I've mentioned ERVs and their relationship to the nested hierarchy of evolution,
and this has also been covered in other videos by cdk007, DonExodus, and many others.
This is extremely powerful science, and I strongly recommend
you watch at least one of these videos before continuing with this one.
This should be a slam-dunk for evolution,
but of course creationists will do anything they can to avoid accepting it.
In this case, they keep claiming that ERVs violate the nested hierarchy
while not understanding how this is actually happening.
This is partly the fault of myself and the other video makers--
we have to simplify things to fit in the time we have for the video
and we also assume that people have the basic knowledge they should have.
But they don't, at least, not to the extent required to properly understand
why the creationist arguments are specious.
An ERV sequence known as HERV-K-GC1 has been found in both gorillas and chimpanzees,
but is not present in human DNA.
Gorillas diverged from an earlier common ancestor to us, and chimps from a later one,
so this ERV must have been present in that earlier common ancestor
as the chances of the virus inserting itself twice in the exact same place is astronomical.
According to the creationists, this means that humans must have the ERV,
and we don't, so we're not related.
This is based on a complete ignorance of genetics and how evolution works.
Genes have what are known as "alleles."
For example, a flower might have only a single gene for color,
but many alleles: red, yellow, white, etc.
This is the reason why humans have different colors of eyes,
hair, skin tone, and many other features.
Eukaryotes (such as plants and animals) are diploid,
meaning they get two different sets of genes, one from each parent.
In our flower example, it might get a red allele from both parents and be red,
yellow from both parents and be yellow, red from one and yellow from the other and be orange,
red from one and white from the other and be pink, and so on.
Often, though, they don't combine.
Consider human earlobes. Most humans have an earlobe that dangles,
but some have an earlobe that attaches.
These represent two alleles for the same gene.
If both alleles are for the dangling earlobe, it dangles;
if they're both for the attached earlobe, it's attached.
But there's nothing in between. There's no such thing as a half-attached earlobe.
So what happens if someone has one gene for attached earlobes and the other for dangling?
The allele for dangling earlobes is dominant,
so someone with both alleles--known as heterozygous,
as opposed to homozygous if they're both the same--will have a dangling earlobe.
But--and this is important--they will still carry an allele for an attached earlobe
and can pass it on to their progeny.
An ERV is an allele.
In the case of HERV-K-GC1, one allele is for the sequence being there,
and the other for not being there at all.
Since ERVs are generally deactivated by mutations and are thus neutral sequences,
there is no selection pressure for or against them.
There's no particular reason for them to dominate,
but no particular reason for them to be selected out, either.
Looking at extant organisms, we find this sequence in gorillas and chimps, but not humans.
But here's the thing: we also don't find it in orangutans,
old-world monkeys such as macaques, or anywhere else.
Therefore, this is a very recent insertion.
The ERV inserted itself into a germ-line cell of one particular organism some 10-15 million years ago,
but when this happened, it was only the luck of the draw that it persisted.
If that organism had had no offspring, or its offspring had had no offspring,
the ERV sequence would not have survived to this day in ANY ancestral line.
It just isn't the case that this event causes the sequence
to instantaneously appear in every single member of the population, as the creationists seem to think.
So the sequence must grow over time, and it becomes one allele in the population,
with the absence of the sequence being the other allele.
Both the earlier common ancestors of gorillas, chimpanzees, and humans,
and the later common ancestors of chimpanzees and humans,
would have had both alleles in their populations to varying degrees.
Remember that evolution is the change in the allele frequencies over generations.
Also keep in mind that speciation events are often driven by a small segment of the parent population
that has been separated in some way--
by space, or by whichever resources are being competed for, or whatever.
In a smaller subset of the population, it is statistically less likely that their allele frequencies
will be representative of the population as a whole,
and given that these separations generally happen
with subsets that are more closely related to each other, the frequencies can vary widely.
So, for the human line to lose the allele for this ERV,
there is only one simple thing that needs to have happened:
the subset of the population that separated and led to humans
had a very low frequency of the ERV allele
Over time, through genetic recombination (recombination can eliminate ERV sequences)
and the luck of the draw, the allele disappeared.
This kind of thing happens all the time, and is no surprise to anyone educated on the subject.
Now, if this had been a much longer-established ERV sequence, there would have been trouble.
Say the ERV happened before the time of the dinosaurs.
Say it's there not only throughout all of mammalia,
but also in all of the birds, crocodiles, and other archosaurs,
as well as many current species of reptile.
Then the nested hierarchy might be called into question,
as it's unlikely that a sequence would survive for that long
in every single branch of descent without becoming fixed in the population.
The further back the allele goes, the less of a chance of it being weeded out;
but the more recently it occurs, the greater the chance of the allele
disappearing in one population or another.
So the absence of this allele in humans does nothing to falsify the nested hierarchy of evolution.
Quite the contrary, we can easily tell that the genetic sequences
flanking the ERV in chimps and gorillas is an incredibly close match to humans,
exactly what we would expect--1.6 to 1.7% different.
Trying to make humans out to be unrelated because of the missing ERV sequence
is ludicrous given the vast similarities at the insertion site,
and the relative probability of the ERV allele disappearing from the early human population.
Really, all this tells us is that chimpanzees, bonobos, and gorillas
are more closely related to each other than any of them are to humans,
and that they all diverged in a relatively short time--
but then, we knew that already,
so this is simply a confirmation of the amazingly consistent nested hierarchy
predicted by the theory of evolution.