What makes us human? In a step toward finding biological answers, scientists have deciphered the DNA of the chimpanzee (search), our closest living relative, and made comprehensive comparisons to the human genetic blueprint.

There are no firm answers yet about how humans picked up key traits such as walking upright and developing complex language. But the work has produced a long list of DNA (search) differences with the chimp and some hints about which ones might be crucial.

"We've got the catalog, now we just have to figure it out," said Dr. Robert Waterston of the University of Washington School of Medicine (search) in Seattle. "It's not going to be one gene. It's going to be an accumulation of changes."

He is senior author of one of several related papers appearing in Thursday's issue of the journal Nature and being published online Thursday by the journal Science.

In one paper, Waterston and colleagues present a draft of the newly deciphered sequence of the chimp genome, for which scientists identified virtually all the roughly 3 billion building blocks of chimp DNA.

"It's a huge deal," said Dr. Francis Collins, director of the National Human Genome Research Institute, which provided support for the project. "We now have the instruction book of our closest relative."

He said the work will help scientists analyze human DNA for roots of disease.

But as for revealing what it takes to be human, he cautioned that's "more than a biological question, that's also a theological question." DNA studies may not reveal much about things like "how do we know what's right and wrong," he said.

While the new DNA comparisons don't firmly identify specific differences that played a big role in producing humans, they do indicate promising areas, said Bruce Lahn, who studies human evolution genetics at the University of Chicago but didn't participate in the project. Lahn said the research refutes a few previous ideas while providing new and better evidence for others.

Humans and chimps have evolved separately since splitting from a common ancestor about 6 million years ago, and their DNA remains highly similar.

They are about 99 percent identical in regions the two species share, and about 96 percent identical if one also considers DNA stretches found in one species but not the other, researchers said.

It's the differences — some 40 million — that attract the attention of scientists.

Waterston and colleagues, for example, looked for genes that apparently have changed more quickly in humans than in chimps or rodents, indicating they might have been particularly important in human evolution. They found evidence of rapid change in some genes that regulate the activity of other genes, telling them when and in what tissues to become active, for example.

It would make sense that changes in these regulatory genes could have a broad impact on how organisms develop, playing a key role in human evolution, Waterston said.

With help from the chimp DNA, his team also uncovered several regions of human DNA that apparently contain beneficial genetic changes that spread rapidly among humans within the past 250,000 years. One area contains a gene called FOXP2, which previous work has suggested is involved in acquiring speech.

Svante Paabo of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and colleagues report in the Science paper that genes active in the brain have changed more in the human lineage than in the chimp lineage.

In a telephone interview, Paabo said that in general, "I'm still sort of taken aback by how similar humans and chimps are" in their DNA. "I'm still amazed, when I see how special humans are and how we have taken over this planet, that we don't find stronger evidence for a huge difference in our genomes."

He said he believes the key differences between the species will prove to be subtle things such as patterns of gene activity and how proteins interact.

In fact, Waterston and co-authors said they hoped documenting the overall similarity of chimp and human genomes will encourage action to save chimps and other great apes in the wild:

"We hope that elaborating how few differences separate our species will broaden recognition of our duty to these extraordinary primates that stand as our siblings in the family of life."