Events are sometimes organized at which thousands of people hold hands and form a human chain, say from coast to coast in the US, in aid of some cause or charity. Let us imagine setting one up along the equator, across the width of our ‘home continent’ Africa. It is a special kind of chain, involving parents and children, and we have to play tricks with time in order to imagine it. You stand on the shore of the Indian Ocean in southern Somalia, facing north, and in your left hand you hold the right hand of your mother. In turn she holds the hand of her mother, your grandmother. Your grandmother holds her mother’s hand, and so on. The chain wends its way up the beach, into the arid scrubland and westwards towards the Kenya border.
How far do we have to go until we reach our common ancestor with the chimpanzees? It is a surprisingly short way. Allowing one yard per person, we arrive at the ancestor we share with chimpanzees in under 300 miles. We have hardly started to cross the continent; we are still not half way to the Great Rift Valley. The ancestor is standing well to the east of Mount Kenya, and holding in her hand an entire chain of her lineal descendents, culminating in you standing on the Somali beach…
The daughter that she is holding by her right hand is the one from whom we are descended. Now the arch-ancestress turns eastward to face the coast, and with her left hand grasps her other daughter, the one from whom the chimpanzees are descended (or son, of course). The two sisters are facing one another, and each holding their mother by the hand. Now the second daughter, the chimpanzee ancestress, holds her daughter’s hand, and a new chain is formed, proceeding backwards towards the coast. First cousin faces first cousin, second cousin faces second cousin, and so on. By the time the double-back chain has reached the coast again, it consists of modern chimpanzees. You are face to face with your chimpanzee cousin, and you are joined to her by an unbroken chain of mothers holding hands with daughters.
If you walked up the line like an inspecting general - past Homo erectus, Homo habilis, perhaps Australopithecus afarensis - and down again the other side, you would nowhere find any sharp discontinuity. Daughters would resemble their mothers just as much (or as little) as they always do. Mothers would love daughters, and feel affinity with them, just as they always do. And this hand-in-hand continuum, joining us seamlessly to chimpanzees, is so short that it barely makes it past the hinterland of Africa, the mother continent.