Helen Briggs, "Reading the chimp book of life" (2005)

By Helen Briggs > <span class="byd">BBC News science reporter <span>

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Scientists have deciphered another book in the library of life - the genetic recipe of our closest living relative, the chimpanzee.

It is arguably the most valuable genetic blueprint for determining what makes us human.

Researchers have already begun to analyse the parts of the life code that are unique to each species.

Buried within the 3 billion DNA "letters" are the changes that put our ancestors on the pathway to humanity.

Wealth of new evidence

It is more than a century since Charles Darwin recognised that humans and chimps are related. A wealth of evidence has emerged since then, including the discovery of the first known chimpanzee fossil revealed this week.

Researchers hope the comparison of the chimp and human genomes will shed light on the past six million years or so of evolution, since the two species diverged from an apelike common ancestor.

In this brief eye blink of evolutionary time, the features that make us human emerged; among them our large brain, complex speech and the ability to walk upright. At the same time, we lost many of the features we associate with our ape cousins such as their dense body hair.

How are alike are we? The number of genetic differences between a mouse and a rat is about 10 times more than between a person and a chimp The number of genetic differences between a person and a chimp is about 10 times more than between two humans The number of genetic differences between a human and a chimp is about 60 times less than between a human and a mouse

Alterations in the sequences of the chemical "letters" (base pairs) along our DNA should account for the differences.

It turns out that we are both more and less like apes than previously thought. This apparent paradox is a hallmark of the complexity of the mammalian genome.

If you take the most meaningful parts of the genome - the genes that code the proteins that build and maintain our bodies - the genetic sequences of man and ape differ by a mere 1% in terms of single letter changes to the genetic code.

But in more poorly-understood parts of the genome - regions of DNA that regulate our genes, for instance, or so-called junk DNA with as yet unknown functions - we are somewhat more divergent than we once thought.

Duplications and shuffling of stretches of DNA add a further difference of about 3% so, when you compare the two genomes as a whole, we share about 96% of DNA.

Disease defences

Many of the 35 million single letter (nucleotide) differences that set us apart from chimps lie in the genes that make proteins involved in our immune response. This is no great surprise, since chimps and humans would have encountered different diseases during evolution.

> <div> Scientists decode Clint the chimp's genetic sequence

Intriguingly, changes to parts of the human genome may have made us prone to certain diseases. A gene that seems to protect other animals against Alzheimer's appears not to function in our genome. Duplication of others stretches of DNA in humans are implicated in the development disorders spinal muscular atrophy and Prader-Willi syndrome.

In terms of what makes us human, the most promising areas for exploration are six regions that show very little variation among humans but more variation in chimps suggesting they were important in the human line of evolution. One of these regions contains a gene called FOXP2 that seems to be important in speech.

But as yet there is no smoking gun - a protein involved in regulating brain function, say - that may have caused our ape-like ancestors to branch off from chimps.

With the genomes of other primates, such as the orangutan, nearing completion, there will soon be other members of the family available for comparison.

The researchers who decoded the chimp genome hope that elaborating how few differences separate the species will broaden recognition of our duty to protect apes in the wild.

Only a day after their study was published in the journal Nature, the UN's environment and biodiversity agencies warned that some of the great apes - chimps, gorillas, and orangutans - could be extinct in the wild within a human generation.