In the past few years, the pace of publishing genomes has been fast and furious. Researchers have sequenced our pets (dogs), our food (cows and chickens), our closest relatives (chimps) and of course the most popular laboratory animals. So why should the genome of the opossum, an obscure marsupial, grab attention?

The divergence of marsupial and placental mammals represents a key time point in evolution — one that was missing from the comparative genomics map until now, explains Tarjei Mikkelsen, doctoral student at the Broad Institute in Cambridge, Massachusetts. Through the efforts of his 60 collaborators, this addition to the evolutionary timeline provides the power to distinguish which genome changes are specific to placental mammals.

When comparing two species' genomes from one evolutionary group, researchers need a third genome from outside that group to measure them against. But before now, if comparing say, mouse and human, both placental mammals, then the next closest outside genome would be chicken, which diverged from mammals about 310 million years ago. But that split is too long ago to trust whether your comparison is catching true differences or just shared sequences that have become unrecognizable over time.

Conversely, if you were to compare humans and chimps (both primates), and use the mouse genome as the outlier, the mouse and primate groups, which diverged from each other in the past 100 million years, would be too close in time to pick up small changes. Marsupials diverged from placental mammals about 180 million years ago, which in evolutionary time is about halfway between the aforementioned divergence events. “Before, even if something was conserved from chickens to humans, we may not have recognized it. The opossum is distant enough to have striking differences, yet close enough to ensure we did not accidentally miss any similarities,” says Mikkelsen.

The team has used the opossum data to boost the biological importance of conserved non-coding elements (CNEs) — DNA that doesn't give rise to proteins — in mammalian genomes. Using the opossum as the measuring stick, the authors discovered that one-fifth of CNEs is an invention of placental mammals' evolution.

In addition, the opossum sequence allowed the team to classify a larger fraction of relatively recent CNEs as originating from transposons, so-called junk DNA. These pieces of DNA were once thought to be merely selfish bits, replicating and inserting themselves into genomes, but current thinking has them transforming the regulatory sequences of mammalian genes. The finding throws more weight behind the idea that “evolution is recycling”, says Mikkelsen. “It's much easier to repurpose old parts from junk DNA, than to invent new ones from scratch.”

Mikkelsen has worked previously on the human, chimp and dog genome projects. “When I started seven years ago on the human genome, we knew very little about how genomes were organized — it was a big, uncharted text file made up of four letters,” says Mikkelsen. What makes us human is not written down in that code per se, it's the journey from ancient organisms to us that matters: what did we keep, what did we change and how does it work differently in us? That story can only be told by collecting other genomes, and now Mikkelsen and colleagues can check off the marsupial.