Ask 10 people what makes humans human and you’ll probably get 10 different answers — and then some. From our biased perspective, it’s seemingly simple to come up with many qualities that define the human experience. We love, we laugh. We form deep personal bonds and complex societies. We use language to communicate, art to express ourselves and technology to accomplish complex tasks. As Aristotle pointed out, although we can often be irrational, we have the ability to reason. We formulate ideas, contemplate and test them, and draw conclusions about our world. We can reflect on our own lives and begin to imagine death.
But if we really want to be scrupulous (and we do at Science News), we would have to admit that very few of these qualities are sufficient to set us apart from all other organisms. There are plenty of tool-users and innovators in the animal kingdom. Neither laughing nor loving are limited even to the primate branch of the evolutionary tree. When it comes to mating especially, many creatures appear to have an appreciation for the visual arts. And elephants, some scientists have claimed, have been observed trying to cope with death.
So to define ourselves, we have to get more specific. Linguists study the details of how we structure our sentences and communicate our ideas. Neuroscientists investigate connections between brain cells. Geneticists have perhaps the most promising approach: Though it’s messy to untangle, a lot of what makes us human definitely resides in our DNA. But looking at genes alone is not enough, as molecular biology writer Tina Hesman Saey has long reported. Chemical modifications of DNA play a big role in how genes turn on and off in the human body (SN: 2/14/09, p. 5). Other molecular players, lincRNAs for example, orchestrate many of the processes of life (SN: 12/17/11, p. 22).
In the May 27 issue, Saey explores yet another underappreciated shaper of humanness: transposons, or “jumping genes.” Much of what sets us apart from our chimpanzee relatives comes from bits of DNA that hop (or have hopped) around our genomes. In fact, nearly 50 percent of our genome is made up of transposons. Depending on where they land, they can cause trouble or, in lucky cases, add some genetic machinery that evolution can work with. Transposons have been shown to play a role in directing embryo development, in fighting immune invaders and perhaps in how we think. “Transposable elements have been with us since the beginning of evolution,” as evolutionary biologist Josefa González says.
It wouldn’t be scrupulous, at least not now, to conclude that transposons should get the credit for making us who we are. But could they have had a contributing part in driving humans to build great houses like those at Chaco Canyon or to domesticate horses? Might transposons have some role in the creativity required to invent an artificial womb? Maybe so. It’s most certainly true that they are yet another valuable route for scientists to explore. And it’s that exploration that gets us beyond simple answers to a fuller understanding of our world and ourselves.