Fragments of DNA left by viruses that infected our distant ancestors may be 'firestarters' for new human life, new research finds.

"Our results illustrate how recently emerged… genes can confer developmentally essential functions in humans," Stanford University biologist Raquel Fueyo and colleagues write in their paper.

Fueyo and her team used a ball of stem cells induced to mimic a blastocyst, the phase of embryonic development about five days after fertilization. This 3D model, or blastoid, replicates the developmental stage just before the embryo implants into the uterus's lining.

When the researchers disabled a group of remnant virus genes known as LTR5Hs, the embryonic model either turned into a disorganized clump of cells or died. Without the LTR5Hs, the middle layer (epiblast) of the three-tissue-layered blastoid did not form properly.

Up to 9 percent of our DNA is composed of genetic material from ancient viral invaders. These endogenous retrovirus remnants infiltrated the genetic material of our ancestors' reproductive cells millions of years ago and are now permanently integrated into our genetic blueprints.