MadSci Network: Evolution

Re: Is there a recognized DNA segment that designates human DNA?

Date: Tue May 8 13:35:14 2001
Posted By: Brian Foley, Post-doc/Fellow Molecular Genetics
Area of science: Evolution
ID: 985373623.Ev

	Human DNA is actually close to 98% identical to chimpanzee DNA.  The human
genome has now been completely sequenced, and the chimpanzee
genome has not been sequenced, but large sections of it have been.  Some
recent papers that address this issue are:

Kaessmann H, Wiebe V, Paabo S.
Extensive nuclear DNA sequence diversity among chimpanzees.
Science. 1999 Nov 5;286(5442):1159-62.
PMID: 10550054

Kaessmann H, Heissig F, von Haeseler A, Paabo S.
DNA sequence variation in a non-coding region of low recombination 
on the human X chromosome.
Nat Genet. 1999 May;22(1):78-81.
PMID: 10319866

Samonte RV, Conte RA, Verma RS.
Localization of human midisatellite and macrosatellite DNA sequences 
on chromosomes 1 and X in the great apes.
J Hum Genet. 1999;44(1):57-9.
PMID: 9929980 

Zhao Z, Jin L, Fu YX, Ramsay M, Jenkins T, Leskinen E, Pamilo P, 
Trexler M, Patthy L, Jorde LB, Ramos-Onsins S, Yu N, Li WH.
Worldwide DNA sequence variation in a 10-kilobase noncoding region 
on human chromosome 22.
Proc Natl Acad Sci U S A. 2000 Oct 10;97(21):11354-8.
PMID: 11005839

Metzler-Guillemain C, Usson Y, Mignon C, Depetris D, Dubreuil G, 
Guichaoua MR, Mattei MG.
Organization of the X and Y chromosomes in human, chimpanzee and 
mouse pachytene nuclei using molecular cytogenetics and three-
dimensional confocal analyses.
Chromosome Res. 2000;8(7):571-84.
PMID: 11117353

	As far as I know, there is no region of the human genome
that is really distinct from chimpanzees.  A good discussion of
human/chimp evolution is the book: "The Third Chimpanzee" by
Jared Diamond.  But it is much more about cultural evolution and
changes in behaviors than about changes in the DNA.  
	In my opinion, the most likely source of the major differences
between humans and chimpanzees come not from "new genes" or significant
changes in existing genes, but from chromosomal rearangements.  If
my memory is correct (I don't have time to look this up right now)
there have been something like 8 or 9 non-homologous chromosomal
crossover events between the last common ancestor of chimps and
humans, and modern chimps and humans.  These are events where something
like chromosome 14 fuses with chromosome 66 to create a new larger 
chromosome, or chromosome 3 crosses over with chromosome 5 to
create twon new chromosome that are 3/5 and 5/3 hybrids.  Each of
these events can instantly create a new species if the individuals
with the new chromosomes can no longer mate with the individuals 
with the old chromosome arrangement.  As far as I know, it is not
yet know how many of the 8 events occurred on the human lineage
vs how many occurred on the chimp lineage.  For example all 8 might
have been on the human lineage and the chimps have had no chromosomal
rearrangements since the common ancestor with humand, or 5 might
have happened in the human lineage and 3 in the chimp lineage.  All
I know is that there are 8 total differences, and sorting out which
happened in which lineage is difficult without complete sets of
chromosomes from ancestors which no longer exist.
	For a number of different reasons, I suspect that the 
chromosomal rearrangements are very important.  One is the observation
that chromosomal rearrangements observed in humans almost always
result in large changes.  For example, trisomy 21 (also known as
Down syndrome) which introduces no new genes and no chromosomal
rearrangement, but just one extra copy of one small chromosome, 
results in many changes in both looks and behavior.  Likewise, many 
chromosomal translocations result in distinctive syndromes in 
the carriers.  A second reason is the "genetic bottleneck" that
such an event creates if it results in a speciation event.  If
just a few individuals carry the new chromosomes and they can only
mate with each other and not with the parental population, they will
be founding members of a new species and any traits they carry will
not get "diluted out" into a huge population.
	Just as with point mutations and other small changes in genes,
most chromosomal rearrangements would tend to be detrimental rather
than clearly beneficial to the carriers.  With some 6 billion people
on the planet today, there are tens of thousands who currently carry
chomosomal translocations, and it is not likely that any of them are
the founders of a new and improved species of human.  
	It also takes more than just good genes to survive in any
society, and ever more so in the increasingly complex human societies.
I really smart chimpanzee might be killed of by his troop if he was
not also strong, or if he was just strange looking to the members
of his troop.  Maybe the super-intelligent chimp would be "too
smart for his own good" and die because he was curious about what
the leopard looked like up close.  To found a new species, the 
individuals would most likely have to leave their parental group
and go off to start a new group.  In almost all competition-driven
evolution, it is within-species competition that is more intense
than between-species competition.  The elk's large antlers are not
primarily good at fighting off wolves, but primarily good at
fighting rival male elk for mating rights with female elk.
	Anyway, there is actually much more that we still don't know
about evolution and speciation, than we know.  It is certainly
not like Darwin had all the details figured out in the mid-1800s.
Studying molecular genetics, evolution, and other areas of biology
are still very exciting and ripe for new discoveries every day.
It will take a combination of genetics, paleontology, archeology,
sociology, and many other disciplines working together to uncover
the origins and explanations of human evolution.
	In my opinion, what make us "human" is not in our genes but
in our behaviors.  It is social evolution, which is largely Lamarkian
rather than Darwinian, that has so greatly influenced the last
50,000 years of our journey.  We have learned to cooperate and work
together to solve problems that individuals or smaller groups could
not solve.  Language and other tools have helped that to happen.

Brian Foley, PhD

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