In the 1960s, the popular wisdom about man's origins was that our ancestors had been hunters. It was a splendid, romantic image: early man exploring the plains of Africa, going after big, dangerous animals, bringing them down, taking hunks of meat to take home to feed the women and children. Something almost seductive there. More appealing still was that it matched how we saw ourselves as a species then: fearless explorers reaching to the moon and beyond.
But as a theory, "Man the Hunter" failed to explain something crucial. Hunting was a male activity. Why should the evolution of a species -- its females too -- be driven by what males did? After all, today women's brains are just as large as men's. What were our female ancestors doing when the men were hunting, and how could it be that it did not contribute to evolution at all?
Thing is, women had never been important players in the reconstructions of our past. Artists' renderings of the life of early humans had females nursing their young, or digging for roots, or other equally insignificant pursuits.
But were they really so insignificant?
In the '70s, a whole new theory answered that. Feminist anthropologists suggested that the life of early humans revolved around the mother and her child. The female gathered food -- eggs, roots, honey, nuts -- and developed tools to help. She shared the food with her young and others in the family. This was the main source of nutrition. What of hunting? It was secondary at best, because it could never bring in a reliable, steady supply of food.
So rather than hunting by males, it was gathering by females that drove the evolution of man, the increase in brain size and intelligence. Or so this theory said.
And of course, it went well with the feminist temper of the '70s.
But eventually, there were questions here too. By replacing a bias in favour of one sex with a bias in favour of the other, how do we advance understanding of our origins? If male hunters could not satisfactorily explain how humans developed intelligence, why should female gatherers be any more satisfactory an explanation?
Naturally, there have been subsequent theories too. Much like human beings, theories about human beings also evolve. Yet in that very evolution are some subtle lessons.
One lesson is in how the theories reflect contemporary times: the "Killer Ape" in the aftermath of the massacres of the World Wars, "Man the Hunter" in the space-exploring '60s, "Woman the Gatherer" during the feminism of the '70s. We think we see the past when we dig up fossils. We try to assign meaning to them that will tell us what that past was like. Yet meaning is rooted in the present day, in modern settings. Inevitably, our ideas of the past are coloured by our ideas of the present.
An example of what I am talking about: some palaeontologists have remarked on the near-perfect symmetry of stone "axes" found in prehistoric sites. They speculated that when attached to a wooden handle, the symmetry makes the axe easier to balance and wield, and that's why they are so symmetric.
A thoroughly sensible hypothesis. But also a thoroughly modern one.
We know that modern axes have such handles, that they would benefit from such symmetry. But nothing tells us that primitive axes had handles. Of course, any wooden handles they might have had would have crumbled away long ago, with no trace that they ever existed. But the point is that we have no evidence of handles. The only reason palaeontologists remark on the symmetry is that they know about modern axes and their use.
By itself, the primitive axe says very little: not even that it is an axe at all.
What do we learn here? That when scientists propose theories, they must look hard at every one of the assumptions their theories are built on. They must critically evaluate whether those assumptions fit the evidence. The ones that do not fit -- like those axe handles -- must be discarded.
That is, the primary task of a scientist trying to explain evidence is to identify, understand and discard what he does not know to be true, even if it is plausible. What's left after this process might be nothing at all. On the other hand, it might be a much stronger, truer theory.
In palaeontology, this leads to another lesson about pronouncing on the past. Put forward most vocally by Lewis Binford, a controversial, argumentative, unpopular but influential palaeontologist at the University of Chicago, this lesson says: it is wrong to assume that everything found close to obviously human artifacts is therefore a result of human activity.
For example, the "Killer Ape" theory of our origins was essentially based on artifacts found at one site. There were human-like bones, bones of other animals including 42 crushed baboon skulls, and many tools. This much led Raymond Dart to suggest that our ancestors had battered victims to death with huge clubs.
But Binford would ask: why should we assume that it was the early men who had killed the baboons? There are various other ways these bones might have come together. They might have been brought there by a stream; or collected by other carnivores.
In fact, a later study showed that it was possible that a leopard, sitting in the branches of a tree and feeding on both baboons and ancient men, had dropped bones below. Forget being a Killer Ape eating animals on the plains -- our ancestors had been among the eaten!
Understanding what life was like for our ancestors is hard work. Digging up fossils and bones is easy, but making sense of them is much harder. Does this mean those theories were worthless? Not at all. Each teaches us new techniques, tells us more about our origins. Most important, each rids us of more of our ignorance. We may never have a full picture of how our ancestors lived. But we'll get closer all the time, and on the way we'll learn a lot about ourselves.
"The question 'Is this true?' doesn't lead anywhere," Lewis Binford once said. "The question to ask is 'Does this open up new learning opportunities?'"