The difference between man and chimpanzee in genetic terms is, as we know, very small – about 1.5 per cent. Yet the different outcome between the two species, which share a common ancestor, is very large. Investigating why is extremely interesting. It is also complex, but evolutionary biologists appear to have opened a promising door on to a lengthy passage of exploration. You will forgive me, I trust, for ruthless simplification; we are not a science journal. And I am not here writing of the infusion of the soul, but rather the biological changes which made our species fit for the integration of the soul.
The story starts with the identification of DNA sequences which have changed most since our lines diverged. The most dramatic discovery was a section of DNA which had remained stable, with only two changes, in the line from the chicken down to the last ancestor we share with the apes. A period of 300 million years. Since then (six million years) this section of DNA in the hominid line has undergone 18 changes. It is known as Human Accelerated Region 1 (HAR1). The rate of acceleration may seem small to us but in relative terms it is immense.
Ironically, HAR1 consists of what was once known as “junk DNA”, but it is now understood that some of this plays an important part in modifying the behaviour of genes, which, in turn, relate to other genes. As a result, a relatively small change here can bring about very large changes elsewhere. In this case it relates directly to the cerebral cortex – the outside crinkly part of the brain – which processes our higher and complex brain activity. Incidentally, more than half the genes near to other HARs are concerned with brain function.
The size of the human brain, which generally correlates with cognitive ability, has increased by more than three times since the split from our common ancestor. Brain size is apparently controlled by just four genes. If they had expressed themselves differently then childbirth (also made difficult by our conversion to bipedalism) would have been a very straightforward matter. But then the mothers giving birth and the babies emerging would have been a very different matter too.
There are several other HARs identified, and they are ranked according to the number of differences which have occurred in hominids. HAR2, for example, controls the foetal development of the wrist and thumb – significant since the flexible use of our hands has been important to human development from the construction of tools to the mastery of playing a piano concerto.
The ability to digest starch varies between humans, according to the number of copies of a particular DNA sequence. These appear to have multiplied at the time when the use of fire for cooking and the (much later) development of agriculture provided diets containing quantities of starch.
The ability to control fire appears to have been crucial. Hominids are unique in being able to control and use it. The oldest known hearth dates from about 800,000 years ago, pre-dating homo sapiens by some 600,000 years. It has been argued that the ability to provide light through fire changed the action of hormones pushing us along the line to modern man.
But fire has another important use. Apart from providing warmth and protection from predators, it enabled our forebears to cook meat, allowing us to digest it more easily, and to derive greater nutrition from it. Such advantages would have changed hunting patterns – allowing more time for other pursuits, and increased survival capacity. And this would probably have been accompanied by adaptations in the alimentary system to maximise the advantage. Much closer to our own time, around 9,000 years ago, we developed a version of a gene which enabled human adults to digest lactose, thus enabling us to benefit from milk products derived from herded animals. This occurred in European and African populations, but not in Asian and Latin American populations. These last groups, where they still carry an ancient primate version of the lactose gene, remain intolerant.
Unsurprisingly, changes in the hominid immune system have been many, since the efficiency of the system has a direct bearing on survival. Viruses, which play an important part in the evolutionary process, abound. Retroviruses, which can insert their genetic material into our genomes, are insidious. Fortunately many which can be identified have lost their potency, but others still lurk. Animal immune systems have evolved to deal with different circumstances. A major example here is the Aids virus against which we, unlike non-human primates, are far from immune. However, the characteristics of this difference may eventually show us how to provide human immunity.
The action of viruses reminds us that the development and spread of variants in the human genome are by no means always the result of the classic random mutation which Darwin described. We might add to the list genetic drift, which results from the natural variability occurring through the sexual reproduction (as in non-identical siblings), and which may give survival advantage resulting from certain characteristics. Much variation is also thought to occur, not simply through selection, but through the location, movement and interbreeding of ancient populations.
As they peer through the door, evolutionists naturally argue about aspects of what lies ahead. But at least they are agreed on one thing: we only know a tiny fraction of what there is to learn. The exploration of the passage which invites us will be exciting indeed.
So come and comment. We need your thoughts. Contributions from those with specialised knowledge would be valuable, as indeed would be queries about any faith questions which you think could arise.