[ There are interesting questions on the
[ physical nature of DNA which gives rise to evolution. For
[ example, the presence of transposons, retroviruses, and exon
[ shufflings add new variables to questions in human evolution.
[ Transposons might be responsive to the external environment and
[ not just random chance. Carnes talks about this, but with
[ bacteria. Is anything Carnsian going on with mammals?
[ bye
[ Jon
This is certainly true! If one where to analyze where most of the
5% difference in DNA sequence exists between homo sapiens and
chimpanzees then you would find that it is not in the unique
coding sequences but in the repetitive DNA. In particular
satelitte DNA, B-1, HERV, Alu, line-1, and other repetitive element
families show marked differences between these two lineages.
Molecular clock analysis suggest that the timing of the "mutations"
which caused these differences were punctuated in character (ie.
they did not occur gradually). In fact most of these elements
display properties of retroviruses, retrotransposons, or gene converters
in terms of their sequence similarities to LTRs and their inferred
behavior.
What is even more amazing is that this form of "mutation" (ie.
viral integration, symbiogenesis, retrotransposition, or whatever
you would like to call it) may be THE major form of genetic change
which is the driving force of evolution. For instance:
JR Harris states in his paper entitled The evolution of placental
mammals; Febs letters, 1991 Dec 16, 295(1-3) "Based on morphological
virological, biochemical and molecular biological data, it is proposed
that the presence of endogenous retrovirus particles in the placental
cytotrophoblasts of many mammals is indicative of some beneficial
action provided by the virus in relation to cell fusion, syncytio-
trophoblast fromation and the creation of the placenta. Further,
it is HYPOTHESIZED THAT THE GERM LINE RETROVIRAL INFECTION OF SOME
PRIMITIVE MAMMAL-LIKE SPECIES RESULTED IN THE EVOLUTION OF PLACENTAL
MAMMALS.
There are also other scientists who study endogenous retroviruses,
who believe that they play a major evolutionary role. I hypothesize
that the presence of some repetitive elements represents the genetic
fossil record of prior viral symbiogenic relationships which were
responsible for the development of new species. Therefore, "mutation"
is not a gradual endogenous process but can actually be created by
by extra-organismal retroviral like factors.
The evolutionary record has shown us other potential kingdom creating
symbiogenic events such as eukaryotes developing from pre mitochondrial
bacteria "infecting" an advanced prokaryote. or the speculated
development of land plants from a joining of primitive fungi and algae.
Can anybody give other potential examples of symbiogenesis leading
to the creation of a new species, genus, family, or order?
Has any work been done to attempt viral archeaolgy? (ie. take
antartic core samples and assay various strata for viral particles by TEM?)
Sincerely, Foster P. Carr MD 9/24/94
