Retroviruses reproduce by inserting their genetic code into a cell's DNA. In rare cases, they happen to do this to one of the host organism's germ line cells (which become sperm or egg cells). When that organism reproduces, the retrovirus' genetic code is passed down to future generations. The retrovirus is now an ERV, which then lies dormant in the genome, slowly accumulating mutations just as the rest of the DNA sequence does. ERVs are essentially genetic fossils, remnants buried not in the ground, but deep within every cell of the body.
We know ERVs are actual remnants of viruses—and not just because they look exactly like viruses, although that should be evidence enough. In 2006, a team of French scientists actually revived an ERV from the human genome, which they dubbed "Phoenix." When introduced to a cluster of human cells, Phoenix was able to infect them—which would of course be impossible if Phoenix wasn't a real virus that actually infected one of our distant ancestors.
Even if ERVs didn't demonstrate our common ancestry with other animals (I'll get to that in a minute), they'd still be completely incompatible with a 6,000 year old earth. They make up almost 8 percent of the human genome in the form of 98,000 fragments from 30,000 retroviruses—and everyone's ERVs are more or less the same, give or take a few mutations. For that many viruses to insert themselves into just the right sperm or egg cells and spread evenly throughout the entire population would take several orders of magnitude longer than the young-earth paradigm allows.
So why are ERVs important evidence for evolution? Simple: they're arranged in patterns called nested hierarchies, and there's no reason to expect to such patterns unless common descent is true. If an ERV shows up in one species, which later splits in two, it will show up in both "daughter" species. For example, in the graph below, some specific ERVs are found in gibbons, orangutans, gorillas, chimps, and humans, because the virus infected the common ancestor of all five. Some ERVs are found in just the latter four, some in just the latter three, and so on. What's more, we find them in the exact same location in the genome of each species, meaning that the virus didn't happen to somehow infect them independently.
At no point should we ever expect the same ERV in the same location in, say, humans and gibbons but not chimps—and we don't. The same goes for any two species that don't share a direct common ancestor, and the pattern holds each and every time. Let no one ever tell you that evolution is unfalsifiable; if the above examples turned to be true for some ERV, it would be completely unexplainable by common descent. However, the nested hierarchies we do find beautifully match the ones formed by other types of evidence.
|From Lebedev et al 2000. Time goes left to right.|
Arrows represent ERV insertion at specific points in the genome,
which carry over to all subsequent species in that lineage.
So there we have it: ERVs point powerfully to an old earth, human evolution, and common descent in general. Naturally, creationists have tried to undermine this evidence, and as usual they fail miserably. I've already answered one objection (that they were never viruses at all), but these two blog posts by a graduate student specializing in ERVs offer a better response than I ever could. It saddens me that so many people—40% in America—believe in strict creationism, when most of them have never so much as glanced at the extraordinary evidence available.