Are human fossils our ancestors?
New Optimist Jack Cohen — also reproductive biologist and Honorary Discworld Wizard — has sent me his surprising response to a recent Nature article, The evolutionary context of the first hominins by Bernard Wood and Terry Harrison. Here it is:
Are human fossils are ancestors? If it seems obvious to you that the answer is Of course they are! reflect for a moment about the 1924 discovery of an ancient 3-year-old child’s skull in South Africa, named Australopithecus africanus by Raymond Dart. Obviously, this Taung child was nobody’s ancestor! But was its species ancestral to ours?
This is a very interesting question, and posed by Wood and Harrison (see Nature, 17th February 2011, Review pp 347-352). The ancestry of man is not a ‘straight line’, with fossil species representing steps on the ladder towards us. It is like a bush, various species representing side branches. It can’t be that each of these earlier species, occasional fossils, were on the road to us, can it?
Let’s look at what these species were. There are, most recently, Homo neanderthalensis (Neanderthal Man) and Homo ‘denisova’ (a finger-bone found in the Denisova cave in Siberia, 48-30, 000 years old), and the little ‘hobbits’ (Homo floresiensis, fossils found very recently on the island of Flores, tiny people) and archaic Homo sapiens of course originally living in Africa and the ancestors of all of us.
A million years earlier Homo erectus had erupted from Africa into Asia: from the Turkana Boy (probably better considered as Homo ergaster, 2.8 million years old) through to Java Man, just over a million years old. They had Palaeolithic culture, quite well-fashioned tools, and fire. This was successor to Homo habilis in Africa, without fire, poor tools, a small brain. Sitting below these are Australopithecus species, from the Taung skull to Lucy and those delightful footprints. Some of them were ‘robust’ forms, Paranthropus (‘Nutcracker Man’ was one) many were ‘gracile’, more like bonobos (‘pygmy chimps) or indeed us (still ‘gracile’ unless too tempted by today’s plentiful supplies of fat and sugar).
Hold on a moment, this is the time to emphasise that the “ape-like ancestor of man” can equally well be called “the man-like ancestor of the apes”! (Much better than “we came from the apes”; apes are our cousins, not our ancestors.)
Apes have diverged as much from that common ancestor as we have.
Our split from the chimps was about seven million years ago, from the gorilla perhaps ten million, from the orangs 15-17 million years ago. Ardepithicus, Orrorin, and Sahelanthropus are three of the earliest fossil ‘men’. A. ramidus at about 5 million years is a possible “stem” hominin – i.e. close to the stem of the bush. But the problem is that creatures often develop similarly after a split. Perhaps they are evolving in response to a common problem, like the forest giving way to grassy plains, so that everybody has to learn to stand upright …
A case of this kind that Wood and Harrison make is the Oreopithecus, who was probably a swamp dweller at the time of the split; not unreasonably, it seems to have been walking upright-ish – because it was up to its waist in water! But, although labelled a hominid, it wasn’t; it was almost certainly an aberrant form, with no interesting descendants.
But a good case has been made that human(oid)s spent some considerable time on the sea-shore, wading in the sea and catching and eating sea-food that gave us surplus essential fatty acids, which we needed to build our big brains. Perhaps some Oreopithecus wandered into the sea . . . No, there’s too good a case for those australopithecines being on the road to us. The marine episode must have come much later, when our brains were enlarging.
There’s a wholly different issue, that’s very relevant and not in the Wood and Harrison article. Perhaps all these forms were interbreeding, more like one species than like a flock of them! Modern DNA testing tells us than most non-Africans have about 4% Neanderthal genomes, and, as it’s been discovered New Guinea people have 4.8% ‘denisova’ genomes, it seems highly likely that so, too, will other Asian peoples.
Perhaps all these forms were interbreeding, because people – and our ancestors – are/were like that! If so, we’ll never discover – even with the best DNA data – just who were the Adam and Eve.
After all, as you go back the number of ancestors increases (two parents, four grandparents, and so on . . . till you include everyone in the species that had progeny) so it’s possible that all of these species were somewhat ancestral to us!