The Robertsonsian fusion that formed human chromosome number two (from
ancestral 2A and 2B, as it is preserved in the other apes) should have
caused a serious reproductive barrier, overcome only by consanguity of
the highest order; mating amongst first-degree relatives. Any breeding
outside the immediate family would have lead to unacceptable
chromosomal imbalances.
No aneuploidy of human #2 (p or q or all) has ever survived.
I cannot imagine a scenario in which this fusion (2a and 2b) could have
survived, and become fixed in our species, unless immediately followed
by consanguity in our lineage.
I often mention this paradox to my Genetics students and I have
searched the literature but I have never found a better explanation nor
a statement in a textbook that all of us (humans) are descendants from
an incestuous family. Yet it is apparent from the facts (explained
above) that we are.
The explanation that the "complete" chromosome two is ancestral and
that the lineages of the other apes (gorilla, chimpanzee, orangutan)
have each and independently experienced centromeric breaks, is not
parsimonious and not accepted by experts in evolution. (It might
comfort those wishing to blame incest on the non-human apes but it is
not very scientific.)
If this fusion occurred recently there should be an excessive amount of
homozygosity (autozygosity) of genes on human chromosome two as
compared with the genes on other chromosomes (assuming that, after the
consanguity, the descendants eventually outbreed and the "new"
number two chromosome somehow became fixed in our descendants). I do
not know if anyone has surveyed the SNPs and alleles for these
chromosomal comparisons but such data might allow a molecular clock
method to determine if the event occurred recently (Homo sps) or
farther back in time (Australopithecus sps).
I am not suggesting that this fusion is what makes us human. Indeed, I
doubt that it is. However, maintaining a balanced chromosomal
complement would have provided a strong reproductive barrier. Mating
with someone with the "old" 2A and 2B karyotype would have resulted
in reduced fecundity due to lethal aneuploidy and errors in meiotic
disjunction. We often speak of humanity as having evolved through a
bottle neck of a few dozen individuals. Perhaps, at least with regards
to our number two chromosome, that bottle neck was one male and one
female from the same family. Placed in the context of fairy tales, I
conclude that Adam and Eve were brother and sister or parent and
offspring.
I look forward to your discussion.
Sincerely,
Dr Jamie Love
