DNA: Chimpanzee And Human

The similarity of DNA between chimpanzee and humans (Proceedings of the National Academy of Sciences) may be about 95% and not the previously held estimate of >98.5%, when insertions and deletions are included. This has been misleading because it depends on what is being compared.

At the end of each chromosome is a string of repeating DNA sequences called a telomere. Chimpanzees and other apes have about 23 kilobases (a kilobase is 1,000 base pairs of DNA) of repeats. Humans are unique among primates with much shorter telomeres only 10 kilobases long, but there are a number of significant differences that are difficult to quantify.

• ‘Cytogenetic differences are those in the type and number of repetitive genomic DNA and transposable elements, abundance and distribution of endogenous retroviruses, the presence and extent of allelic polymorphisms, specific gene inactivation events, gene sequence differences, gene duplications, single nucleotide polymorphisms, gene expression differences, and messenger RNA splicing variations.

Specific examples of these differences include:

• Humans have 23 pairs of chromosomes while chimpanzees have 24. Evolutionary scientists believe that one of the human chromosomes has been formed through the fusion of two small chromosomes in the chimp, instead of an intrinsic difference resulting from a separate creation.

• While 18 pairs of chromosomes are ‘virtually identical’, chromosomes 4, 9 and 12 show evidence of being remodelled. In other words, the genes and markers on these chromosomes are not in the same order in the human and chimpanzee. Instead of ‘being remodelled’ as the evolutionists suggest, these could, logically, also be intrinsic differences because of a separate creation.

• The Y chromosome in particular is of a different size and has many markers that do not line up between the human and chimpanzee.

• Scientists have prepared a human-chimpanzee comparative clone map of chromosome 21 in particular. They observed ‘large, non-random regions of difference between the two genomes.’ They found a number of regions that ‘might correspond to insertions that are specific to the human lineage.’

These types of differences are not generally included in calculations of percent DNA similarity

In an extensive study that compared human and chimp DNA, >19.8 million bases were compared. It represents slightly less than 1% of the genome. Differences of 1.23% (98.77% similarities) were calculated . Other studies only considered substitutions and did not take into account insertions or deletions. A nucleotide substitution is a mutation where one base (A, G, C, or T) is replaced with another. An insertion or deletion (indel) is found where there are nucleotides missing when two sequences are compared.

A	G	T	C	G	T	A	C	C
|	|	|	|		|	|	|	|
A	G	T	C	A	T	A	C	C

Substitution: G -> AAGTCGTACC becomes AGTCATACC

Looking at 779 kilobase pairs and carefully examining differences between chimpanzees and humans, 1.4% of the bases had been substituted and is in agreement with previous studies (98.6% similarity). However, a much larger number of indels (Insertion/Deletions) was found. Although there were a few that were > 1000 base pairs long, most were only 1 to 4 nucleotides in length.

Surprisingly, the indels added 3.4 % of base pairs that were different.

Previous studies have focused on base substitutions and may have missed perhaps the greatest contribution to the genetic differences between chimps and humans. Missing nucleotides from one or the other appear to account for more than twice the number of substituted nucleotides. Although the number of substitutions is about ten times higher than the number of indels, the number of nucleotides involved in indels is greater. These indels were reported to be equally represented in the chimp and human sequences. Therefore, the insertions or deletions were not occurring only in the chimp or only in the human and could also be interpreted as intrinsic differences.

• The similarity of chimpanzee and human DNA has been reduced from >98.5% to ~95%, but is there a common acestor? If 5% of the DNA is different, this amounts to 150,000,000 DNA base pairs that are different between them.

• The DNA sequences for all people are so similar that scientists generally conclude that there is a ‘recent single origin for modern humans, with general replacement of archaic populations’. The estimates for a date of a ‘most recent common ancestor’ (MRCA) has this ‘recent single origin’ about 100,000-200,000 years ago.

• These estimates have been based on comparisons with chimpanzees and the assumption of a chimp/human common ancestor approximately 5 million years ago, though studies that have used pedigrees or generational mtDNA comparisons have yielded a much more recent MRCA - perhaps just 6,500 years.