Common descent

Common descent is a scientific theory in Biology it can be applied on two levels, with a clad or group of species being said to share a recent common ancestor, or the broader theory which states that all organisms on earth originated from a single common ancestor. The theory is usually used in support of the Theory of Evolution.

Darwin wrote:

"When I view all beings not as special creations, but as the lineal descendants of some few beings which lived long before the first bed of the Silurian system was deposited, they seem to me to become ennobled."

Evidence for common descent

There are several lines of converging evidence for common descent. These include but are not limited to:

• Anatomical homologies- The phenotype of organisms often share similar structure. This structure often doesn't serve a specific design function and is most easily explained by constraints of common descent.

• Endogenous retroviral insertions - These are inactivated viral genes that were inserted by ancient retroviruses. Having the same retrovirus inserted in the same location between two species is best explained by common descent.

• Pseudogenes - These are genes that no longer code for a protein due to a mutation. Species often share the same pseudogene, with the same inactivating mutation. A famous example of this is the L-gulonolactone oxidase which synthesizes vitamin c, all simians including humans share this pseudogene but the guinea pig which also has an inactivated L-gulonolactone oxidase gene has a different mutation. This is best explained by common descent.

• The convergence of phylogenetic trees using anatomical homology, DNA homology, pseudogenes, endogenous retroviral insertions, and many other methods all converge on a similar looking tree. There are slight differences but the general relationships of the tree are intact. If all of these methods were flawed you would not expect each of them to converge on the same tree.

• Embryology - The pharyngula stage of embryonic development appears to be highly conserved over time. At this stage it is difficult to tell the difference between various vertebrate species.

Controversy over common descent

Dr. Jonathon Wells published a book in 2002 entitled Icons of Evolution. Dr. Wells contends the book shows that "the best-known “evidences” for Darwin’s theory have been exaggerated, distorted or even faked." Also, Jonathan Wells in his book Icons of Evolution made the claim that many current biology textbooks use distorted pictures of vertebrate embryos to convince students that vertebrates share a common ancestor. (Wells) However, this statement has been heavily criticized by many scientists who have reviewed his book as being misleading in some cases and false in others. [1] [2]

On the other hand, noted evolutionist Stephen Gould wrote the following regarding Ernst Haeckel's work in a March 2000 issue of Natural History:

"Haeckel’s forceful, eminently comprehensible, if not always accurate, books appeared in all major languages and surely exerted more influence than the works of any other scientist, including Darwin…in convincing people throughout the world about the validity of evolution... Haeckel had exaggerated the similarities [between embryos of different species] by idealizations and omissions. He also, in some cases — in a procedure that can only be called fraudulent — simply copied the same figure over and over again.…Haeckel’s drawings never fooled expert embryologists, who recognized his fudgings right from the start. Haeckel’s drawings, despite their noted inaccuracies, entered into the most impenetrable and permanent of all quasi-scientific literatures: standard student textbooks of biology... Once ensconced in textbooks, misinformation becomes cocooned and effectively permanent, because…textbooks copy from previous texts.... [W]e do, I think, have the right to be both astonished and ashamed by the century of mindless recycling that has led to the persistence of these drawings in a large number, if not a majority, of modern textbooks!"

Creationist scientists have written regarding the nature of Haeckel's work and how a prestigious German science journal published a work that they believe was invalid.

Most textbooks used in modern biology classes use photographs of real embryos now rather than the drawings.