CODEX Entry 1211: Human DNA
The primary difference between humans and great apes, is that humans have 23 pairs of chromosomes and other great apes have 24. All members of Hominidae except humans, Neanderthals, and Denisovans have 24 pairs of chromosomes. In the human evolutionary lineage, two ancestral ape chromosomes fused at their telomeres, producing human chromosome 2. Chromosome 2 represents 8% of the building material of DNA. There are nine other major chromosomal differences between chimpanzees and humans. Chromosome segment inversions on human chromosomes 1, 4, 5, 9, 12, 15, 16, 17, 18. In fact, only 13 Chromosomes are the same.
These differences at the gene level are beginning to be understood, and the differences between individual humans and common chimpanzees are about 10 times the typical difference between pairs of humans. For example, the forkhead-box P2 transcription factor, which is involved in speech development, is different in the human lineage. Several genes involved in hearing are also different. The genetic differences between human and chimpanzee genes are thirty-five million single-nucleotide changes, five million insertion/deletion events, and various chromosomal rearrangements. Gene duplications account for most of the sequence differences between humans and chimps. Single-base-pair substitutions account for about half as much genetic change as does gene duplication.
Evidence has been discovered of non-mutation and not vertical DNA evolution from sequences of proteins, RNA, and DNA, indicating that organisms have been involved in exchanges of genetic material across diverse groups in processes known as horizontal gene transfer or HGT. The true extent of HGT in bacteria and archaea has now been firmly established, with 80 percent of their DNA showing signs of horizontal transfer. Bacteria and archaea are the oldest life forms on earth and represent 90% of earth’s biomass. The most likely agent of this genetic shuffling is currently believed to be viruses, which effectively cut and paste DNA into cells from one genome into another, often across great taxonomic distances. In fact, by some reckoning 40 to 50 per cent of the human genome consists of DNA imported horizontally by viruses, some of which have taken on vital biological functions. Some old earth creationists believe that HGT is proof of a creator slicing and splicing DNA from a number of sources to make the first man.
It is famously indicated that 98.5% of our DNA is the same as that coded in chimpanzees. But if the two genomes are first lined up with the parts of each genome that are similar next to each other, after this alignment, only 2400 million of the human genome’s 3165 million ’letters’ align with the chimpanzee genome, or 76%. Adding artificial gaps, and places where there are single letters, or 2 to 1 genome match differences, this drops to 70%. Some scientists have argued that the 24% of the human genome that does not line up with the chimpanzee genome is useless ”junk DNA”. However, it now seems that this DNA could contain over 600 protein-coding genes, and also code for functional RNA molecules. At present we cannot fully assess the difference in structure of the two genomes, because the human genome was used as a template, or ”scaffold”, when the chimpanzee draft genome was assembled.