The tree of our species has many thousands of branches, almost all of which withered and died. To date, palaeontologists have invented countless names for the bone fragments they have pulled from the ground: Homo habilis, Zinjanthropus, Rampithecus, to name but three more. Primatologists, meanwhile, take our origins back past the bonobo, to squirrels and beyond. So, why pick the emergence of Homo erectus from Australopithecus as due any special note? Is there anything, moreover, to be learnt from our ancestors of more than 80,000 generations ago?
"There is a current trend to try to find the origins of the human mind on the Pleistocene savannah," the dental expert Walker cautioned me. "And I have this perennial question: why then? Why not go back to a fish ancestor, or your grandmother, or a few generations ago. Why pick that as the ancestral thing you are interested in?"
The answer, I suppose, is the speed of the change and the depth of the impact it produced. When Emma Mbua, curator of hominids at the Nairobi museum, showed me a selection of famous skulls, I could see straight away that Homo erectus represented a quantum jump. And measured on the clock of evolution, the 200,000 years or so that it spent diverging from its predecessor was so rapid that it almost makes you want to talk about the day we came down from the trees.
Bones, however, have little to say about the way that their owners behaved - allowing images to be conjured about them that are driven by more recent preoccupations. All of the Australopithecus finds have been in Africa (the latest in Chad) and, until Homo erectus specimens turned up in Asia, fossils were exclusively retrieved by white adventurers in some past or actual European colony. Dart, an Australian working in Johannesburg, made his name during apartheid's construction. The Leakeys have been prime exponents of white settlers controlling the sites. Even American expeditions in Ethiopia have had a peculiarly imperialist feel.
The killer ape narrative appealed to such folk, for whom the most sophisticated scientific techniques were deployed on fixing their beloved Land Rovers. It was almost as if through palaeontology they felt they could prove objectively that their kind's supremacy was historically preordained. If old Australopithecus murdered his way out of the trees, it could be argued, how natural and welcome is civilisation's advance to the polite domination of today?
It was Ardrey, once more, who got the words down - and in whose footsteps I might almost literally have trod on my journey around the sites. The fossil hunters and zoologists were like two "wings" of a "revolution", he wrote, which dovetailed into a third: "The African independence movements are rapidly converting a continent into something approaching a political state of nature, where primitive human behaviour may be observed not as we should wish it to be, but as it is."
Attitudes are changing, however, and science is mirroring that. In the same cultural shifts that see aboriginal peoples probed for ancient wisdoms, so Homo erectus is being rehabilitated as something more than a beast. While lonely skulls and stone tools once stressed individualism, large-scale land surveys and evidence of fire now point to a community life. And we now see that it was not aggression, but fear which dominated our earliest lives. Fear, which a philosopher might see as the source of all human evil, is the default emotion of our evolutionary line. It just never went away.
That archaeology and other sciences should now offer us such notions, may say as much about ourselves today as it does about our ancient past. Hardly more than a generation ago, in the heady euphoria of the booming 1960s, the image of the times was the ultimate weapon: the mushrooming nuclear bomb. In the pessimistic, economically fragile 1990s, another image is taking hold in our collective mind: the secure, life-giving tree.
As our planet's atmosphere sours, as it did for other reasons before, and the last tatters of the primeval forest are threatened with destruction, maybe we see something of our present predicament in the melancholy, scavenging way of life that Homo erectus really knew. "Humans are losing confidence," Robert Foley, director of the Duckworth Laboratory at Cambridge University, pointed out to me. "We have started to see ourselves as the victims, rather than the masters, of nature."
Some sense of that loss was weighing on me when I drove on the plain that Sunday. Our origins, of course, is a story of winners. But my emotion - maybe a genetic vibration - was with all those branches of our evolutionary tree that shrivelled. In the fickle, shifting, African mosaic, there were always more losers than winners. Some perished when their rivers turned to mud and then dust. Others were caught by cats. Some were too hairy to stay cool in the day. Some so brainless they simply got lost.
From the wheel of my red Hyundai, I looked across the yellow savannah that afternoon and felt for a moment that I was seeing this landscape as one of these hominids might have done. I wondered if, two million years ago, as the nutritious, protective forest dwindled to islands in the grass, maybe he and his band had made a break from a patch of wilting, fruitless trees. Thirsty, hungry, confused and frightened, they had set off for the empty horizon in search of food and shade.
But they had started too late, had found no water and as the sun rose above them, they had slumped, defeated, in the merciless tropical heat.
They never fell victim to killer apes. Like us, they were nature's prey.