DNA, genes and human history

Cells

Cells are the basic building blocks of all living things – the human body is composed of trillions of them. They perform a wide range of functions, including giving structure to the body, converting nutrients into energy and containing hereditary material.

Most forms of life, including humans, have eukaryote cells, that is, cells with an organised nucleus which serves as the cells command centre, as shown in the diagram.

Proteins and amino acids

Proteins are complex molecules which make up about a third of most cells, and perform many important functions. Each protein is comprised of somewhere between a hundred and several thousand ‘amino acids’, each of which may be composed of about a dozen or more of mainly carbon, hydrogen, oxygen and nitrogen atoms. There are 20 different amino acids, half of which humans can make themselves, and half of which are obtained from food. The sequence of amino acids in a protein determine what function it will perform.

DNA and how it works

DNA is an abbreviation for deoxyribonucleic acid, a complex organic molecule composed of two chains of chemical building blocks called nucleotides, wound together to form a spiral, or ‘double helix’, as shown in the diagram. The key components or bases are four nitrogen-based chemicals labelled A, C, G and T (abbreviated from adenine, cytosine, guanine and thymine). The two sides of the spiral are complementary – that is, an A is always opposite and joined to a T and likewise with C and G.

Most DNA is stored in the nucleus of cells, coiled up into a compacted form known as ‘chromosomes’, but a small amount of DNA, known as ‘mitochondrial DNA’ is located outside the cell nucleus. There is identical DNA in every cell in our bodies, apart from red blood cells, and “cornified” (i.e. “dead”) hair, nails and skin cells.

DNA contains the biological instructions that control how our body grows, by providing code to make the different protein molecules which make up different parts of the body. Most of the DNA code will be the same for all humans, but some sections of the code (which specify individual characteristics such as hair or eye colour, height, facial shape, blood group, etc) will be different.

Chromosomes and Genes

Most of our human DNA is contained 46 chromosomes, each in the double helix spiral form, but of varying lengths. The chromosomes form into 23 matching pairs; one of each pair is inherited from each parent.

The sequences of the A, C, G & T components determine the instructions given by the DNA. Each such sequence is called a gene, and each gene can be anything from about a thousand of these bases to a million. The full human DNA, known as the ‘genome’, contains about 3 billion bases – more than 20,000 genes on 23 pairs of chromosomes – and was only mapped about 10 years ago by a team led by Francis Collins.

The role of DNA in cell division (mitosis)

When a cell prepares to divide, the chromosome pairs separate. Each of the 46 chromosomes splits into its two complementary strands, each of which is used to build a second strand, making 46 new DNA molecules identical to the first ones. The cell then splits in two, creating a second identical “daughter” cell with identical DNA. Only part of the whole DNA sequence is relevant for each type of cell, and that sequence of information determines the characteristics of that cell, say hair colour if it is a hair cell.

The way the information is transferred and the cell built is quite complicated. Enzymes (one type of protein) read the information in a DNA molecule and copy it into messenger ribonucleic acid, or mRNA. The code carried by the mRNA is used to construct the sequence of amino acids in the required protein molecule. Each group of three bases in the DNA sequence specifies a particular amino acid.

The role of DNA in reproduction (meiosis)

In reproduction, the chromosome pairs separate, the nuclear DNA splits into the complementary strands, and the DNA is replicated, as in mitosis. However after the cell divides, it divides a second time, forming 4 cells each with only 23 chromosomes. These will be the male or female gametes, which combine with a cell of the opposite sex to create a new cell with the full 46 chromosomes.

During this process, after the DNA is replicated, a process known as “recombination” occurs, in which the pairs of each chromosome swap genetic information (i.e. they swap a long sequence of base pairs) in what appears to be a random manner. This means that while each child inherits 23 chromosomes from each parent, the make-up of each chromosome will likely be different, contain segments from grandparents in different combinations.

The 23rd chromosome determines gender. The mother will always pass on an X chromosome, but the father may pass on either an X or a Y – XX makes a female and XY makes a male.

Mutations and random variations occur to varying degrees in the different parts of DNA. Many of these can lead to genetic abnormalities such as trisomy or Downs syndrome.

Mitochondrial DNA

Mitochondria are small structures outside the cell nucleus, and are used to help the cell use oxygen to create energy. Mitochondria have their own DNA, a very much smaller and different molecule from the nuclear DNA, and this DNA is passed on by the mother to all children, but males do not pass their mitochondrial DNA to their children. This makes this DNA important for tracing human history, as we will see.

Using DNA to solve problems

DNA testing now allows many different types of problems to be resolved:

• Genetic testing can be used to identify propensity for certain diseases and conditions.
• Because no-one has exactly the same DNA (except identical twins), DNA testing is used in criminology to identify blood or other cells left behind at a crime scene.
• Uncertain paternity can be resolved via DNA testing of the alleged parent and child.
• Almost all of human DNA is exactly the same, but a small amount varies. Each of us inherits long segments of DNA from each parent, shorter segments from each grandparent, etc. So the length of segments two people have in common is an indication of possible relationship – long identical segments indicate the two people share a common ancestor only a few generations back, but short identical segments indicate any common ancestor is likely many centuries back. Thus DNA can be used in genealogy.

Historical DNA studies

The same principles that are used in Genetic genealogy can be used to identify ancient ancestry, and thus identify lines of human evolution and migration.

Ancestry and mitochondrial DNA

Mitochondrial DNA is passed down from mother to child. Normally DNA is passed down exactly, but occasionally a mutation occurs in our DNA and one of the bases is changed – it has been estimated that a mutation occurs in this part of the DNA about once in 10,000 years, although this rate can vary in different regions. By tracking the occurrence of these mutations, ancestry of modern day people can be determined. A sequence of about 500 bases in the mitochondrial DNA is used for this. Therefore, several conclusions can be drawn from mitochondrial DNA studies:

• If two people today have the same sequence, they have a common female ancestor, probably fairly recently.
• If two people have similar sequences, with only one base different, their ancestry is closely related, a mutation occurred, probably in the last 10,000 years, and their common female ancestor lived earlier than that.
• Where there are several differences due to several mutations, the common female ancestor was probably longer ago.
• Where similar DNA sequences cluster geographically, the migration path of people long ago can be traced. Since mutations virtually always cause the DNA sequences to become more dissimilar with time, the direction of migration can also be estimated. The number of mutations that cause the different sequences can give an estimate of the time since the common ancestors.

Ancestry and Y chromosomes

Women pass on only X chromosomes, but men pass on both X and Y. If a Y chromosome is present, then that person will be male. This enables a second way of determining ancestry, this time through the male line. However mutations in the Y chromosome don’t occur so often as changes in mitochondrial DNA, so Y chromosomes are a less useful means of tracing ancestry.

Comparing genomes

A comparison of the human genome with that of other animals, especially the great apes, allows common ancestry to be investigated.

Out of Africa

DNA studies have tried to establish when the common ancestors of all humans lived, but estimates still vary widely. It appears we all have a common female ancestor living in Africa about 100,000-200,000 years ago – latest studies suggest about 150,000 years ago. Known as ‘mitochondrial Eve’, she was almost certainly not the only woman alive at that time, but the only one to have an unbroken maternal line from then until now.

‘Y-chromosomal Adam’, the common paternal ancestor of all people today, also lived in Africa, apparently more than 200,000 years ago. ‘Adam’ was almost certainly not the only man alive at the time. And this Adam and Eve were probably not a couple, for it is considered unlikely they lived at the same time and in the same place.

Modern humans began migrating from Africa about 60,000 years ago, spreading eastwards through Asia and eventually through Alaska and down into North and South America. Others migrated westwards into Europe, gradually replacing the older Neanderthals, who had been there for several hundred thousand years. It appears that there was a small amount of interbreeding between humans (homo sapiens) and Neanderthals, perhaps about 50,000 years ago in Asia Minor, so most humans alive today probably have some Neanderthal DNA. Nevertheless, Neanderthals are considered a separate species.

Humans have much DNA in common with other animals and even plants – for example 20% with some plants, 40-50% with some insects, 60-90% with most animals, and almost 99% with great apes.

DNA and God: difficult questions

It isn’t hard to see that these discoveries and historical reconstructions raise some difficult questions for christian believers.

If DNA similarities can demonstrate connections between our relatives, the same principles hold true for more distant relatives among animals. Thus DNA becomes a significant support for evolutionary science and the concept of common descent, in three ways:

1. Genetic Diversity: The rate of change in DNA from generation to generation allows predictions to be made of variations between species over long time periods. Using non-genetic evidence, the common ancestor of humans and chimpanzees was estimated to have lived about 6 million years ago. The calculation from genetic differences gives a similar figure.
2. Genetic “scars”: The insertion or deletion of segments of DNA remains as a “scar” on the DNA which is passed on to subsequent generations. If humans and chimpanzees have the same scar, but orangutans don’t, then we know humans and chimps have a more recent common ancestor that we don’t share with orangutans. Thus the family tree can be defined.
3. Genetic synonyms: These are different genetic codes that produce the same proteins. Again the existence of synonyms in two different species indicates a common ancestor.

Francis Collins says: “I would say we are as solid in claiming the truth of evolution as we are in claiming the truth of the germ theory.”

Who were the first humans?

Three species or sub species are of interest:

• Homo erectus lived from about 1.8 million years ago to about 300,000 years ago.
• Neanderthals lived between maybe 250,000 years ago and 30,000 years ago.
• Modern humans, Home sapiens, first appeared more than 200,000 years ago possibly a lot more.

Paleoanthropologists classify all groups as “human”, but they are sometimes referred to as separate species (which infers that they could not interbreed) and sometimes as sub-species (which infers that perhaps they could). I have seen studies which suggest Neanderthal-Homo sapiens interbreeding in Asia or the Middle East as recently as 50,000 years ago, so we may perhaps consider Neanderthals as “human”.

For christians, the essence of humanity is more than just biological, but spiritual. Thus christians may only classify as human those who had a spiritual capability, and we don’t know whether that evolved or was a special endowment by God at one time – the latter seems more reasonable.

Thus a christian might believe that the first spiritual humans capable of responding to God were Homo sapiens, perhaps 100,000 years ago. This raises the issue of what God was doing between then and his interventions in human affairs recorded in the Bible, about 4,000 years ago. Perhaps for this reason, some christians believe the first spiritual humans may have been much more recent than that.

However it is quite possible that God revealed himself in appropriate ways to these people, who may be fewer in number than we would expect. (I have used current estimates of human population and life expectancy to construct a spreadsheet model of the number of people who died in time intervals since 100,000 BCE and now, and this suggests that only about 10% of all people lived and died in the enormous period from 100,000 BCE to 2000 BCE.)

Adam and Eve?

If this DNA reconstruction is true, the Genesis story of Adam and Eve should probably be interpreted mythologically, which many christians now do. But some christians, including molecular biologist Denis Alexander, believe that there could have been a literal Adam and Eve living as spiritual humans in the Middle East alongside spiritually unaware fellow Homo sapiens

Theological difficulties

Without a literal Adam and Eve, the traditional theological doctrines of the Fall and original sin are thrown into question. These issues are discussed in Evolution and God.

Conclusion

It seems foolish to pretend to know exactly what happened. All ‘explanations’ appear to have some problems. I personally believe the Adam and Eve story is mythical, and am willing to accept an old date for the first humans, but that is only a guess.

Arguments supporting the existence of God

Three arguments from the DNA history seem to me to offer support for the existence of the christian God.

Common ancestry

There are many creation myths around the world. The Genesis story of a common ancestor leaving the land of her birth and her children spreading out all over the world does seem to have some connection with the DNA story. I’m not sure how significant this connection is.

The beginnings of life

The process of cell division is complex, requiring a genetic blueprint and complex molecules to both provide the raw materials and construct the new cell from the blueprint.

The raw material of cells is mainly made up of water and 4 types of complex organic molecules – carbohydrates, lipids, proteins, and nucleic acids. Each of these has to be made up from smaller constituents. We have seen how proteins are made up of amino acids, which are made from carbon, hydrogen, oxygen and nitrogen atoms. Nucleic acids, and most carbohydrates are formed by the joining of hundreds or thousands of nucleotides and simple sugars, respectively.

The blueprint is provided by DNA. When a cell prepares to divide, the DNA splits into two strands, each of which is used to build a second strand and thus a second cell. Only part of the whole DNA sequence is relevant for each type of cell, and that sequence of information determines the characteristics of that cell, say hair colour if it is a hair cell. Enzymes (one type of protein) read the information in a DNA molecule and copy it into messenger ribonucleic acid, or mRNA. The code carried by the mRNA is used to construct the sequence of amino acids in the required protein molecule. Each group of three bases in the DNA sequence specifies a particular amino acid.

The first cells (prokaryotic) are simpler than the human cells we have been discussing (eukaryotic), but similar complex processes are required for them to form.. For life to begin in the first place (abiogenesis), this complex process had to evolve without any previous blueprint, something that is extremely difficult to believe happened by chance (it was before natural selection could operate). Science has not yet established how it happened, despite a lot of attention being given to this research. It is no wonder that many argue that abiogenesis couldn’t have happened naturally.

However christian biologists, such as those at Biologos, caution against this conclusion. They suggest it is better to say that God created the universe in such a way that this evolution of life from non-life could occur “naturally”.

Nevertheless, the complexity of DNA and the difficulty of explaining how life with such complexity arose is a challenge to non-believers. It is truly amazing that life began at all.

DNA as information

DNA is essentially a code carrying an enormous amount of information that is required for life to develop and grow. Each DNA molecule carries about 1.5 Gb of information, enough to write a large computer program. The human body’s cells thus are capable of storing about 150 zettabytes (150 million million Gb) of information. (Recently scientists have begun to develop ways of storing computer information on DNA, and it looks like all the information in the world could be stored on genetic material that would fit on the back of a single truck!)

Stephen Meyer suggests that the operating system embedded in the genome includes nested coding, digital processing, distributive retrieval and storage systems, an amazing claim for a single-celled creature!

It can be argued that DNA is a code, a language, that encodes information, just like computer code, and that such information code has never occurred naturally, but always requires design. Therefore, it is argued, DNA points to a designer. Non-believers see things differently. They say that DNA isn’t a code, it is nothing like computer code, it is a molecule.

But information theory is the science and mathematics of information. It has been well developed and utilised in modern computing, and has been shown to be valid in considering DNA.

Conclusion

So is it possible that such complex and detailed information could be developed from less complex information by natural selection, or that the initial information could arise from chemistry at the beginnings of life? Many believe that such information-bearing code requires a programmer – for example, former atheist Antony Flew gave this as one the main reasons why he decided God existed after all.

Christian and science website Biologos counsels against assuming too much about genetic code and its origins. But granted the amazing information properties of DNA, I think we can conclude that DNA presents a challenge to naturalism.

Conclusion on DNA and God

The evidence seems to be ambivalent. On the one hand, DNA indicates that humans have evolved from animals, and thus throws into doubt traditional christian views about creation, Adam and Eve and the distinctiveness of the human race. However many christians find little difficulty with this.

On the other hand, the amazing properties of DNA and the difficulty of explaining how life could evolve such a complex process provide some support for the belief that God designed the DNA needed for life, though many sceptics don’t accept this.

I would assess the pro-God arguments as being the stronger, but doubtless each person will make a different assessment.