The human genome contains between 20,000 and 25,000 genes. Most of these pre-date our species by millions of years, and have counterparts in chimps, mice, flies, yeast, and even bacteria. But some of our genes are ours alone. They are human-specific innovations that arose within the last few million years.
These genes might have contributed to the distinctive traits that make us human, but ironically, they are also very hard to study and often ignored. Many are missing from the reference human genome, which was supposedly “completed” in 2003.
One such unique human gene is HYDIN2. It first appeared around 3.1 million years ago, as a duplicate of an existing gene called HYDIN. During the duplication process, “the head got chopped off and the tail got chopped off,” explains Max Dougherty from the University of Washington. It was as if someone had transcribed a book but neglected the prologue and epilogue. That should have been a fatal mistake since the prologues of genes contain sequences called promoters, which switch them on or off. The new gene should have been dead on arrival—a book that couldn’t be opened.
Instead, as luck would have it, it fused with a copy of another gene, which gave it a new lease on life. The fusion, which Dougherty described at the American Society of Human Genetics 2015 conference, created an entirely original gene, which looks like HYDIN but with a new prologue and a new first chapter. And while HYDIN, like most of our genes, exists in many other animals, its wayward daughter—HYDIN2—is a human-only innovation.