A recent article by David Derbyshire, in the Daily Mail, claims that a new genetic study shows humans and orangutans share 97% of the same DNA. This is true, but the statement does not carry the significance that it appears to have at first sight.
It is five years since a Swedish team completed research on the genetic similarity between man and what they called “the other great apes”. Orangutans are non-aggressive, very intelligent, tool-making primates, but in fact it is difficult to say how much of our genetic heritage we have in common. A fuller explanation by David Irons, who works with the Borneo Orangutan Survival Foundation, confirms that “their anatomy is very similar and their systems work, in most cases, practically the same as ours.” But DNA is more complex than that.
DNA, or Deoxyribonucleic Acid, comprises only four chemical building blocks, adenine, thymine, guanine and cytosine, usually referred to as AGCT. Mankind shares those four blocks with all living creatures, plant and animal alike. The difference lies in the sequencing of those blocks and the consequent order of protein-building switches they activate. The protein-building switches are the genes which are composed of three-part nucleotides, called codons, which code for the proteins to be built. Differences in the order in which the switches are activated produce the end product, be it oyster, fruit fly, orangutan or a banana.
It has been seriously claimed that mankind shares 60% of its genes with a banana, 17% with a lemon, and 60% with a fruit fly as well as large amounts with chickens, rats, pigs, oysters and puffer-fish. But the simple fact of sharing does not take into account the significance of the sequencing of the DNA blocks. The apparent matches actually illustrate the importance of the difference between partial or exact sequences.
When you also consider that there are thousands of pairs of these AGCT protein-building blocks in a single chromosome and that humans have 23 pairs of chromosomes, the 2% accounts for a lot of building blocks. You should also know that long sequences of those blocks are not activated. The ones that are activated, in turn activate long chains of other proteins, sugars and chemical transmitters.
Some of these creatures also differ in chromosome count. The significance of chromosome count is not easy to understand when we find out that we have the same chromosome count as some rosebushes, but only half that of a diploid apple tree which belongs to the same plant family as the rose. The 23 pairs of chromosomes inman are the long strings of genes that carry the instructions to produce certain proteins. MOst mammals have between 70,000 and 100,000 genes, containing billions of nucleotide pairs. Some mammals like the muntjak deer have as few as three pairs of chromosomes, whilst the black rhinoceros has 67.
The human genome project has mapped all the blocks and sequences of the human genome or genetic map, but only about 60% of some other animals have been mapped, by matching short sequences at a time, overlapping them to fit where necessary. Sweeping pronouncements, like sharing 97%, or 98%, or even 99% of our genetic material or DNA, could be very misleading. A Times reporter made this clear when he remarked that the 2% difference between men and chimpanzees was obviously doing “more than its fair share of the work”.
It is as much as 7 million years since the human line split from the common stock it shares with other apes. In that time, certain genes have been lost and others changed in structure. Still more remain the same but have been switched on or off.
If you accept that the loss of only one gene out of 30,000 accounts for the comparative lack of hair between man and ape, you can see that there are far more than enough variables left to account for the tremendous changes in body and brain structure. This is particularly significant since the human genome project suggests that almost half of human DNA has no known function. This is sometimes referred to as “junk” DNA. It may be material left over from previous genetic mutations, or it may have functions we do not yet recognise.
There is therefore a possible permutation of several billion sequences and combinations, all of which may be switched on or off at different times. It is obvious that the simple similarities in genetic structure between man and apes which have been documented, are far less significant that the complex differences we do not even begin to understand.
References:
The Chimpanzee Sequencing and Analysis Consortium (2005). “Initial sequence of the chimpanzee genome and comparison with the human genome”. Nature 437 (1 September 2005): 69–87
Wang, Xiaoxia; Grus, WE; Zhang, J (2006).
