The discovery of a possible ape-hominid transitional fossil, Australopithecus sediba, is of course incredibly interesting. It may be the earliest direct ancestor of modern humans yet discovered. It hints interestingly that humanity’s first steps – or, rather, first bipedal steps – in southern Africa rather than, as previously suspected, east Africa. It gives new and exciting insights into the evolution of humanity’s anatomy and brain structure.
What it is not, however, is a “missing link”.
It’s a term that many newspapers, including this one, love because it is so recognisable and freighted with a century and a half of meaning. It’s also used by creationists and other people ignorant of evolutionary biology to suggest that the fossil record shows gaps. It’s just a shame it’s complete nonsense.
The problem is that it implies that something is missing. I suppose in one sense something is: the fossils of 99.99999 per cent of all the animals and plants that have ever lived. But in the specific sense of “a fossil that is neither entirely an ape nor entirely a human but somewhere between the two”, there are lots and lots. Wikipedia, the journalist’s friend, has a wonderful list of so-called “transitional fossils” (my favourite is Tiktaalik, the halfway house between a fish and an amphibian), and in its “Human evolution” section, it lists:
• Ardipithecus, “Intermediate between the last common ancestor of chimps and humans, and the australopithecines”
• Australopithecus, “Intermediate between extinct quadrupedal and bipedal apes. While the relationship between some species are being revised, Australopithecus afarensis is considered to be, by most experts, the ancestor to all later hominids.”
• Homo habilis, “Perfect intermediate between early hominids and later humans, possibly ancestral to modern humans.”
• Homo erectus, “Ancestral to modern humans and neanderthals.”
Where among these do the apes stop and the humans begin? Some want to call Australopithecus a human, some an ape, although as you can tell from the Latin names (Australopithecus, “southern ape”; Homo habilis, “handy man”) the consensus is the latter. There is a serious campaign to get modern chimpanzees reclassified into the Homo genus, because they’re so similar to us; the line between humanity and ape is completely arbitrary. Australopithecus sideba is later than Australopithecus africanus, and shares many features with early Homo. So it is, if you must, the missing link between Australopithecus africanus and Homo habilis.
But now we have a problem. There’s a missing link between Australopithecus sediba and Homo habilis! And – look – another between Australopithecus africanus and sediba! Now there are TWO gaps!
This point has been made before, but it’s a serious one. The fossil record is remarkably full, given the hugely unlikely set of circumstances required to create a fossil. Humanity, being essentially a dry-land species, is inconsiderately prone to not dying in the wet places that lend themselves to fossilisation. And yet we have a patchy but highly revealing fossil history of humanity’s primate ancestors going back 36 million years, to before the split between Old World and New World monkeys.
They’re called “transitional fossils”. But even that, I would suggest, is a slightly misleading term. Every transitional fossil was a fully operational organism, not a prototype for a later model. Australopithecus sediba did not wander around waiting to become Homo habilis, it just went about its day, eating, mating and using for-its-time revolutionary new tools, like sticks. And every organism is transitional, because species are not fixed: we feel like we’re pretty set as humans, but our descendants in 2.33 million years, assuming we leave any, may be as different from us as we are from A. sediba.
So by all means shout about the discovery of Australopithecus sediba. It’s fascinating and wonderful. But for pity’s sake don’t call it a “missing link”.