>John asks a penetrating question:
>
>>I often hear that a given primate species (say, chimpanzees) are 98% or 99%
>identical to human wrt the genome.
>>
>>Is this literally true? What, exactly, is the claim being made here? How
>do they know without sequencing the whole thing? By "identical" do they
>just mean "similar" or compatible or something more vague like that, or do
>they mean base-pair-for-base-pair identical?
>>
>>And how identical are two randomly selected humans? (I've heard informally
>over 99.6%) or two siblings?
>>
>>If you know a better question that I would be asking if I knew more about
>it, feel free to answer that one instead or (preferably) in addition.
>
>I only recently heard the claim that humans and chimps share 90-something%
>genomic identity. I heard this from Prof. R.J. (Sam) Berry, a Christian
>geneticist from the Univ. of London, during a talk he gave on this campus
>entitled, "Developing a Christian Mind". As I recall, the number comes from
>someone at UCLA who proposes that humans and chimps should be considered
>part of the same genus (Homo) but different species. Currently, chimps
>belong to a different genus (Pan).
>
>Unfortunately, I do not know how such an estimation of genetic relatedness
>was made. The same questions John raised above, I also have. The problem,
>I beleive, lies in what is meant by "identity".
The locus classicus for chimp/human genetic near-identity is M.C. King and A.C.
Wilson, "Evolution at Two Levels in Humans and Chimpanzees," _Science_ 188
(1975): 107-116, where electrophoretic mobility of proteins, amino acid
sequence, and immunologic comparisons were all used to estimate "that the
average human polypeptide is more than 99 percent identical to its chimpanzee
countepart" (pp. 114-115).
King and Wilson argued however that this must near-identity applies only to
structural proteins, because the two species
differ far more than sibling species in anatomy and way of life.
Although humans and chimpanzees are rather similar in the structure
of thorax and arms, they differ substantially not only in brain
size but also in the anatomy of the pelvis, foor, and jaws,
as well as in relative lengths of limbs and digits. Humans and
chimpanzees also differ significantly in many other anatomical
respects, to the extent that nearly every bone in the body of a
chimpanzee is readily distinguishable in shape and size from its
human counterpart. Associated with these anatomical differences
there are, of course, major differences in posture..., mode of
locomotion, methods of procuring food, and means of communication.
Because of these major differences in anatomy and way of life,
biologists place the two species not just in seperate genera but
in separate families. So it appears that molecular and organismal
methods of evaluating the chimpanzee-human difference yield quite
different conclusions. (p. 113)
King and Wilson conclude that regulatory mutations, not single-base pair
changes in structural proteins, must account for the differences between
humans and chimps.
The other major source for the claim of chimp/human near-identity at the
molecular level is DNA-DNA hybridization data, a "distance" method which
eliminates -- as part of its experimental protocol -- much of the repetitive
DNA in both the human and chimp genome. See the publications of Charles Sibley
and Jon Ahlquist (but see also the extensive critiques by Jon Marks, Vincent
Sarich, and others).
Both King and Wilson and the DNA hybridization conclusions are estimates.
As John Rylander noted, no one has actually compared, base pair by base pair,
the entire genomes of Pan troglodytes and Homo sapiens. Whether one regards
the existing estimates as accurately reflecting the actual (still to be sequenced)
nucleotides will depend on how reasonable one finds several underlying
assumptions about the comparative methods. Personally I'd like to know
what all that repetitive DNA is doing, among other things.
Paul Nelson