Researchers have discovered a stretch of “junk” DNA that may have contributed to humanity’s evolution of opposable thumbs. When genetically engineered into mice, the human DNA seems to activate genes in the budding wrist and thumb. Chimp and monkey versions, on the other hand, seem only capable of switching on genes in the developing shoulder [New Scientist].
In a new study, the research team combed through the vast regions of human DNA that do not contain code for making proteins. Formerly dissed as “junk DNA,” sections of these non-gene regions are now known to play a regulatory role, dialing down or cranking up the activity of actual genes [Science News]. Researchers first found a long sequence of DNA that had barely changed during the entire evolution of backboned creatures, and then zeroed in on a smaller stretch of code that had accumulated 16 changes since the ancestors of humans and chimpanzees split, about 6 million years ago.
The human version of the genetic sequence that they isolated, known as HACSN1, was then injected into single-celled mouse embryos. Eleven-and-a-half days into the experiment, they found that the gene was active throughout the embryos’ developing forelimbs, in areas analogous to the thumb and first couple of fingers of humans. When the team injected embryos with comparable sequences from chimps and macaques, the gene activity was confined to the base of the limb. That suggests that HACSN1 could be a “molecular component of humanness,” says [lead researcher James] Noonan [ScienceNOW Daily News].
The results, published in the journal Science [subscription required], don’t prove that this genetic sequence caused early hominids to develop opposable thumbs; researchers caution that the results are very preliminary, and also say that many genes would be need to refine a complicated mechanism like the human hand. “One has to expect that in the remodeling of human limbs, brain or skeleton that many, many, many genes were involved over a long period of time,” says [study co-author Sean] Carroll. “If you consider this a piece of the genetic puzzle, we don’t know how big this piece is, or where it fits in” [Science News].