Obelisks: New RNA Entities in Human Microbiomes Challenge Definition of Life

Researchers have discovered a vast family of previously unknown RNA elements inside bacteria that live in and on humans. Dubbed obelisks, they replicate without encoding the machinery to do it themselves and appear across the globe in staggering variety—tens of thousands of distinct types—raising fresh questions about the boundary between life and non-life.

The work, led by Ivan Zheludev and colleagues, used large-scale RNA sequencing and computational screening to find circular RNA molecules about 1,000 nucleotides long. These obelisks fold into rod-like structures across their length and encode a novel protein called Oblin; some also carry hammerhead self-cleaving ribozymes. They were reported in a 2024 preprint and later in peer-reviewed literature, and summarized in a 2025 perspective in Biomedical Odyssey.

Where Obelisks Show Up

Roughly 30,000 distinct obelisks (at 90% sequence identity) have been detected across seven continents in diverse environments. In people, they show up in about 7% of stool samples and about 50% of oral samples. One type has been linked to the bacterium Streptococcus sanguinis. They colonize bacteria the way viroids colonize plants: they rely on the host for replication and do not fit neatly into existing categories of virus or viroid.

Obelisks share traits with viroids—small, circular, non-coding RNAs that infect plants—but form their own phylogenetic group with no clear kinship to known viruses, viroids, or other subviral agents. They are among the simplest possible replicators: RNA that persists and spreads by co-opting host enzymes.

Why the Definition of Life Is in Play

Whether obelisks are "alive" depends on how life is defined. They replicate and evolve but lack metabolism and the kind of genetic toolkit that cells use. Under lab conditions they do not appear essential for bacterial survival; their effect on bacterial fitness and ecology in the wild is still unknown. The discovery extends the roster of viroid-like colonists of human and environmental microbiomes and revives debate about the RNA world and the minimal features of life.

What's Next

Scientists will probe how obelisks spread between bacteria, whether they affect human health, and how they relate to other minimal replicators like viroids and ribozyviruses. As sequencing improves, more such entities are likely to appear, further blurring the line between organism and replicating molecule.