Our ancient ancestors: When did they become human?

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.