Again from Lucy: The Beginnings of Humankind today, Don Johanson describes his thoughts upon the question of whether to place the Hadar jaw remains (later attributed to Au. afarensis) in the genus Homo:
With a kind of mental blink I acknowledged that I had arrived at the point where other anthropologists before me had arrived, where each had had to ask the old question: where did one draw the line?
I had never had to ask myself that question. I had had no fossils of my own to force it on me. Now, for the first time, I became aware of what a problem it might have been for others to draw the ancestral human thread thinner and thinner, back farther and farther, until it was not a human thread at all. This could be an emotional problem as well as a scientific one. The investigator would find himself pressing ever deeper to pass beyond the origins of man, and yet not be able to give man up. There would always be a human ancestor: a little older, a little more primitive---but still human.
I really like that passage. It appears immediately after Johanson recounts a long conversation with Richard Leakey, during which both ponder the significance of Homo-like characters and the Leakeys’ discovery of the KNM-ER 1470 skull.
To be honest, really only Louis Leakey, Phillip Tobias and John Napier had faced the same question before in the same way, with their diagnosis of Homo habilis in 1964. In every previous discovery, the discoverers (Dubois, Dart, Broom, Black, von Koenigswald) had declined to extend the thread of Homo into a more primitive fossil form, instead naming a new genus for each primitive hominin they found: Pithecanthropus, Australopithecus, Paranthropus, Sinanthropus, Meganthropus, even Zinjanthropus. Leakey, Napier and Tobias for the first time deliberately extended the genus, embracing fossils with brain sizes smaller than any normal living human (“Leakey, Tobias, and Napier on the definition of our genus”).
In light of Johanson’s later narrative, his text here might be read to criticize the Leakeys for their “emotional problem” pressing Homo deeper than it should really go. But I read it straight. It’s a problem we inevitably face when considering the evolution of our own genus. We might cast the diagnosis of Homo habilis today as a radical act. Homo must have a beginning, one that Darwinian mechanisms will inevitably allow us to see only in a very foggy outline.
Those of us who have been involved in the interpretation of fossils found after the 1970s have faced just this question, for some of the same reasons as Johanson.