What makes a human different from a chimpanzee? Not much, but the little genetic differences clearly count for a lot, said scientists who have mapped the complete chimp genome and compared it to the human gene map.

They said their findings, published in the journal Nature on Wednesday, would shed light on why people get Alzheimer’s disease, certain cancers and even AIDS, and chimpanzees do not.

The researchers said the findings were yet more proof that evolution is real and works through natural selection, just as Charles Darwin predicted a century ago.

“As our closest relatives, they (chimpanzees) tell us special things about what it means to be a primate and, ultimately, what it means to be a human at the DNA level,” Dr. Francis Collins, head of the National Human Genome Research Institute, which funded the studies, told a news conference.

Dr. Robert Waterston of the University of Washington in Seattle and colleagues sequenced the DNA of a chimpanzee named Clint, who is now dead.

They compared it to the human genome sequence and did a letter-by-letter comparison of the DNA base pairs - the A, C, T and G nucleotides that make up the human and chimp genetic codes.

Out of 3 billion base pairs that make up both genomes, only 40 million differ between human and chimp, they found.

Most are changes in a single letter - for instance a human has an A where a chimp has a T.

In addition, humans have some extra DNA that chimps do not have and vice-versa.

All these differences add up to 4 percent of the total genomes - meaning humans and chimps are 96 percent genetically identical.

BASIS FOR HUMANITY

“If you see a difference between a chimp and a human, it is clearly the result of a single evolutionary event,” Waterston said.

“Within those 40 million events, we clearly have the basis for what makes us human.”

Humans and chimps evolved separately from a common ancestor that lived about 6 million years ago.

Three different types of genes seem to be evolving rapidly in both humans and chimpanzees, said Washington University’s Dr. LaDeana Hillier - those involved in reproduction, smelling and immunity.

“The vast majority of these 40 million changes are probably not relevant to what makes us human because they are in junk DNA,” said Tarjei Mikkelsen, a graduate student at the Broad Institute, a joint venture of the Massachusetts Institute of Technology and Harvard University who led one study.

He said only about 5 percent affect proteins that are likely to have a large effect on biology.

Among them, a parasite related to sleeping sickness that infects chimps but not humans, and one gene for sialic acid, which is found on the surfaces of cells and is used by some viruses to infect them.

There is also an enzyme called caspase 12, which is mutated in humans and appears to make our species susceptible to the brain-wasting Alzheimer’s disease, Mikkelsen said.

The researchers said their findings clearly contradicted an increasingly vocal movement in the United States that disputed the science of evolution and instead called for teaching “creationism” or the “intelligent design” to school children.

“To me, looking at this - we are looking at evolution in action,” Waterston said.

“I couldn’t imagine Darwin hoping for a stronger confirmation of his ideas when we compare the human and the chimpanzee genome.”

But, added Collins, the study did not address philosophical or religious questions. “It may very well not tell us about other aspects of humanity, such as how do we tell right and wrong,” Collins said.