Posted by: Lister | February 24, 2007

Common Descent

A more to-the-point follow up to my chicken-joke analogy, here’s some info from talk-origins. [See original for links to scientific articles]

Using a ubiquitous gene such as cytochrome c, there is no reason to assume that two different organisms should have the same protein sequence or even similar protein sequences, unless the two organisms are genealogically related. This is due in part to the functional redundancy of protein sequences and structures. Here, “functional redundancy” indicates that many different protein sequences form the same general structure and perform the same general biological role. Cytochrome c is an extremely functionally redundant protein, because many dissimilar sequences all form cytochrome c electron transport proteins.

[…] Cytochrome c is absolutely essential for life – organisms that lack it cannot live. It has been shown that the human cytochrome c protein works in yeast (a unicellular organism) that has had its own native cytochrome c gene deleted, even though yeast cytochrome c differs from human cytochrome c over 40% of the protein.

In fact, the cytochrome c genes from tuna (fish), pigeon (bird), horse (mammal), Drosophila fly (insect), and rat (mammal) all function in yeast that lack their own native yeast cytochrome c.

Furthermore, extensive genetic analysis of cytochrome c has demonstrated that the majority of the protein sequence is unnecessary for its function in vivo.

Only about a third of the 100 amino acids in cytochrome c are necessary to specify its function. Most of the amino acids in cytochrome c are hypervariable (i.e. they can be replaced by a large number of functionally similar amino acids).

Importantly, Hubert Yockey has done a careful study in which he calculated that there are a minimum of 2.3 x 1093 possible functional cytochrome c protein sequences, based on these genetic mutational analyses. For perspective, the number 1093 is about one billion times larger than the number of atoms in the visible universe. Thus, functional cytochrome c sequences are virtually unlimited in number, and there is no a priori reason for two different species to have the same, or even mildly similar, cytochrome c protein sequences.

In terms of a scientific statistical analysis, the “null hypothesis” is that the identity of non-essential amino acids in the cytochrome c proteins from human and chimpanzee should be random with respect to one another. However, from the theory of common descent and our standard phylogenetic tree we know that humans and chimpanzees are quite closely related. We therefore predict, in spite of the odds, that human and chimpanzee cytochrome c sequences should be much more similar than, say, human and yeast cytochrome c – simply due to inheritance.


Humans and chimpanzees have the exact same cytochrome c protein sequence. The “null hypothesis” given above is false. In the absence of common descent, the chance of this occurrence is conservatively less than 10-93 (1 out of 1093). Thus, the high degree of similarity in these proteins is a spectacular corroboration of the theory of common descent. Furthermore, human and chimpanzee cytochrome c proteins differ by ~10 amino acids from all other mammals. The chance of this occurring in the absence of a hereditary mechanism is less than 10-29. The yeast Candida krusei is one of the most distantly related eukaryotic organisms from humans. Candida has 51 amino acid differences from the human sequence. A conservative estimate of this probability is less than 10-25.


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