Anthropologists study humanity and our evolution, but they don't agree on when we crossed over the threshold to status as full "human beings."
The question crops up often, as it did this month over some new scientific articles describing 2-million-year-old fossils of humanlike creatures from South Africa.
Called Australopithecus sediba, members of this group had brains little bigger than those of chimps, but they walked upright and possessed humanlike hands - which scientists say looked as if they were good at making and using tools.
If they were good tool users, they might need to be renamed as homo something - a member of our wider group, the genus Homo, which is Latin for human. The earliest known member of this group, Homo habilis, lived 1.8 million years ago and earned this qualification because their remains were found along with stone tools.
Does that make them human?
In the popular imagination, humans are often separated from other living things by some barrier or "missing link," said Princeton University anthropologist Alan Mann.
In scientific circles, the definition of humanity is ultimately an arbitrary one, since the answer depends on what traits we decide make up a human. Some consider it a matter of brain size and tool use, while others use evidence of artistic expression and language.
On Thursday, in a talk titled "What Does It Mean to Be Human?", Mann will contrast our special self-image with the scientific evidence. The event, at 7 p.m. at the Free Library of Philadelphia, is sponsored by the Freethought Society.
A main source of the idea that we humans are above the rest of the living world is religion. Even religions that accept evolution espouse a kind of human exceptionalism.
In spring, for example, Pope Benedict XVI said that the Catholic faith should accept evolution, but that natural processes can't explain the human mind: "It is not the case that in the expanding universe, at a late stage, in some tiny corner of the cosmos, there evolved randomly some species of living being capable of reasoning and of trying to find rationality within creation, or to bring rationality into it."
Mann points out that our self-image as "higher" beings crops up in popular culture as well. The apelike creatures in 2001: A Space Odyssey must touch a mysterious monolith to become transformed into humans.
One problem with this belief in a phoenixlike change, said Mann, is that "there would have to be a generation of humans whose parents were not human."
Not only is that scenario weird, but also there's no evidence that it happened. The scientific story shows a gradual transition, with different features linked to humanity originating at different times.
There are several traits that anthropologists tend to use to characterize humanity, said University of Wisconsin anthropologist John Hawks. One is tool use, which goes back about 2.5 million years. Another is brain size. The relatively new South African fossils of Australopithecus sediba have some but not all of these human qualities. They had small brains but very human hands, and their remains may yet be linked to tools.
When it comes to the brain, size isn't everything, said Columbia University anthropologist Ralph Holloway. He uses casts of fossilized skulls to show that after our lineage split from that of chimps, our ancestors' brains stayed the same size but showed signs of being reorganized to a more human configuration.
Long before our lineage qualified for the genus Homo, for example, the association cortex, which helps identify objects, moved to a different site and became more developed.
Other experts are more particular about defining humans as users of symbolic communication - language and art. But that tighter set of qualifications probably does not exclude Neanderthals, said Princeton's Mann.
They are clearly members of the genus Homo, having branched off from our ancestors 400,000 years ago. Though there's new genetic evidence showing they interbred with our ancestors, they are still considered a separate lineage that went extinct about 25,000 years ago.
There's also evidence they decorated themselves with pigments and feathers, similar to what our more direct ancestors did in Africa starting 80,000 years ago, said Mann.
Homo sapiens officially appeared in Africa around 200,000 years ago, he said, and yet the evidence for more-elaborate art doesn't start until 37,000 years ago.
Those first art objects take the form of headless female carvings known as Venus figurines as well as a small figure of a human body with a lion's head. It's assumed they were made by modern humans and not Neanderthals, but there's no proof of that, Mann said.
And even scientists can make judgments based on looks, said anthropologist Janet Monge of the University of Pennsylvania Museum of Archaeology and Anthropology.
The brow ridges and receding chins that characterize Neanderthal faces make some people reluctant to include them in the human family. And yet, if you look at members of our own lineage, before about 60,000 years ago, they looked weird, too. Some had brow ridges and other primitive-looking features. The only distinction, Monge said, is that Neanderthal features were all different in the same way, whereas our African ancestors differed from us in many ways.
It's true that it was our ancestors, modern Homo sapiens, who made the elaborate cave paintings of about 32,000 years ago found in Europe. Neanderthals may never have achieved that level of artistic genius, but Hawks questions whether this should be used to define humanity.
If humanness requires the talent to create such masterful paintings, after all, most of us wouldn't qualify.