'Barack's voice was just like his father's - I thought he had come back from the dead'

As Lake Victoria disappeared to the south, the road rose and fell, winding gently through the boulder-strewn hills of far western Kenya. Men rode heavy single-speed bicycles with sacks of charcoal strapped to the back; women walked, buckets of bananas balanced perfectly on their heads. Where a sign pointed the route to the Senator Obama Secondary School - Motto: Endeavour to excel - the asphalt gave way to bumpy red dirt. An American flag hung in front of the New Apostolic Church, providing a further clue to the identity of the village's favourite son.

A few miles further on, an 86-year-old woman sat on a plastic chair under a mango tree outside her simple three-bedroom house with a pale-blue corrugated iron roof. Television crews waited their turn to interview her. As chickens squawked, and cows grazed nearby, Sarah Obama appeared perplexed as to what the media fuss was all about.

"It's just a job, a government job," said "Mama Sarah", step-grandmother of the man who this week became the Democratic candidate in the US presidential race, during a recent visit. "I am not going to make a big deal out of it or pretend that it's anything really great."

Her comments would surely amuse Barack Obama, who first met his "granny" in his 20s when he came to Kenya in search of his roots. For while they show a refreshing modesty and honesty in a presidential campaign full of hyperbole, they also hint at the distance travelled by the Obama family over just three generations.

The story begins a few metres away from where Mama Sarah was sitting. There, an unpainted concrete headstone marks the resting place of her husband, Hussein Onyango Obama, herbalist, farmer and village elder. As Mama Sarah recounted in Barack Obama's 1995 memoir Dreams from My Father, Hussein was one of the first people in the village to wear trousers and a shirt rather than the traditional goatskin.

As a restless young man - "it is said of him that he had ants in his anus", in Mama Sarah's words - Hussein learned to read and write English, and walked for two weeks to the capital, Nairobi, where he found work as a cook and housekeeper for British settler families. After his first wife, Helima, discovered she could not bear children, Hussein outbid another man for the hand of a beautiful young woman called Akumu, paying her father a dowry of 15 cattle.

They had two daughters and a son, Barack Obama Sr. But Hussein's harsh, even abusive, manner - his grandchildren would later refer to him only half jokingly as "the Terror" - and his constant demands for a spotless house, drove Akumu to leave. Mama Sarah, Hussein's third wife and decades his junior, raised Akumu's children alongside her own. By then, Barack Obama Sr was nine, and already showing the same stubborn, independent streak.

"He refused to go to the local school, where the teacher was a woman," said Mama Sarah. "When the pupils were naughty, they would get spanked. He told me 'I'm not going to be spanked by a woman.'"

Instead, Obama Sr enrolled at a primary school six miles away. Though he often bunked lessons when Mama Sarah did not give him a lift all the way on her bicycle, he still came top of his class. His marks earned him a rare place at secondary school, but he was soon expelled for bad behaviour.

Hussein, who served as a cook in the British army in Burma during the second world war, and was later detained by the colonial administration in Kenya in the lead up to independence, was livid with his son, believing he had thrown away the one opportunity to better himself.

"When my husband heard later that Barack [Obama Sr] wanted to study in America, he said it was impossible as there was no money," said Mama Sarah. "But Barack told him he would manage it on his own."

Obama Sr moved to Nairobi, where he took a correspondence course to complete his schooling, studying at night while working as an office clerk by day. He wrote to dozens of US universities. The University of Hawaii replied, offering him a scholarship. Leaving behind his infant son, Roy, and his young wife, Kezia, who was pregnant with their daughter, Auma, he flew to America.

Being the university's first African student did not discourage him. He graduated top of his economics class. He married a white student from Kansas, Ann Dunham, and they had a son, who he gave his full name: Barack Hussein Obama.

Obama Sr continued his studies at Harvard, but his marriage soon faltered. When he returned to Nairobi a few years later it was with a different white American woman, Ruth. Together, they had two children. But Obama Sr was still seeing his first wife, Kezia, on the side, and she bore him two more sons. Later, as ethnic rivalries hampered his career as a senior government official - the Obamas are Luo, while the government was largely Kikuyu-dominated - and as he increasingly turned to drink, he had an eighth child by another Kenyan woman.

The Obama siblings in Kenya grew up hearing stories from their father about their half-brother who remained in America, a boy they knew as "Barry".

"That's what my father always called him in front of us," said Auma Obama, a no-nonsense woman who returned to Kenya last year from the UK, where she ran a children's trust. "When I finally got to meet Barack in America, the first thing he told me was: 'Don't call me Barry! My name is Barack!'"

A few years after that meeting in 1987, Barack made his first trip to Kenya, travelling to the village in Kogelo that his siblings had always called "Home Squared": their second home. There, next to the headstone of his grandfather Hussein, he found the grave of his father, who had died in a car accident five years earlier. Since Barack had never learned Luo, and Mama Sarah speaks no English, they could not communicate directly. Still, she knew immediately that this was the son of the boy that she had helped raise.

"Barack's voice was exactly like his father's. It made me think that his father had come back from the dead."

In the 1990s, Barack travelled to Kenya again, this time with his fiancee Michelle, to introduce her to his Kenyan relatives. When he returned in 2006, by which time there was talk of him being America's first black head of state, he received a presidential-style welcome, with thousands turning out to see him.

Since then the path to Mama Sarah's home has become well trodden by local and foreign journalists, usually arriving unannounced with satellite dishes and cameras in tow. She has received them happily, inviting them into her living room, which is decorated with family pictures, including one of Barack carrying a sack of vegetables beside her.

But by January, with the primaries in full swing, the intrusion had become too much, even for a woman who still tends her crops of maize, sweet potatoes, beans and cassava for several hours a day and only recently stopped riding a bicycle. Her son, Said, a genial man who lives in the lakeside town of Kisumu, 90 minutes' drive away, now regulates media access, together with Auma, who works for an international charity based in Nairobi. Most of Barack's other half-siblings live abroad.

"Though we had a family meeting to discuss the implications for us all in Kenya before Barack announced he would run for president, we did not imagine it would ever become this big," said Auma, who helped out on the Obama campaign in the US in December and January.

She keeps in close contact with Barack's media team, regularly discussing "campaign strategy and how to handle certain questions" about the family's past. One particularly thorny area that is bound to resurface in coming months is religion. Hussein converted to Islam, and Mama Sarah calls herself a Muslim, even though she wears her faith lightly. A photograph circulated in the US earlier this year, which showed Barack wearing customary Somali dress bestowed upon him by local elders during his Kenyan visit in 2006, was seen by many Democrats as an attempt to smear Obama by suggesting he was not a Christian.

"Things are dwelt on that in our family are non-issues," said Auma. "His [Barack's] father was never a Muslim although he was born into a Muslim family with a Muslim name. He [Barack] himself was never a Muslim."

There have also been insinuations that Barack has somehow neglected Mama Sarah, as she does not have access to mains electricity or running water. Yet she has solar power, a water pump in the garden and a solid, brick house, making her far better off than most people in the area. And though she recently received a 21-inch flatscreen television from friends so that she could follow the presidential campaign, one sensed she would have been equally content listening to the news on local radio.

Yes, she will attend the presidential inauguration, if it comes to that, she said, but nobody should expect her to live in America or change her lifestyle.

"What should I do?" she said with a laugh. "Become a young woman and start all over again?"