Visions of new life: How migration led to the formation of human society

Beyond all our Christmas homecomings beckons the hope of turning a new page. The New Year is a vision –muzzy, perhaps, but still a vision - of new life. We'll give up smoking and go to the gym every day.

It's not surprising that these resolutions follow our seasonal migrations back to the family hearth, for that's what migration is all about - new life. The word "migrate" comes from the Latin migrare, to move from one place to another. It embraces many different activities, human and biological, and is related to the English "mutable" and the Greek ameibein, "to change". It comes in two basic forms - Go-and-Stay, and Go-and-Come-Back.

Go-and-Stay is what we do when we emigrate, immigrate or colonise someone else's home. It is what refugees and asylum-seekers have to do, but the process also operates at the micro-biological level: our own cells migrate in our bodies. Cells migrate in us for two reasons, to heal infection – the immune system is based on cell migration - and to form a foetus. To repair existing tissue, in other words, or to create new life.

No one knows how the first cell arrived on earth, but once here, whether formed in a deep-sea biochemical accident or blown in from outer space, that cell changed everything. Cells of blue-green algae set off into new life and the adventure of self-replication as if they heard God say on the fifth day of creation, "Go forth and multiply".

The beginning of life was the beginning of migration. What we come from is the Go-and-Stay of primeval slime. Go-and-Come-Back, the more evolved form, is a response to the spinning of the globe. Life is on the move all the time. As our tilted sphere spins, temperatures rise and fall in different places, food comes and goes, and life follows it. Migrant workers follow seasonal work, and thousands of life-forms, from whales to toads aiming to breed, migrate to raise their young.

Some migrations are breathtaking feats of endurance. Bar-headed geese nest in Central Asia but winter the other side of the Himalayas. They fly over Everest to get there. If a storm sweeps in and a youngster drops to snow-shrouded crags below, the mother must keep going, or she'll die too.

Migration is very wasteful but, every spring and autumn, most get through. How? The oxygen a bird needs to fly is 20 times what it needs on the ground. At 33,000 feet, there's only a quarter of the oxygen available at sea level. But their haemoglobin absorbs oxygen faster, and their capillary veins penetrate especially deep into their muscle. So oxygen reaches further quicker and they get more from each breath. Over millennia, they have adapted their bodies.

They and their route are older than the Himalayas. When rocks blocked their flight-path, they didn't go round. They flew over, changing their haemoglobin and capillaries to cope with the increasing altitude.

The biggest-scale Go-and-Come-Back migration is vertical. It happens every night and morning in the sea. At dusk, billions of jellyfish rise to the surface to graze on surface phytoplankton, which have been processing sunlight all day. Other life-forms follow, like sea butterflies, lantern-fish and squid, feeding off and depending on each other, then sinking back down at dawn.

This vast marine respiration, up and down, night and morning, is not that different from human commuting. Jellyfish go up and down for phytoplankton. We, morning and evening, go in and out of cities for work: which we convert, as phytoplankton convert sunlight into energy, into nourishment for our families and warmth from the fuel bills we pay.

For swallows, jellyfish or human beings, migration is going where the thing is which you need and don't have; it is reaching whatever helps you and your children to survive. It was how we began. We walked out of Africa looking for food, safety, water, shelter, a better climate, new territory. Like blue-green algae, we spread over the globe. Migration made the world biologically, but also created human civilisation.

Maybe this is why the idea of Homo viator - man on the road, human life as a pilgrimage - is deep-grained in Western thought. "The soul is a wanderer and fugitive," the pre-Socratic philosopher Empedocles said, "driven by decrees and laws of gods".

The Fall gave that idea a new depth. Medieval Christianity said we were strangers in this world, searching for the spiritual homeland to which we originally belonged. We weren't meant to be here; we were put in a garden and this went wrong, so now we are wanderers between two worlds, wayfarers on the via, the way, of life: looking for, or trying to regain, our true home. In theology, psychology and society, home, homelessness and migration belong together, two sides of the same ancient coin. Home is where you start from and what you are looking for. But you must travel to find it.

Home is also what you make, as the whelk makes its shell, throughout your life. It is the housing of your particular self, your individual wicker-work of defences, impulses and memories. Then that word "mutable" strikes. Things change - in yourself through age and disease, and in the world. Rivers dry, invaders grab your land, you lose your home and have to go elsewhere.

Human history is the story of the nomad giving way to the settler, but when people are unsettled they have to migrate. So there are always two factors behind migration: the push and the pull. Birds that nest in the Arctic to feed on summer insects leave, when the cold comes, not only because the insects have gone but because they would freeze to death if they stayed. Need and escape, hope and new life, are names for the same God.

Of all the push factors behind human migration, war is one of the biggest. The Second World War created 40 million refugees. Goya's painting The Colossus, of refugees fleeing from a giant roaming the Spanish countryside, sums up these push factors. If your home is destroyed, or something terrible has entered it, all you can do is run.

Environmental destruction is another Colossus. Outside the Senegalese capital Dakar is the suburb of Thiaroye, once a fishing village. The seas have been fished out by European trawlers so now Thiaroye is the starting-point for thousands of West Africans desperate to get to Europe for work. The traffickers send them across the Sahara and out from Libya in open boats.

In March 2009, 400 people from Lagos, Accra, Addis Ababa, Nairobi, Yaounde, Banjul; from Egypt, Somalia, Ghana, Nigeria, Tunisia, Algeria, Morocco, Gambia and Cameroon; from Bangladesh, Syria and Pakistan, sitting packed into three small fishing boats, drowned in one night. They were hoping, said one of the few survivors, now in a Libyan prison, for a better life.

Thousands more land up in the Canary Islands, often too weak to walk. When a boat approaches, the harbour fills with tents, police, wheelchairs and helicopters. Littering the harbour are piles of skiffs in which a hundred people have sat for many days. Sometimes dead bodies are found in them. Others arrive in Sicily, Lampedusa or Malta hoping to reach Europe and that dream of new life.

They face the same hostile planet as the birds - desert and sea, mountain and storm. If you super-imposed human and avian journeys on the map, the lines would mesh. Long-winged birds cannot flap over the sea because they rise on thermals of hot air and there are no thermals over an ocean, so they seek out the shortest sea crossing and do it in one long glide. Shortest crossings are what human migrants try for, too.

In 1992, the Brazilian photo-journalist Sebastiao Salgado began a project of photographing, through 47 countries in Africa, Europe, Asia and the Americas, hundreds of millions of people doing what that Latin verb migrare stands for: moving from place to place, displaced like this by war, by environmental ruin (from multi-nationals investing in vast-scale agriculture and mining), by "natural" disasters like drought and famine brought on by what we are doing to the planet, and by the widening gap between the rich and poor. Twenty years on, with seven billion of us in the world, this means unprecedented levels of migration and demographic change. Animal or human, the point of migration is survival.

Nous voulons de l'air pour nos enfants, said graffiti in the detention centre of Sangatte outside Calais. Stories of endurance matching the bar-headed geese lie behind all migrants discovered in lorries at Dover, and the 16-year-old Vietnamese girl recently found by the UK Border Agency, curled up behind the dashboard of a car in sweltering conditions, surrounded by electrical wiring and clutching a stuffed rabbit.

But cells also migrate, let's not forget, to heal living tissue and create new life. As Hungary criminalises homelessness and Britain worries whether its Border Agency is keeping out enough migrants, maybe the New Year is a good time to apply this idea to society, too.

'The Mara Crossing' , Ruth Padel's book of poems and prose on migration, animal and human, is published by Chatto &Windus next week