Are human beings ‘special’ in a way which makes them distinct from all other creatures on earth? A scientist will look at this question and ask you to define ‘human’. A linguist will look at the same question and note that ‘special’ comes from the same root as ‘species’, while to call something a creature implies it was created by God!

The ‘lies-to-children’ version of biology says that two animals belong to different species if they cannot interbreed with one another. But in the real world, hybrids are indeed possible – just among the big cats, we see tigons, ligers, and lepjags, o my! The more sophisticated ‘lies-to-teenagers’ version says that two creatures are different species if, when they do interbreed, their offspring is infertile. But there are even counter examples to that; and how does that help define ‘species’ for bacteria which multiply by budding rather than breeding?

Each living creature is the outworking of the unique genetic blueprint, or genome, within each of its cells. (For more on how descendants relate to ancestors, see the previous article on genealogy). The question of arranging creatures into species then becomes like the problem of classifying books in a library. How do you group together ‘similar’ things?

This goes back to an ancient problem in philosophy where the ancient Greeks argued about whether physical things were examples of some abstract reality (do all dogs represent particular examples of the ‘ideal dog’?) or whether there are only individual dogs which can be grouped together insofar as they had some useful property in common. A similar question is implicit in the language of the Bible: Genesis 1 tells how God created the animals on earth, each ‘after their kind’ (the Hebrew word is min). But sometimes this word is used in Scripture as a very broad grouping and at other times in a very specific way; we should be wary of translating it to refer to what we now mean by ‘species’. In Genesis 2, which seems to be a separately composed account of creation, the word kind is not used, but the first human is charged with naming (and therefore classifying) all the other living creatures.

We also find a law, in Leviticus 19, against interbreeding different kinds of animals, but this uses a different Hebrew word, kil’ayim. Possibly this means not to try breeding animals which we know cannot successfully interbreed; mules (horse-donkey hybrids) were known in the Bible and King Solomon was even seated on one to be acclaimed in I Kings 1. Some modern Jews, though, do take this verse as a prohibition on breeding mules.

What we now know of biology helps us to see that whether two creatures can interbreed depends on how their genes are arranged into chromosomes. Because they come in matching pairs, each offspring receives one copy from each parent, and this protects the offspring from genetic disease because there are two copies of every blueprint needed – if one gets corrupted, the good copy can be used to rectify things. But across different species, the number of chromosomes, or the arrangement of instructions within them, may not match, and that is the reason why the genetic template of a hybrid might not work to grow the offspring to maturity; or if it does, why the offspring might be infertile.

What do we find within the genetic blueprint, or genome? There will be instructions on how to build, power and reproduce individual cells. We will find a template for an overall body plan – all vertebrates share a basic pattern for a spine, head, and four limbs. There will be instructions for the different organs needed – heart, kidneys, liver, stomach and intestines. There will be the code which makes that particular species unique. And there will be code for those features which vary within species – whether a human has blue eyes or brown hair, whether a dog is a Doberman or a dachshund. This is why around 20% of the genes found somewhere in human beings are also found in bananas; and when we match not only the presence of genes but the way the sequences are ordered, we find we are 1% similar to zebrafish, 32% similar to mice and 91% similar to chimpanzees (figures: Dessimoz Lab). 40% of our whole genome seems to be padding which is very likely the remnants of ancient viruses; and another 8% is clearly composed of inert viruses which invaded our cells and now get copied along with the required blueprint! 

In a previous essay, I argued that our status as humans made in God’s image depends on us being descended from the first human ancestor to have the brain capacity to know right from wrong, who also became the Original Sinner. But when in history might that ancestor have lived? Archaeological evidence points to an ancestor, Homo erectus, which walked upright, thriving between 2 million and half-a-million years ago. Inferring the presence of a conscience from cultural artefacts is difficult; some scholars argue a sense of morality would have been needed for co-operative hunting as long as 400,000 years ago; others would only affirm that morality is certainly present once humans have culture, which can be traced back at least 45,000 years, when we have evidence for art.

Our own body shape, which we class as Homo sapiens, is seen back to 300,000 years ago in Africa, and a distinct form, Homo neanderthalensis, lived in Europe about the same time. Neanderthals are now extinct, but genetic evidence suggests they interbred with Homo sapiens when the latter reached Europe, and the modern inhabitants of Europe and Asia – so possibly even the Blessed Mother and Our Lord – carry 1-2% of genetic code from Neanderthal ancestors.

One answer to the question, “What is it to be human?” is that it simply means being a descendant of the Original Sinner. Any human offspring, no matter how afflicted by developmental deformity or genetic defects, therefore, qualifies as human and must be treated with the utmost dignity. There are no other species living on earth with which humans are interfertile, and so even the narrowest reading of Leviticus 19 would reaffirm the Catholic Church’s philosophy that it would be offensive to human dignity to attempt to cross-breed a human with another species.

Cross-breeding, however, implies a wholesale attempt to merge two different body plans and identities. More complex questions arise when it is a question of inserting a single human gene into another species. Can we distinguish those genes in our whole sequence which make us ‘specifically human’ from those which just happen to be part of our heritage? Our ability to genetically engineer cells means that we can feasibly grow animals with organs whose surfaces will ‘look human’ to our immune cells if those organs are transplanted into a human body. But a pig engineered to produce a human-compatible kidney is still, to outward appearance and behaviours, a pig. 

Can Scripture help? Psalm 8 identifies human beings as ‘a little lower than the angels’ and notes our ability to control the rest of nature. Human beings are not the planet’s only engineers – birds build their nests and beavers their dams – but our influence on planet Earth is so profound that geologists ponder whether we need to declare the start of a new geological age, the Anthropocene, since our entire globe now bears the hallmarks of human activity.

The Book of Job, in the midst of reflecting on human suffering, includes many asides about God as the creator of the natural world. One comment in Job 32 identifies human beings as those creatures filled by God’s spirit, and therefore able to reason and argue. Ephesians 2 notes that human beings, in general, are subject to the ‘cravings of the flesh’ – like other creatures we have animal instincts driving us to seek food, find comfort and procreate. 

The genes which make us distinctively human, then, are those which allow us to reason, take care of the world around us, and rise above our fleshly instincts. As long as we do not use these genes to try to raise the intellects of other living creatures, we might not be forbidden from making use of other genes which are only ‘incidentally’ human.

Psalm 103 acknowledges our mortality. Humans flourish for a season, and then are gone. A literal way of reading Genesis 3 would be that the first human was subject to neither death nor temptations of the flesh until the first sin was committed. As a scientist I find it more consistent to read this as a parable of grace – that had we not sinned, God would have offered us the miraculous gifts of being preserved from both bodily temptation and physical death. But rather than worrying about what might have happened in the past, we know that God has a rescue plan for humans in the future: We alone can profess faith in Christ, in whom we shall not pass away, but be redeemed and raised up to eternal life in Heaven. Thanks be to God!

Revd Dr Gareth Leyshon is a parish priest in the South Wales Valleys, who read physics at Oxford and completed a doctorate at Cardiff (a study of the structure of distant galaxies) before entering seminary.

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