‘Junk’ DNA, once thought unimportant, found to be crucial in human evolution
When the entire human genome was sequenced eight years ago, the news came as a shock. The shock came not from what scientists had found, but what they didn’t find. Most scientists had assumed that a sophisticated complex creature like a human would have 100,000 genes.
They were wrong. It turns out we have just over 20,000 genes—about equal to a simple roundworm.
This finding begged new questions:
• If we only have 20,000 genes, then what is it that makes us human, as opposed to a roundworm?
• What else besides genes could be influencing human evolution?
• Are we really that special after all?
What is ‘junk’ DNA?
Scientists have found that genes aren’t the only way we pass on characteristics to our offspring. More than 90 percent of genomes consists of DNA long considered “junk.” This DNA, which does not create proteins, had unknown functions or was considered a molecular record of our past.
It turns out this “junk” DNA serves a very important purpose. It regulates how genes make proteins, even preventing proteins from being made. It can be influenced by the environment, so outside influences (like radiation, or the ability to digest certain foods) are inherited. The field of study around “junk DNA,” known as epigenetics, has made significant discoveries in how most of our genome does not make proteins, but tells our genes whether and how to.
‘Junk’ DNA in humans and chimpanzees
A study at Stanford University in 2011 found that epigenetics may be what is separating us from our closest animal relatives, the chimpanzee. It certainly isn’t our genomes—we share 96 percent of our DNA with chimpanzees. The researchers found that out of the three billion combinations of DNA that make up our genes, about 510 are present in chimps but missing in humans. Many of these missing genes performed regulatory functions that don’t occur in humans anymore.
The researchers found two key areas that weren’t regulated in humans (but are in chimps). One strip of DNA sits near a gene that creates a male hormone receptor. If the regulatory DNA is there, then the animal grows sensory whiskers and has altered male genitalia. Without that DNA strip, humans were free from growing whiskers, and free to develop traits that allowed for monogamous bonding.
The other DNA strip sat near a gene called GADD45g, which inhibits cell growth in the brain. Humans don’t have this strip of DNA, and thus could grow larger brains.
How could these differences have arisen? Another group of researchers at the Georgia Institute of Technology found that these changes can arise from random insertions and deletions of tiny bits of DNA, which shut down regulatory functions of genes that lead to the evolution of larger brains, less facial hair and other traits that make us human. The Georgia researchers found 68,000 such alterations, indicating a fertile field for making changes outside our “genes,” but within our DNA.
So, the merger of modern genetics with anthropology research methods is slowly giving us a picture of how we came to be human.
Polavarapu, N., et al. (2011). Characterization and potential functional significance of human-chimpanzee large INDEL variation Mobile DNA, 2 (1) DOI: 10.1186/1759-8753-2-13
McLean, C., et al. (2011). Human-specific loss of regulatory DNA and the evolution of human-specific traits Nature, 471 (7337), 216-219 DOI: 10.1038/nature09774