What makes us human? Was it the discovery of fire, the industrial revolution, or the development of the Internet? Or are we human thanks to ancient bacterial and viral genes that are dotted throughout our DNA?
Every cell in our body contains around 3 billion DNA bases, but our genes only make up approximately 1 percent of this. So what about the rest?
Nearly half is made up of jumping genes, which are also called mobile DNA or mobile genetic elements. These stretches of DNA have the ability to move from one location in the genome to another, an ability that normal genes lack.
Most jumping genes are not technically genes, as their DNA does not contain the code to make functional proteins. However, they are thought to affect the expression and actions of other genes.
Long hailed as "junk DNA," scientists are getting closer to figuring out the importance of jumping genes in shaping the modern human as well as their role in disease.
Remnants of evolution
Bacteria make frequent use of jumping genes. This allows them to adapt to environmental pressures such as gaining antibiotic resistance.
Genes can also jump when bacteria or viruses infect humans. Although our cells have mechanisms to counteract such events, some mobile DNA fragments become established in our cells, where they add genetic diversity.
The first jumping genes in our evolution can be traced back as far as 600 million years ago, to the human ancestor Giardi lamblia, a primitive parasite.
Once mammals arrived on the scene, insertions of mobile DNA into their genomes really took off. This happened between 40 and 12 million years ago.
Exactly how these ancient jumping genes contributed to the development of the modern human is unclear. Scientists think that their step-wise integration coincided with the emergence of an increasingly complex brain structure, possibly giving us a crucial advantage during primate evolution. And their influence can still be felt today.
Not just along for the ride
Today, we know that jumping genes are important for placental development and actively regulate gene expression during early embryonic development.
The jumping gene known as HERVK is thought to be a remnant of an infection by an ancient retrovirus that took up residence in the genome around 200,000 years ago.
HERVK is switched on at the very early stage of human embryonic development and triggers a precise antiviral response, even though no virus is present.
Scientists think that this event may provide the developing embryo with some level of viral resistance, which is, of course, a favorable trait.
Jumping genes are also known to play crucial roles in brain function. One such gene contains a regulatory RNA molecule that is important for normal human brain development. If this is mutated, it causes infantile encephalopathy.
While we now know that jumping genes contribute to normal body functions, they also have the potential to wreak serious havoc with our genes.
Jumping genes and disease
Mobile DNA can jump to another location on the same chromosome or a different chromosome each time a cell divides. If this happens in sperm or egg cells, it will be passed on to the next generation. The current estimate of such events occurring ranges from 1 in 20 to 1 in 1,000 births.
Other cell populations also seem particularly prone to mobile DNA rearrangements. Several epithelial cancers, such as those lining the gastrointestinal tract, are known to harbor mobile genetic elements at diverse locations.
Whether these events are at the root of the cancer or a side effect is not currently known, and the human genome is much more complex than previously thought. While jumping genes are just one part of the puzzle, scientists are beginning to appreciate the genetic contribution that microbes make to human diversity and disease.