In the 1970s scientists found inactive copies of a gene sequence in Xenopus (African clawed frog). Subsequently scientists called any non-protein-coding copies of genes pseudogenes. Scientists have since labeled many sequences in many organisms as pseudogenes. Naturally these were included under the banner of “junk DNA”. More recently a number of pseudogenes have been identified which actually do translate into proteins. However, while no specific function has as yet been identified for the majority of pseudogenes, that situation is changing. There is growing evidence that many RNAs transcribed from pseudogenes perform essential regulatory functions other than translating into proteins in the cell.
But there is one pseudogene that is non-functional that has been the subject of a lot of controversy. That is the human GULO pseudogene. The story goes like this. Vitamin C is essential for the health of many creatures including humans. Its synthesis requires the action of 4 enzymes. The human genome does have three of the required genes, however, the fourth gene is present only in part and its sequence does not translate into a protein. Thus, the human genome contains a vitamin C pseudogene. It so happens that some primates also exhibit vitamin C pseudogenes. This situation has long been considered by many to constitute proof that humans descended from the primates because they share this pseudogene, (presumably an error made in a primate and passed on to human descendants).
The final enzyme in mammals in the catalysis of glucose to vitamin C is L-gulano-Y-lactone oxidase or GULO for short. (Yeast, plants and many animals use different pathways to manufacture vitamin C, so there is no clear ancestral pattern of descent with modification.) However, loss of the GULO gene is observed in guinea pigs, great apes and humans. In humans there are 6 exons left of the gene. Absolutely none of these exhibits any transcription at all. [An exon is a piece of DNA sequence that is expected to be part of a functioning gene. An intron is a piece of DNA sequence that is omitted from the functioning gene.]
It does seem as if the human gene was once fully functional but now is broken. Compared to a fully functional sequence assumed to represent the mammalian condition in general for this gene, humans and apes have lost exons 1,2,5,7 and 10. The remaining six human exons vary wildly in sequence identity compared to five different ape taxa. When scientists tried to plot evolutionary trees for each of the remaining six exons, only one showed close similarity with the primates. One creationist commentator declares: “The other exon sequences all produce markedly different trees that are decidedly taxonomically incongruent [don’t fit] according to the inferred human-ape evolutionary tree.” [p. 96] When one compares the entire GULO sequence, comprised of 28,800 nucleotides, only the one small segment of 564 bases is similar to the primates. In fact, the entire sequence is only 84% similar to chimpanzees and 87% similar to gorilla. [Jeffrey P. Tomkins. 2014. The Human GULO Pseudogene – Evidence for Evolutionary Discontinuity and Genetic Entropy. Answers Research Journal 7 pp. 91-101. Available on Camp List see matters relating specifically to human origins].
The one feature which humans have exactly in common with the great apes is that they all exhibit a deletion at nucleotide position 97 in the fifth exon. Apart from humans, orangutang, chimpanzee and macques, no other tested animals display such a deletion. This evidence however has to be taken within the total picture which suggests that it is a coincidental loss at a mutation hotspot. Jeffrey Tomkins therefore suggests that “The individual six exon regions which are generally very similar among humans and various apes, each independently exhibit completely different and discordant phylogenetic patterns of similarity for all but one exon [the fifth].” [p. 98] The overall picture does not lend itself to conclusions of common descent.
There are many supposed pseudogenes in the human genome which apparently are fully functional. The GULO pseudogene is not one of them. Such situations however are expected under the model of genetic entropy as explained by John Sandford in his book Genetic Entropy. The human genome has been declining in vigour since the fall of Adam.