structures through mutations and sorting them out by natural selection--if it works, it survives. Tinkering, more broadly, is the process of making repeated trials, experiments, tests, etc. to find a desired result or solution. Culture and choice is a third driving force in the development of humans. However, both depend on the society one lives in. Note: This is different from the so-called laws (actually observed regularities) of nature that determine much of the physical world. Instead, evolution of all plants and animals is chance and necessity, and also culture and choice at least in the case of humans. Are random genetic mutations a good thing? Modern humans' existence was made possible by mutations that have powered their evolution from single celled organisms over the last 3,500 million years. However, the species is virtually unchanged for the last 100,000 years, yet gene mutations still occur, and they are for the most part perniciously harmful. Here are some of the better known hereditary diseases: Breast cancer, cystic fibrosis, Down's syndrome, hemophilia, Huntington's disease, muscular dystrophy, schizophrenia, sickle cell anemia, and spina bifida. What would be a clear example of natural selection at work? The classic example is the peppered (white with black speckles) moth (Biston betularia), which is one of about 20,000 moth species found in the United Kingdom. This population of moths was mostly light colored (grayish). But there was also a small number of dark-colored individuals on account of a single gene. The dark-colored moths against light-colored tree trunks were eaten by birds who easily located them visually. However, when the countryside became blackened from smoke, the light-colored moths lost their camouflaged background, and they now became easy pray for birds and diminished. The dark-colored moths, on the other hand, flourished because they were more difficult to locate against the darkened tree trunks. Then, once more, a reversal occurred when a reduction of smoke emissions resulted in lighter-colored tree trunks. This change favored the light-colored moths again, and they increased in numbers while the dark moth population was reduced. What is the evolutionary science of Neo-Darwinism? It is the updated theory of Darwin that includes new discoveries. Modern Evolutionary Synthesis is a more recent name designating Neo-Darwinism. It is an organized body of knowledge that started with a theory by Charles Darwin (1809-1882) in his work On the Origin of Species by Means of Natural Selection (1859). He demonstrated that the major mechanism for evolution came from descent with modification and a subsequent natural selection or "survival of the fittest" as Herbert Spencer (1820-1903) phrased it. Since then, Darwin's findings have been verified, explained in detail, and augmented by various scientific disciplines. In particular, Darwin's theory has been augmented by the key findings of modern genetics and the concept of emergence. Genetics started with the rediscovery of the work of Gregor Mendel (1822-1884). Mendel had discovered that characteristics are transmitted across generations in discrete units, now known as genes. Neo-Darwinism claims: 1. Every organism develops according to a program encoded in its genes, that is, its DNA. 2. DNA (deoxyribonucleic acid) is the hereditary material that transmits traits from organisms to their descendants. It has made possible the wide diversity of life forms that developed throughout the course of evolution. 3. New or modified traits arise because of DNA mutations, that is, inheritable changes in genes occur randomly and spontaneously with no apparent external cause. 4. Natural selection produces microevolution within a species by favoring survival of the fittest and thereby increasing the frequency of their genes in the population. 5. Also, natural selection produces macroevolution, that is, transitory and new species. It is thought of larger scale changes over longer periods of time due to the compounded effects of microevolution. The concept of emergence covers the way complex systems and patterns arise out of a multiplicity of relatively simple interactions. Life is a major source of complexity, and evolution is the major principle or driving force behind life. It seems certain that evolution is the main reason for the growth of complexity in the natural world. If we speak of the emergence of complex living beings and life-forms, we refer therefore to processes of sudden changes in evolution. The most astonishing emergence is probably that of the human mind. It is more than than the sum of its parts, e.g., 50 billion nerve cells of the brain and each with up to 10,000 synaptic connections. The emergent property of mind cannot be demonstrated by its parts and cannot be predicted, let's say in the form of a deduction, even if the parts are completely understood. What is the scientific evidence for evolution? There is a chain of life forms that started with single celled organisms 3,500 million years ago and gradually evolved into the first modern humans about 160,000 years ago. Noteworthy is that no reversals have been found. Evidence supporting evolution is overwhelming and has been provided by the following: 1. "Written in stone," the fossil record* contains hundreds of thousands of fossil organism. Found in well-dated rock sequences, these organisms prove successions of forms through time and demonstrate many evolutionary transitions. There are no important missing links, that is, intermediary forms. They have been found: 1. Between fish and amphibians (see 1st illustration below) 2. Between amphibians and reptiles 3. Between reptiles and birds, for instance, Archaeopteryx (see 2nd illustration below) 4. Between mammal-like reptiles and mammals 5. Between apes and modern humans, for instance, Australopithecine *Established by Paleontology, the science that deals with life forms from the past, esp. prehistoric life forms, through the study of plant and animal fossils. As such it lies on the boundaries of biology an geology.
The Development of Limbs from Fins (Photo source: Wikipedia): In Late Devonian vertebrate speciation, descendants of pelagic lobe-finned fish – like Eusthenopteron – exhibited a sequence of adaptations:
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Panderichthys, suited to muddy shallows;
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Tiktaalik with limb-like fins that could take it onto land;
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Early tetrapods in weed-filled swamps, such as:
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Acanthostega which had feet with eight digits,
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Ichthyostega with limbs
For instance, and with reference to the picture below:
Throughout the tetrapods [vertebrates having four legs or limbs, including the mammals, birds, and reptiles], the fundamental structures of pentadactyl limbs [having five fingers or toes on each hand or foot] are the same, indicating that they originated from a common ancestor. But in the course of evolution, these fundamental structures have been modified. They have become superficially different and unrelated structures to serve different functions in adaptation to different environments and modes of life. This phenomenon is clearly shown in the forelimbs of mammals. For example:
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In the monkey, the forelimbs are much elongated to form a grasping hand for climbing and swinging among trees.
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In the pig, the first digit is lost, and the second and fifth digits are reduced. The remaining two digits are longer and stouter than the rest and bear a hoof for supporting the body.
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In the horse, the forelimbs are adapted for support and running by great elongation of the third digit bearing a hoof.
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The mole has a pair of short, spade-like forelimbs for burrowing.
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The anteater uses its enlarged third digit for tearing down ant hills and termite nests.
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In the whale, the forelimbs become flippers for steering and maintaining equilibrium during swimming.
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In the bat, the forelimbs have turned into wings for flying by great elongation of four digits, while the hook-like first digit remains free for hanging from trees (From Wikipedia article Evidence of Common Descent).
(Image source: Wikimedia Commons)
6. Humans and other animals have degenerate, atrophied, or rudimentary organs or parts that were more fully developed or functional in an earlier stage and are still fully developed and functional in some animals. These vestigial structures can be found in a variety of animals such as snakes (pelvic girdle and leg bones of walking ancestors), fish (cave dwellers with eye sockets but no eyes), whales (a set of bones that obviously corresponding in structure with the pelvis of all four-limbed vertebrate), and in humans. For instance, the appendix in humans has become redundant and is no longer an essential organ in our daily function. However, it is still an important organ to plant eating animals as it is responsible for the breaking down of cellulose which composes the cell wall. Likewise, though infrequently, a child is born with a "soft tail," which contains no vertebrae, but only blood vessels, muscles, and nerves. Modern procedures allow doctors to eliminate the tail at delivery. The longest human tail on record belonged to a twelve-year-old boy living in what was then French Indochina, which measured nine inches or 229 mm (source: Wikipedia). 7. The family of great apes (Hominoids) which includes humans, bonobos, chimpanzees, gibbons, gorillas, and orangutans show great similarities in their bone structures. Common ancestry, together with adaptation to different conditions through descent with modification, explains the stunningly similar bone and vestigial structures in animals. As one observer notes: With Darwin (1809-1882), humanity discovered that no biological disjuncture exists between itself and the ape. Even more, no real metaphysical discontinuity exists between humanity and the ape -- that is, there is no radical difference of essence, no true qualitative leap. Darwin himself showed, in his The Descent of Man (1871) and in The Expression of the Emotions in Man and Animals (1872), that there is no difference of kind between man and other animals, but only of degree. Rather than an unbridgeable gulf, Darwin demonstrated that there is a gradual change not only between man and other animals, but between all organic forms which is a consequence of the gradual change continuously and cumulatively operating over time.
The hominoids are descendants of a common ancestor.
(Photo source: Wikipedia)
8. Comparative Embryology*
has discovered that closely related organisms go through similar stages
during their embryonic development.
As an organism passes through embryonic
development it actually re-traces every stage of its evolutionary
ancestry. This idea became known as "Ontogeny recapitulates Phylogeny,"
which literally means "Development is a replay of Ancestry."
Studied from the time of conception, all vertebrate embryos go through a stage in which
they have gill pouches on the sides of their throats, a post-anal tail, and
segmented muscles. During the early stages of development these embryos are stunningly
alike.
However, as development continues, the various groups of vertebrates diverge
considerably taking on the distinct characteristics of their classes. Among
fishes, the gill pouches become gills. In humans, they develop into the
slender tubes between the middle ear and the pharynx, which serves to
equalize air pressure on both sides of the eardrum.
Again, common ancestry, together with adaptation to different conditions through descent with modification, explains the early but later diverging similarity between embryos ranging from fish via birds and other mammals to humans.
*Comparative
Embryology is the
branch of biology that investigates and compares the formation and
development of embryos of different animals.
