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Evolution: Genetic Evidence - Endogenous RetroVirus

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Uploaded on Aug 15, 2009

Evolution: Genetic Evidence - Ubiquitous Proteins

Excerpt from The Cassiopeia Project; "Facts of Evolution". The Full series can be found
at: http://www.cassiopeiaproject.com/vid_...

(Transcript of Video Below)

A third type of structure found within the genome
is also direct evidence of genetic relationships.

Retroviruses like HTLV1 (which causes a type of
leukemia) and AIDS make a DNA copy of their
own viral genome and insert it into their host's
genome. If this happens inside of sperm cells
or egg cells the retroviral DNA will be inherited by
descendants of the host. And these copies of
virus DNA are called endogenous retroviruses.

In human DNA, there are about 30,000 endogenous
retroviruses. There are at least seven distinct
instances of identical retrogene insertions shared
between chimps and humans.

The phylogenic tree for cats provide another
example. The standard phylogenetic tree has
small cats diverging later than large cats.
The small cats (e.g. the jungle cat, European
wildcat, African wildcat, blackfooted cat, and
domestic cat) share a specific retroviral gene
insertion. In contrast, all other carnivores which
diverged earlier lack this sequence.

Retrovirus Example

Let's examine how Endogenous RetroViruses
or ERVs would behave within a model of
evolution by common descent.

Suppose an ancient creature, let's call
it Primus Mammalius, is the common ancestor
of all modern mammals and is infected by a
retrovirus that becomes endogenous.

All of the Primus descendants would be
expected to carry the same ERV lets call it
ERV1 in the same chromosomal location.

Fast forward 30 million years. Different
lineages have evolved and diverged from the
original common ancestor and there are
now many different types of mammal in existence,
all carrying ERV1. A small rodent, let's call it
Secundus Mousus, is the common ancestor of
mice and rats and once again is infected by a new
species-specific retrovirus that becomes endogenous.
This is ERV2.

In a different line, Secundus Apus, the common
ancestor of all great apes acquires a third
retrovirus, ERV3.

Moving forward 30 million years again, a
fourth ERV appears in mitochondrial Eve,
the common ancestor of all modern humas
Lets call it ERV4.

As early humans spread out, a fifth ERV
arises in a population that is isolated in Australia,
so ERV5 does not spread to other human populations.

So what would we expect?
Humans, chimps, mice and rats should
all possess ERV1.

The mouse and rat genomes will also
contain ERV2, the virus that infected their
common ancestor, but not the
primate-specific ERV3, ERV4 or ERV5 insertions.

All great apes will share an identical ERV3
insertion; all humans will also possess an
ERV4 insertion that is NOT found in chimps
or other apes.

In addition, some, but not all, humans will
carry an insertion of ERV5. And of course,
the rodent-specific ERV2 insertion will not be
found in any primate species.

Now that several genomes have been
sequenced, we have begun to test these
predictions.

The patterns of ERV insertions observed in
modern species exactly match the predictions
made by the model described above.

Some insertions are shared between
humans and mice and represent truly ancient
viral infections.

Others are found only in primates, and not
in other species, obviously derived from an
infection of the ancestral primate species after
its divergence from other lineages.

More modern insertions are found only in
humans, while the youngest ERVs of all are
found in some humans, but not in all.

We DO NOT FIND ANY examples of ERV
insertions shared by, say, humans and mice,
but not by chimps. Insertions are always
shared by all species, and only by those species,
that have a common ancestor. ERV insertions
therefore provide excellent support for the idea
of common descent.

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