by Scientists once considered much of the human genome “junk” because much of its genetic code does not give rise to any proteins, the complex molecules tasked with keeping cells running. However, it has since been discovered that this so-called garbage DNA plays important roles in cells, and in a new study, researchers report that humans may actually have junk DNA to thank for our exceptionally large brains.
The research, published Monday (January 2) in the journal Natural ecology and evolution (opens in a new tab)suggesting that the genes that made it possible human brains growing large lobes and complex information networks may have originally arisen from junk DNA. In other words, at some point the “junk” picked up the ability to code for proteins, and the new proteins may have been critical for the human brain development.
The findings suggest that such genes “may have a role in brain development and may have been a driver of cognition during human evolution,” Erich Bornberg-Bauer (opens in a new tab)an evolutionary biophysicist at the University of Münster in Germany who was not involved in the research, said Science magazine (opens in a new tab).
Usually, new protein-coding genes are born when cells duplicate and make copies of their DNA. When cells construct new DNA molecules, mutations can appear in the genetic code, and the altered genes can then give rise to slightly different proteins than their predecessors did. Genes born from junk DNA, known as de novo genes, undergo a more dramatic transformation, in that they suddenly gain the ability to make proteins.
Related: More than 150 “made from scratch” genes are in the human genome. 2 are completely unique to us.
To make proteins, cells “read” protein-coding genes and write down their genetic blueprints in a molecule called RNA, which then slides over to a protein building site in the cell, called a ribosome. From there, the ribosome uses the RNA blueprint to build the desired protein. Junk DNA, interestingly, can also be used to make different flavors of RNA, but very few of these RNA molecules can exit the nucleus, the protective bubble where cells house their DNA, the study authors discovered. Their new research suggests that to transform into protein-coding DNA, junk DNA must first start making RNA capable of escaping the nucleus and reaching a ribosome, Science magazine reported.
By comparing the genomes of humans, chimpanzees (Pan troglodytes) and rhesus macaques (Macaca mulatta), a more distant primate relative of ours, the authors found 74 examples of junk DNA being transformed into protein-coding DNA, It was reported by Ars Technica (opens in a new tab). A key step in this transformation was the junk DNA picking up mutations that allowed the RNA to leave the nucleus, they confirmed.
Humans and chimpanzees share 29 of these de novo genes, meaning that the genes appeared after humans and chimpanzees split from the evolutionary ancestor they shared with rhesus macaques. The remaining 45 de novo genes appeared after humans and chimpanzees split from each other about 6 million years agowhich means that the genes are unique to humans.
Furthermore, the team found that nine of these unique genes appear to be active in the human brain, so they investigated the genes’ functions in several experiments. Some tests involved tiny 3D models of the brain grown in lab dishes; two of the genes caused these mini-brains to grow larger than they did without those genes. In genetically modified mice, these two genes respectively drove above-average brain growth and caused human-like ridges and grooves to form in the rodents’ brains, Science magazine reported.
It’s important to note that miniature brains don’t capture all the complexity of full-sized human brains, and that the rodent studies included relatively few mice, experts told Science magazine. But ultimately, the work suggests that junk DNA may have provided some of the key ingredients for what makes us human.
