Common descent

Common descent is a principle of Darwinism which holds that life on Earth evolved from a common ancestor. Darwin's theory of descent with modification led inexorably to this conclusion as is illustrated in the following quote from Prentice Hall Biology (2008). The theory draws support from homology (physical similarities), embryology (developmental similarities), and the geographic distribution of organisms.

Descent with modification also implies that all living organisms are related to one another. Look back in time, and you will find common ancestors shared by tigers, panthers, and cheetahs. Look farther back, and you will find ancestors that these felines share with horses, dogs, and bats. Farther back still are the common ancestors of mammals, birds, alligators, and fishes. If we look far enough back, the logic concludes, we could find the common ancestors of all living things. This is the principle known as common descent.

By contrast, creationists believe that God created many kinds of organisms, and that innumerable species developed from those original kinds through microevolution via inherent genetic variability and natural selection. That tigers, panthers, and cheetahs share a common ancestor is a view accepted by most young earth creationists. However, the belief that biological evolution supports that these felines share ancestors with horses, dogs, and bats is a point where evolutionists and most creationists part ways.

Homology

 * Main Article: Homology

Belief in common descent is largely derived from the existence of homologous structures shared by many forms of life. Homologies may include structural (morphological) components, such as the forelimbs, which illustrate a similarity of construction overall, but are yet unique in other ways, providing varied functions. Cellular and molecular similarities are also considered to be derived from a common ancestor and therefore homologous, as in the existence of common organelles in eukaryotes, and nucleic acid (DNA/RNA) as the universal genetic code. Such homologies (similarities) are used as one of the main proofs for macroevolution and the common descent of all life on Earth.

By Darwin's time, researchers had noticed striking anatomical similarities among the body parts of animals with backbones. For example, the limbs of reptiles, birds, and mammals—arms, wings, legs, and flippers—vary greatly in form and function. Yet, they are all constructed from the same basic bones,... Homologous structures provide strong evidence that all four-limbed vertebrates have descended, with modifications, from common ancestors.

Based on these observed similarities, evolutionists infer that all life is related through one original life form that generated naturally through abiogenesis. Creationists on the other hand infer that life and the natural laws that sustain it were designed by a common Creator, namely God, Who supernaturally created many original kinds of animals. It is important to note that creationists do not reject fully the idea of common descent, only its ultimate evolutionary conclusion.

Embryology

 * Main Article: Embryology

Since the time Darwin published The Origin of Species, embryology has been used to support common descent. In fact, Darwin referred to embryological homology as the strongest single class of facts that existed to support his theories. A theory later put forth by Ernst Haeckel, known as the biogenetic law, asserted that the evolutionary history of an organism was recapped during embryo development. Although the biogenetic law is now discredited, in recent years, embryology has reemerged as a tool used by evolutionary biologists that attempt to establish phylogenetic relationships by identifying developmental similarities between taxonomic groups. Evolutionary developmental biology is a merging of developmental biology and evolutionary biology that is commonly known as "evo-devo".

Challenges
Challenges to the theory of common descent are numerous. It is especially important to realize that modern evolutionists are now refining their original theory of common descent to one that now includes more original life forms at the beginning. They have argued against a single-celled evolution and thus posit the original existence of no less than three "loosely constructed cellular organizations."

Tree of life
Charles Darwin's "tree of life" is not born out in scientific observation. This is concluded through more modern research, on example of such coming from the Proceedings for the National Academy of Science (PNAS). The research found that "there is no independent evidence that the natural order is an inclusive hierarchy" and that "the only data sets from which we might construct a universal hierarchy including prokaryotes, the sequences of genes, often disagree and can seldom be proven to agree."

There are what are called evolutionary "bushes," but according to Public Library of Science (PLoS) research, these bushes do not support the conclusion of a single cause of a complete tree of life as proposed by Darwin.

Abiogenesis

 * Main Article: Abiogenesis

The asserted improbability and impossibility of this completely naturalistic mechanism is a severe problem for modern evolutionists. Abiogenesis is the spontaneous appearance of the first, self-replicating protocell which can be argued has neither been observed nor comprehensively explained.

Morphological gaps in the fossil record

 * Main Article: transitional form

Differences in morphology, or "gaps in the fossil record," exist, across which it is argued there is no fossil evidence of transitional forms. For example these gaps are glaring when the context between reptiles and mammals, reptiles and birds, or apes and men is understood.

Falsifiability
Because of the absence of substantial hard evidence to support belief in common ancestry, creationists and even some evolutionists acknowledge that evolution is a philosophical belief, not scientific in essence. Evolution is referred to as a fact without providing sufficient explanation for the means. The assertions made by evolutionists are often not falsifiable and cannot be observed in nature or tested by experimentation. It therefore, falls outside the boundaries of the scientific method.

History
Common ancestry is an ancient idea, articulated by many pagan philosophers throughout history. In Europe, it fell from prominence when Christianity was dominant, but regained popularity in the 19th century, with secularization. It is an ancient idea, held by ancient pagan philosophers such as Anaximander as early as the 7th century BC, and returning to popularity in 19th century Europe.

Anaximander says that fish and men were not produced in the same substances, but that men were first produced in fishes, and, when they were grown up and able to help themselves, were thrown out, and so lived upon the land. Therefore, as the fire devours its parents, that is, the matter out of which it was first kindled, so Anaximander, asserting that fish were our common parents, condemneth our feeding on them.
 * Plutarch, Symposiacs, Book viii, question viii: Anaximander founded the first school of Greek philosophy and lived circa 610-546 BC.

Could one not say that, in the fortuitous combinations of the productions of nature, as there must be some characterized by a certain relation of fitness which are able to subsist, it is not to be wondered at that this fitness is present in all the species that are currently in existence? Chance, one would say, produced an innumerable multitude of individuals; a small number found themselves constructed in such a manner that the parts of the animal were able to satisfy its needs; in another infinitely greater number, there was neither fitness nor order: all of these latter have perished. Animals lacking a mouth could not live; others lacking reproductive organs could not perpetuate themselves... The species we see today are but the smallest part of what blind destiny has produced...
 * Pierre-Louis Moreau de Maupertuis (1698-1759), Vénus Physique.


 * The Temple of Nature in 1802.
 * Organic life beneath the shoreless waves
 * Was born and nurs'd in ocean's pearly caves;
 * First forms minute, unseen by spheric glass,
 * Move on the mud, or pierce the watery mass;
 * These, as successive generations bloom,
 * New powers acquire and larger limbs assume;
 * Whence countless groups of vegetation spring,
 * And breathing realms of fin and feet and wing.
 * Erasmus Darwin.