It was mid-morning one day in 1967 when Richard Leakey first flew along the eastern shores of Lake Turkana. Below lay a tangle of blackened sandstone layers that looked like the slag heaps of a coal pit.

Leakey asked the pilot to circle. The single-engined plane turned out over the water beyond the sandy spit of Koobi Fora, to make another pass at the height of one thousand feet.

A camera could only have recorded what was there. Leakey saw things differently, however – not exactly an hallucination but rather an inspired guess, a vision of the distant past as it might have been.

First he saw graceful trees and rolling grasslands where other vegetation also grew. Mischievous monkeys gamboled in the thick foliage. Strange elephant-like pachyderms with shortened trucks and thicker but smaller tusks browsed quietly. Nearby some hairy peoples, small but upright, with squat low foreheads, chattered amicably in one group; in another workmen banged and chipped stones to fashion new tools.

Then rivers cut through the grasslands in Leakey's inner-eye, waters swift and clear. Eden was spread out before him.

Even now the forces which shaped Africa's Great Rift Valley are not fully understood. The fault was caused by upheavals immense enough to result in a valley that is sometimes thirty miles wide and as much as 2,000 feet deep. It was first investigated by a young Scot, John Walter Gregory. He marched up to Baringo from Mombasa in 1893 and, hammering out samples of different rock layers, he returned to Britain to proclaim his conclusions – that the 3,500-mile long valley was formed "by the rock sinking in mass, while the adjacent land remained stationary."

Gregory gave it the name of the Great Rift Valley. But it is the name with which it was endowed by Suess which best captures the imagination: graben, deriving from grabe, the grave. In the light of what has been found since, no other image could have been more exact.

All along this majestic flaw, in the walls of its great scarps and on or just beneath the surface of its floor, lies evidence of mankind's beginnings. Much of it has yet to be discovered. The assumption that it exists is based on what has been revealed so far at the two most significant sites in palaeontological history – Olduvai Gorge in Tanzania's Rift Valley and Koobi Fora, Kenya.

Synonymous with both sites is the name of Leakey. Early in the 1960's, Drs. Louis and Mary Leakey found the 1.8-million-year-old fossil remains of a creature they named Homo habilis which means "Handy Man." The bones were in the wall of Olduvai Gorge, which lies between Lake Natron and Lake Eyasi.

Fewer than ten years later, Louis Leakey's son Richard was summoned by his friend Kamoya Kimeu to a spot on the outlying edges of the Lake Turkana excavations. Richard spent weeks piecing together thirty or more fossil fragments. The result of this painstaking work was another skull similar to that of Homo habilis but dating back almost three million years. [Recent geological work dates the find to 1.9 million years old.]

Thereafter, Leakey was often at Koobi Fora, 500 miles north of Nairobi, until a debilitating illness forced him to spend less time there.

When he left school Richard Leakey rebelled against his strong-willed father and vowed he would "never be a paleontologist." Yet surveying Lake Natron in northern Tanzania three years later he found himself unable to resist the tell-tale signs of another rich fossil ground. His parents encouraged him to explore the sedimentary beds and at once Richard struck a fossil lode. In 1964 he assembled a fragment of an Australopithecus boisei, an early form of hominid, which had been found by his friend Kamoya.

Trapped once again by the Leakey "manhunting fever" Richard set off for London to complete his formal education catching up on two years of studies in seven months and qualifying for university. But with nine months to wait before the new term opened, he decided to return to Kenya.

There, at the age of 23, he was appointed co-leader of an expedition into southern Ethiopia's Omo Valley. Teams from America, France and Kenya had been allowed to explore rich fossil beds on the river's lower reaches. Leakey joined them and uncovered animal fossils at least four million years old. He also found two Homo sapiens skulls which suggested that modern man may have existed as long as 100,000 years ago.

Returning to the site after a visit to Nairobi his pilot flew along Lake Turkana's eastern shores to avoid bad weather. The change of route was another case of "Leakey's luck." Borrowing a helicopter from the American Omo Valley team, Richard returned to the sandstone hills near Allia Bay to check on what his inspired vision had promised. The helicopter blades had barely stopped whirling when Leakey picked up a stone-age tool similar to those he had found in Olduvai Gorge as a youngster.

Within months Richard Leakey had assumed the Directorship of Kenya's National Museums and had begun the exploration which was to confirm Koobi Fora's potential as the world's richest treasure-trove of fossils of early hominids.

The tectonic movements which formed the Great Rift Valley established the right formula for preserving fossil bones. Africa's convulsions were spaced over thousands of years, each spasm causing lakes to form and waters to flow along fresh rills and over new-born plains. Each wrack laid down new sediments from these waters, rising in tiers like different fillings in an exotic cake. Tightly compressed in their shrouds of calcium carbonate, within each tier lay the skeletons of several species including the remains of mankind's ancestors.

Initial discoveries in the Koobi Fora fossil beds are made by surface prospecting – locating areas where erosion has left bones and teeth exposed.

The Rift Valley strata have many prehistoric relics of major importance lying open on the ground. Richard Leakey recounts his discovery of the area's first Australopithecus fossil: "There on the sand twenty feet ahead, in full view beside a thorny bush, lay a domed grayish-white object. Halfway to it I sat downed stunned, incredulous, staring. For years I had dreamed of such a prize, and now I have found it – the nearly complete skull of an early hominid.

Fossil recovery at Koobi Fora is an intricate and delicate task. When a discovery is made a geologist visits the site to determine the exact "stratigraphic" level in which the fossil lies. This helps to assess the relic's age accurately. The prospectors then scrape down the entire surrounding surface and put it through a sieve in the hope of recovering more fragments. They also keep an eye open for any that might be held tight in the rock.

Different techniques are used to recover fossils that are partly or entirely embedded in rock. The tools are dental picks and brushes made from camel's hair. The samples are so delicate that they can fall apart if touched by the slightest wind. To prevent further disintegration, the scientists treat their samples with preservative fluid. Even this has its risks: the impact of a falling drop can break a fossil.

The multi-disciplinary approach which has quickened the pace of evaluation and reconstruction at Koobi Fora includes the relatively new science of taphonomy – the study of what happens to organisms after their death.

Anatomy is another important science. It helps in studying bone formations to find similarities between early and modern man. Thus we know that Homo erectus [and all hominids] walked on two legs. He was a biped who had some anatomical mechanisms similar to modern man.

Other scientists reconstruct ancient camp sites and learn how to make and use stone-age tools. Others still test sedimentary layers and fossil content to determine an accurate age for each find.

The richest era of discovery dates between three million and one million years ago. In the past decade [the 1970’s] more than 160 fossil remains of early hominids, more than 4,000 fossil specimens of mammals, and numerous stone-age artifacts have been recovered. Other remains include fish, turtle, tortoise and crocodile species. It is not just the fossils which have given Koobi Fora significance. The sedimentary layers in which the fossils were buried have yielded important evidence of the environment three million years ago and of the animals and plants with which early man and pre-man shared the world.

Leakey believes the layers have provided enough evidence to establish that Lake Turkana's shore were once a well-watered verdant land of forest and grass blessed with an abundance of good things. The plains teemed with wild animals including prehistoric elephants, three-toed ancestors of the horse, sabre-toothed cats, antelopes, giant baboons, rhino-sized pigs, ostriches and many other ancestors of modern species.

And what of the "missing link", that mythical half-ape, half-man? Richard's father, Louis Leakey, said there was no single missing link. In the search for mankind's origins he believed there would be many. Perhaps this is the case; but each time Richard flies over Koobi Fora one thought nags him: "Somewhere down there lies the key."

Until that key is found – if it exists – there are other clues to concern us. If we can learn much from the fossil beds, we can also learn from the nomadic communities who live around the lake's shores – people who have adapted magnificently to their harsh environment and in whose circumstances are to be found reminders of our own beginnings.