It's well known that chimpanzees are the closest living relatives to humans (actually I think it's bonobos, aka pgymy chimps).

However every time I read about how similar our genomes are, I hear a different percentage quoted for how much of our DNA is shared. I've heard percentages ranging from 95 to 99.5 in common, with many figures in between. Every author on evolutionary biology seems to have his or her own number.

Since we have so many experts here at tracking down obscure info and separating fact from myth, can anyone find a definitive and authoritative figure?

My monkey-brained brother-in-law notwithstanding, what is our simian similarity?

Here's what it comes down to. The data has been looked at at least 3 ways:

One way is to look at substitutions. That is, replacement of a single base pair with another. So, ACTGT in a chimp might be ACGGT in a human. This would be counted as a 20% difference. The typical number using this technique (see for example Clark et al., page 1960 of the Dec 12, 2003 Science) is around 98.6%. This figure seems to vary within a range of maybe 98.4-98.8%. The reason is that no one has actually looked at the entire chimp genome and compared it to the entire human genome. There will be a small difference in this number depending which section of the genome you choose to look at.

Roy Britten (Proceedings National Academy of Science 2002, page 13633). Also looked at another type of change, insertions/deletions. So, ATCGT might become ATGT, which would be counted as a 20% difference. Britten found that these added an additional 3.4% variance, for a total of 5% difference / 95% similarity.

Morris Goodman (Proceedings National Academy of Science 2003, page 7181) took the view that not all DNA is equally important and that not all changes are equally important. He found that "junk" DNA (DNA that does not code for proteins but current research says is still very important and makes up most of the genome) and synonymous DNA (that is changes in the code that don't change the amino acid coded for and therefore don't change the protein built) are more likely to have differences. Looking at only the "critical" changes, Goodman found a 99.4% similarity between humans and chimps. This is probably the highest number you will find as it considers only non-synonymous substitutions (and not insertions or deletions) in protein-coding DNA.

Methos, thanks for your very scholarly reply. I see now why the figures differ so much. AP never disappoints!

"Junk" DNA is indeed mysterious -- I am aware that there is much speculation about hidden meaning in the apparent junk -- perhaps as-yet-unknown regulatory function that may turn out to be of key importance.

When I searched on Google using the words in the title of this thread, it mostly turned up creationist rants. Good info is hard to come by.

Just out of curiosity: Did you find these references on-line? Do you live next door to a university library? Are you a supercomputer?

I started with Google. I've forgotten the exact search term, but it was something along the lines of percent DNA chimpanzees. In retrospect, it might have been a shorter search had I used Google Scholar.

At any rate, I spotted a few news articles saying something like "in this month's issue of Science" and then simply grabbed that issue. "Simply," I say because I have online subscriptions to a very large number of journals through my university (so in a sense your comment about living next door to a university library isn't far off). Skimming Britten and Goodman's articles was enough to find out what the normal range was (98.6-ish) and what they did differently that resulted in their figures being out of this range.

As of August 31, 2005...

The first detailed genetic comparison between humans and chimpanzees shows that 96 per cent of the DNA sequence is identical in the two species...

An international scientific consortium publishes the genome of the chimpanzee, the animal most closely related to homo sapiens on Thursday in the journal Nature. It is the fourth mammal to have its full genome sequenced, after the mouse, rat and human being.

Thanks, John. That same item was reported by CNN, but their story says:

Humans and chimps have evolved separately since splitting from a common ancestor about 6 million years ago, and their DNA remains highly similar -- about 96 percent to almost 99 percent identical, depending on how the comparison is made.

The work was done by researchers at University of Washington in Seattle, and published in the journal Nature.

I presume the given range of similarity depends on junk DNA and other factors, as explained by methos above.

Yes, the paper (which I have only skimmed very briefly) gives different comparisons along the lines of what is discussed above.

These extracts seem pertinent:

We calculate the genome-wide nucleotide divergence between human and chimpanzee to be 1.23%, confirming recent results from more limited studies^12,33,34....we estimate that polymorphism accounts for 14–22% of the observed divergence rate and thus that the fixed divergence is 1.06% or less.

the indel differences between the genomes thus total 90Mb. This difference corresponds to 3% of both genomes and dwarfs the 1.23% difference resulting from nucleotide substitutions; this confirms and extends several recent studies^63–67. Of course, the number of indel events is far fewer than the number of substitution events (5 million compared with 35 million, respectively).

These are the first two ways I mentioned in my first post. The answers using the entire genome are about the same as those that have been previously found by taking sections of the genome. It doesn't appear that Goodman's method of neglecting junk DNA was tried with this data.

Aw, come on, whatever y'all are saying, as long as there is not 100% certainty, I ain't no chump, I mean, chimp.

Well, it sounds like there's 100% certainty that humans' and chimps' genomes are 96 to 99 percent identical. Now excuse me, but I've got to go peel another banana. I've been on the go all day and my knuckles are tired!