Scientists once considered most of the human genome to be “junk” because large chunks of its genetic code do not give rise to any proteins, the complex molecules whose job is to keep cells moving. However, it has since been discovered to be so-called garbage GOUT plays an important role in cells, and in a new study, scientists report that humans may actually have junk DNA, thanks to our exceptionally large brains.
The study, published Monday (January 2) in the journal Nature ecology and evolution (opens in a new tab)suggests that the genes that made it possible human brains to grow large patches and complex information networks may have originally emerged from junk DNA. In other words, at some point, the “junk” acquired the ability to code for proteins, and these new proteins may have been crucial to the human brain evolution.
The findings suggest that such genes “may play a role in brain development and drive 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).
Typically, new protein-coding genes are born when cells duplicate and make copies of your DNA. As cells construct new DNA molecules, mutations can occur in the genetic code, and the altered genes can then give rise to slightly different proteins than their predecessors. Genes born from junk DNA, known as de novo genes, undergo a more dramatic transformation where they suddenly gain the ability to make proteins.
Related: There are over 150 “made from scratch” genes in the human genome. 2 are completely special to us.
To make proteins, cells “read” protein-coding genes and write their genetic blueprints into a molecule called RNA, which then moves to the protein building site in the cell, called the ribosome. From there, the ribosome uses the RNA blueprint to build the desired protein. Interestingly, junk DNA can also be used to make different flavors of RNA, but very few of these RNA molecules can leave the nucleus, the protective bubble where cells store their DNA, the study authors found. Their new research suggests that in order to turn into protein-coding DNA, the junk DNA must first start producing RNA capable of escaping from the nucleus and reaching the ribosome, reports Science.
Comparing human genomes, chimpanzees (Troglodyte gentlemen) and rhesus macaques (Macaca mulatto), our more distant primate relative, the authors identified 74 examples of junk DNA turning into protein-coding DNA, Ars Technica reported (opens in a new tab). They confirmed that a key step in this transformation was that the junk DNA picked up the mutations that allowed its RNA to leave the nucleus.
Humans and chimpanzees share 29 of these genes de novo, meaning the genes emerged after humans and chimpanzees diverged from an evolutionary ancestor they shared with rhesus macaques. The remaining 45 de novo genes appeared after the divergence of humans and chimpanzees about 6 million years agomeaning 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 function of the genes in several experiments. Some of the tests involved small, three-dimensional models of the brain grown in lab dishes; two genes caused these minibrains to grow larger than they would without those genes. As Science magazine reports, in genetically modified mice, these two genes respectively drove above-average brain growth and caused the formation of human-like ridges and grooves in the rodents’ brains.
It’s key to remember that minibrains don’t capture all the complexity of full-size human brains, and that the rodent studies involved a relatively small number of mice, the experts told Science magazine. But ultimately, the work suggests that junk DNA may have provided some of the key ingredients of what makes us human.