Clinging to this capricious surface are two peoples who seem to have imbibed the spirit of the land. They live and die like Siamese twins who hate each other. The Hutus and Tutsis are inextricably bound together by history, culture, language and the struggle for survival in the most densely populated countries in Africa. Hardly an inch of cultivable land is unused and the two groups live hut by hut on the same steep hills.
The boundaries of Rwanda, Burundi, Tanzania, Uganda and Zaire were imposed from Europe. Now the ethnic division, not only in Rwanda and Burundi, but in eastern Zaire and south-west Uganda, is tearing the region apart. Periodically in the past the Tutsis have massacred Hutus in their thousands. In the last year the Hutus have turned on their former masters. Somehow nothing can explain such hatred. In scale it is far worse than anything in Bosnia or Somalia. Its volcanic brutality, its mass murder - by hand - of thousands of men, women and children has no contemporary parallel. It is mass madness.
The traditional explanation for this genocide is that the Hutu farmers, ethnically Bantu people, lived there first and were invaded by cattle-keeping Tutsis, Hamitic people from the north who, though inferior in numbers, came to dominate and enslave the Hutu. In their stereotypes they are as different as Scandinavians and Greeks.
The Hutu tend to be round- faced and short, the Tutsi tall, long-faced and lighter-skinned. Some anthropologists, however, believe that they emerged from one people and an aristocracy grew by a process of self- selection, always marrying similar types. Cattle became a symbol of their dominance.
Whatever the explanation, the two groups have mixed so much that it is difficult for outsiders to tell the difference. But the gangs of killers armed with machetes and sharpened bamboo spears on the streets of Kigali think they can. The last scream of many people being chopped to death, however, has been that they are of the same people as their killers.
Visiting a colline - a hill which is also a village chiefdom - in Burundi in February, I got a taste of the callous contempt of the Tutsi overlords as well as the mistrust and fear and the fierce desire for revenge of the Hutu. The village had been the scene of horrific killings a short time before. No one was able to explain how it started and people told me miserably that there had been no local tension. The army, which is now exclusively Tutsi, had swept into the village and murdered scores of Hutu. The soldiers were now protecting the surviving Tutsis in a school on top of the hill. A nun who was a Hutu bravely took me to interview them and act as translator. Her lip quivered as she spoke to the commander, a tall languid man in a green tracksuit. When I asked what had happened he dismissed the question with an arrogant: 'Why do you want to know?'
When the Belgians took over the two kingdoms as protectorates from the Germans after the First World War, they added them to their empire of the Congo basin and reinforced the existing social structure. They left in place the monarchies of Rwanda Urundi, as the region was then called, and concentrated on giving Western education to the Tutsis, whom they regarded as more capable and intelligent than the Hutus. The Hutus remained peasants and serfs, even though they made up about 85 per cent of the population.
No one else paid much attention to these tiny countries. Rwanda and Burundi are at the back end of Zaire, the bottom tip of Uganda, the north-westernmost tip of Tanzania. If they had any strategic importance or produced any precious minerals there might have been more Western interest, but possessing neither they remained two insignificant countries, renowned only for gorillas.
In Burundi the Tutsis remained in power. Virtually all the university graduates, professional classes and army are Tutsis. When multi-party politics led to the end of the Tutsi- dominated one-party state and the election in 1993 of Melchior Ndadaye, the first Hutu president, the Tutsi army officers mounted a coup and killed him. In revenge the Hutus turned on their Tutsi neighbours. About 250,000 people are estimated to have been killed.
In Rwanda the Hutu gained political power after independence. The subsequent revolution in society led to massacres. Hundreds of thousands of Tutsis fled to neighbouring countries, where they have lived ever since. In eastern Zaire the Banya Rwanda gradually established themselves as successful farmers and tradesmen. The local people, the Bahunde and other traditional farmers, grew jealous and felt threatened by a Banya Rwanda takeover. Ethnic war started there last year and still continues.
In Uganda the Rwandese exiles, squatting on precious scarce land, also became a target for local jealousy. They became caught up in Uganda's local politics. A division similar to the Hutu-Tutsi divide exists among some ethnic groups of south-west Uganda.
Yoweri Museveni, the president, comes from an ethnic group which is the local equivalent of the Tutsis. He is a Muhima, one of the aristocratic cattle-keepers. When he began his guerrilla war against the Uganda government in 1981 he was joined by many Tutsi exiles. The Obote government turned on the Rwandese exiles. Hundreds were killed or driven from their homes. Two of Museveni's top commanders were Rwandese. One of them, Fred Rwigema, became his army chief after his victory in 1986. Four years later Rwigema and other Rwandese exiles secretly formed an army to invade their homeland. About 10,000 fighters of the Rwanda Patriotic Front swarmed across the Uganda border and, although Rwigema was killed, they fought their way to the capital, Kigali. Here they were held up by government troops backed by Belgian and French paratroopers, but the country was already in turmoil. Hundreds of thousands of refugees from the predominantly Hutu northern parts of Rwanda fled south.
The RPF, a largely Tutsi army, looked and behaved like a real army compared to the scruffy, ill-disciplined Rwanda soldiers. It could have taken over the capital, but it wanted only a share of power.
After protracted negotiations, a ceasefire was agreed last year in the Tanzanian town of Arusha, which committed President Juvenale Habyarimana to forming a coalition transitional government. But Habyarimana, who had been a dictator for more than 20 years, was unwilling to diminish his power, and cynically delayed the formation of a new government by a variety of trivial diversions. The political temperature rose. Looking across the border at the massacres in Burundi last year, Habyarimana could not have been ignorant of the price of political breakdown. Rwanda is now paying that price - and its first instalment was his own death.
In the anarchy which has followed and which may spark off the war in Burundi again, no one will ever discover who fired the missile which brought down the plane and killed the two countries' presidents last week. What is now certain is that the leadership of these two countries is incapable of working out a sustainable political solution to the fundamental problem of statehood.
In both cases the UN has tried to assist the leaders to reach agreement, but the frustration of the UN representatives in both countries has been palpable. They raged at the deviousness and small-mindedness of many of the politicians in Rwanda and Burundi and their inability to keep to agreements, or turn up at meetings.
After the catastrophe of Somalia it is unlikely that the UN will be willing to step into these two tiny countries, but nothing else can save the lives of thousands, possibly millions, of people who will otherwise die by violence or hunger in the weeks to come.