Dr Denis Alexander is a distinguished scientist who was chair of the Molecular Immunology Programme at The Babraham Institute in Cambridge. From a lifetime's study of the biological sciences, he suggests in a new book that while the cost of existence is extremely high, perhaps carbon-based life is the only way in which intelligent beings can exist who can freely respond to God's love.

Most people know that their bodies are made of thousands of different carbon compounds. And it really is the case that carbon and the other elements essential for life were made in the stars. We are all made of star dust.

Not only is every living thing on our planet made of carbon, but everywhere we look in space with our telescopes we can detect carbon-containing molecules. Space is not as empty as you might think. More than 200 different carbon-containing compounds are swirling around out there, some containing as many as 70 atoms of carbon. In fact the mass of carbon-containing molecules that we can detect in space is far greater than in all the living things found on planet Earth.

So it seems very likely that if or when we discover life on other planets, it will be carbon-based life. Carbon is like a Lego brick with knobbly bits on four of its sides that stick very effectively to other bricks. Perhaps silicon could be made to work, but it's not nearly as good as carbon. Life anywhere in the universe is likely to be carbon-based life. Why does that matter?

As far as we know so far, we humans are the most complex organisms in the universe, certainly the most complex living organisms here on Earth. This is because our brains contain around 100 billion brain cells (neurons) connected to each other by some 100 trillion or more links (synapses), thereby generating a system with extraordinary computing power. Our particular brains are essential for mind, and so for relationships – and so in turn provide the capacity to pray and to enter into a relationship with God. Our free will depends on our brains as well, generating the ability to make moral choices and to give and receive love. We are not robots.

Might there be a different material, apart from carbon, which could be used to create freely-willed intelligent beings who have the capacity to respond to God's love (or not)? It doesn't seem that way from the knowledge of the universe and its chemicals that we have so far.

But surely God could create bodies in other materials, not carbon? Yes, we know that's the case – think about the resurrection body of Jesus, who could pass through walls as if they weren't there (John 20:19). Clearly resurrection bodies are not made of carbon. I am sometimes reminded of this when passing through King's Cross Station in London. There at platform 9¾ there is a continual line of people, not all young by any means, queuing up to have their photo taken pushing a trolley apparently going right through a brick wall. But carbon-based bodies just don't do that, and charging at brick walls with the expectation of passing through them is best not attempted.

But before his resurrection Jesus was very clearly made of carbon, like we are, entering in his incarnation into all the limitations and challenges of being a true human-being (Philippians 2: 6-7). The sacrifice of Jesus on the cross for our sins involved the shedding of real carbon-based blood, followed by a real death.

So why didn't God just make us with resurrection bodies outright so that we could enjoy his presence for ever without pain, suffering or death? The answer seems to be that God only wants people in his kingdom who have chosen to be there – genuine love requiring real free will – and the only way we know that can happen is by being carbon-based persons. So being carbon-based persons represents God's 'phase one' of his grand plan, a time-limited period restricted to the material universe in which we find ourselves, whereas in 'phase two' we receive our resurrection bodies, the only way in which we can enjoy the presence of God for ever.

'Phase one' – our present carbon-based life – is a costly existence. Carbon-based life is impossible without carbon-based death. No multicellular animal can live by deriving all its energy needs from chemical elements, all are completely dependent on the food-chain whereby organic molecules synthesized in other organisms are passed on to them.

Plants can of course derive at least some of their nutritional needs from the sun by photosynthesis. But that comes at the cost of not being able to move around. And it comes at the cost of not having a brain, because brains are very costly in energetic terms: 25 per cent of our own energy as adults is consumed by our brains. If you want to generate organisms with language, mobility, consciousness, relationships, and social organization, you need a complex nervous system and a big brain. Energetically that doesn't come cheap – it requires a food-chain in which complex carbon molecules are synthesized in other organisms and then consumed by the bearer of the brain by eating.

Death is also part of the carbon-based life 'package deal' because it is death that makes space for the next generation. The dead are constantly making space for the living on planet Earth. If it were not so, the planet would run out of space and resources in a very short space of time. Death is written into the package deal. In the very early microseconds after the Big Bang, the physical laws were emerging that define the properties of the chemical elements which in turn define the properties of life. Carbon-based life, together with carbon-based death, is written into the script of God's creation right from the beginning.

Diamonds are made of carbon too, formed in the depths of the earth under conditions of incredibly high pressure and temperature. Precious things often have costly beginnings. For humans, likewise, the cost of existence is very high. God has chosen us to exist in carbon in phase one so that we might enjoy eternity with him in resurrection bodies in phase two. Being made of carbon does seem to be the only way to be a Christian.

Dr Denis Alexander is emeritus director of The Faraday Institute for Science and Religion, St Edmund's College, Cambridge. His book 'Is There Purpose in Biology? The cost of existence and the God of love' is published by Lion, price £9.99.