Human Evolution: Genes, Genealogies and Phylogenies
Ever since I entered university I have been interested in cells and their large molecules – DNA, RNA and proteins. It is said that small things amuse small minds, and I must confess that my interest waned with entities larger than about 20 µm. That’s approximately the diameter of a cell. Naturally, I became a cell biologist.
I was certainly not interested in evolutionary biology. Its subject matter was the wrong size, and besides, I was aware that on-going controversy was associated with this scientific field. Consequently, I wanted nothing to do with it. In a lazy way, I accepted the evolutionary paradigm without going beyond what I was taught in introductory biology courses.
I became involved in cancer research, and in the early 1980s, read avidly to inform myself of dramatic developments in the genetics of cancers. It was then that I came across oncogenic retroviruses. These are a subtype of virus that had a cunning mode of propagating themselves, and they were revolutionizing our understanding of how cancers developed. They brought to light a class of genes known as oncogenes. I struggled to assimilate the deluge of data, totally focused on cancer biology, my professional interest. But to my enormous surprise, I was following a continuous track which led to the point where I found myself reading in the area of evolutionary biology.
Retroviruses provided a way of demonstrating that many cancers are produced from a single abnormal cell. Counter-intuitive though it may seem, the billions of cells that may populate a tumour are the descendants of a single ancestral cell, so cancers are said to be monoclonal. And, almost unbelievably, retroviruses provided a way of showing that multiple species may be derived from a single progenitor species (indeed, ultimately from a single cell). Such related taxa of organisms are said to be monophyletic.
As I read, I found that a large variety of genetic markers established both the monoclonal nature of tumours on the one hand, and the monophyletic nature of groups of species on the other. Humans, chimps, gorillas and orang-utans, for example, share millions of genetic markers that show – unambiguously – that the four species share a common history. The genetic principles applicable to cancer (or immunology or microbiology or whatever) and evolutionary phylogenetics were the same, thoroughly established and non-controversial.
For example, birds lack teeth. But their genomes contain the remnants of genes that once encoded proteins required to make teeth. The bird ‘tooth’ genes are severely degenerated – bits of genetic material have dropped in or out in numerous places. But interestingly, some of these mutational changes are shared by different species of birds. For example, a run of five bases has been inserted into the ruins of the enamelin gene of budgies, ducks, chickens, and turkeys. This disruption is absent from the functional gene of crocodiles. All the species of birds possessing this gene-disrupting insert are descended from the ancestor in which the (unique) gene-disrupting event occurred.
Further examples of genetic markers that demonstrate evolutionary relationships are seen in small lengths of genetic material called transposable elements, which are found in the genomes of most organisms (including humans). These copy-and-paste themselves into new sites in the genome. They reproduce themselves haphazardly, and sometimes generate new copies that disrupt important genes and cause genetic disease. If multiple cells (or organisms or species) have the same transposable element located in the same site of the genome, they can be considered to be the descendent of the one cell (or organism or species) in which that unique copy-and-paste event occurred. Humans share millions of transposable elements with other species of primate. These constitute a colossal weight of evidence that humans and those other species are the issue of the same ancestor – the one in which each new transposable element arose. A particular example of a transposable element is shown below. It shows an insert called an Alu element sandwiched between a 15-base duplication of the DNA (shaded – such duplications are intrinsic to the process). This Alu element, located between recognisably the same duplication, is present in humans, other apes, monkeys, and tarsiers, but absent from the genomes of prosimians and non-primates. This single insert establishes that humans and tarsiers share a common ancestor – the one in which the Alu element was generated.
As I imbibed the astonishing new findings in comparative genetics, I came to feel that I had to tell my fellow-Christians about the power of the new genetic evidence that gave such unambiguous support for evolutionary histories. DNA markers are a native language to any cancer cell biologist, and these same markers showed, with an astonishing degree of coherence, that we humans are an evolved species. DNA markers provided a brand new line of evidence that would have been unimaginable before the days of large-scale DNA sequencing (say 20 years ago). But their testimony was so compelling that they called for an end to the controversies over whether evolution had occurred. I read many scientific articles that I found enthralling – they reminded me just why the information-dense molecule DNA has always been so utterly captivating to me.
Indeed, I started to write about it and the climax has been a book on comparative genetics. It is called Human Evolution: Genes, Genealogies and Phylogenies (Cambridge: Cambridge University Press, 2013). It describes four types of genetic markers that provide compelling evidence that tumours are monoclonal, and that species (humans and other primates, and beyond that, other mammals) are monophyletic.
The book is offered in the hope that it will be accessible to a wide range of interested people, and that it will be used widely in high schools, colleges, and medical schools – and for professional scientists who (like me) are not specialists in evolutionary biology. It is also written for theologians interested in science. It is offered in the hope that it will help calm the conflicts over evolutionary biology that are proving so divisive in the Church and our society.
So how did this demonstration of human evolution fit in with my Christian faith? The biblical concept of creation emphasized that all of reality is given existence by God. This biblical understanding was very important for the development of science during the Middle Ages, because it entailed that creation was ordered and behaved lawfully (and therefore was worthy of investigation). It also meant that the entire universe was creation, distinct from God (and therefore was amenable to investigation). The concept of an independent and lawful creation was foreign to polytheistic or pantheistic worldviews. On the biblical account, biological evolution is merely the history of life, and an aspect of God’s created world.
Old Testament scholars have shown that the creation stories in the book of Genesis were written to present a vision of the nature of God – in his peerless sovereignty, faithfulness, and goodness. This God was wholly different from the violent and petty gods of the ancient Babylonians or Canaanites. When read in this way, the Genesis stories may be seen as one of the greatest-ever revolutions in human understanding. Genesis used the literary forms of the day to introduce its readers to a rational God who cares for people – and whose faithfulness made science possible. Genesis is expressly about theology, not science.
Ultimately, of course, Christian faith comes from God’s revelation in Jesus of Nazareth. As David Hart states, Christianity is the only major religion that arose from a single historical claim. A lifetime of study can only emphasize the authenticity and power of this history. And I have a deep sense of satisfaction that the God who is revealed in human history is also the God of cosmic and biological history. All such histories run freely, but they follow channels that are constrained by divinely-ordained laws, and they have culminated in astonishing climaxes. Science reveals the universe, the solar system, and all living organisms to have an authentic history. In the same way, the Christian faith is irreducibly historical. The wonders of biological development and of Resurrection come from the same faithful God. That’s exhilarating.