A discussion thread on one of my favorite blogs included this fascinating link to a story that is just incredible. The article is a little lengthy, but not too bad, and it's definitely worth the read, especially if you have any interest in biology or science. Thanks to Sid Schwab for providing the link.
Essentially, when geneticists mapped out the human genome, they discovered that while only 2% of it was entirely necessary for life, as much as 8% consisted of "junk" DNA believed to have been incorporated due to infections from retroviruses. Retroviruses, for the uninitiated, are viruses capable of permanently altering the DNA of organisms they infect, occasionally causing the changes that they make to the human genetic code to be passed on to offspring. The most notorious retrovirus, of course, is HIV, the virus responsible for causing AIDS. A few innovative geneticists studied these fragments and managed to sequence the genetic code of one of the retroviruses that might've insinuated itself into the genome of more complex organisms tens or hundreds of thousands of years ago.
Using biotechnological methods for creating recombinant DNA, these scientists effectively resurrected an extinct virus. And their creation was capable of infecting mammalian cells.
According to the article, there's evidence that the incorporation of retroviral DNA into our genome influenced our evolution. The formation of the placenta, for example, may have been a result of retroviral infiltration. And retroviral incorporation may explain why monkeys are carriers for HIV but are unaffected by it; there's a huge chunk of retroviral DNA that they have that we don't. So sequencing and experimenting with "extinct" retroviruses gives us more concrete evidence for evolutionary pathways and may have medical applications.
Read the article. It's totally worth it.
Interesting to think that there are "fossils" of extinct organisms in your DNA, no?