Scientists on what the ENCODE junk DNA study really said

"The Yale team studying the evolutionary history of pregnancy looked at cells found in the uterus associated with placental development. They compared the genetic make-up of these cells in opossums -- marsupials that give birth two weeks after conception -- to armadillos and humans, distantly related mammals with highly developed placentas that nurture developing fetuses for nine months."

"They found more than 1500 genes that were expressed in the uterus solely in the placental mammals. Intriguingly, note the researchers, the expression of these genes in the uterus is coordinated by transposons -- essentially selfish pieces of genetic material that replicate within the host genome and used to be called junk DNA."

If the genome of our remote mammalian ancestors hadn't kept around such junk to co-opt it, we might be marsupials like opossums, or even monotremes such as the echidna.

Of course, evolution doesn't have foresight -- it didn't intentionally stash aside useless DNA in hopes it would one day produce placentalism. It just worked out that way.

As to why the junk DNA wasn't discarded as useless, that's yet another question for evolution studies. Did it hang around because replication didn't impair fitness greatly? Or is a higher-level selective process at work, favoring retention of junk DNA because species with it have more opportunities to evolve adaptations?


