What did scientists think was mammal-only?

The feature in question is an internal cavity that helps organize the developing embryo before a critical stage called gastrulation — the point when the embryo establishes its body plan and begins arranging the tissues that will eventually become organs and other major structures.

Scientists had long assumed that forming this cavity was a uniquely mammalian trait. The new research shows veiled chameleons do it too.

Chameleons offered a practical advantage for studying this phase. When chameleons lay their eggs, the embryos inside are still at a very early stage of development. In many other reptiles, development is much further along by the time eggs are laid, which makes those earliest stages harder to examine.

How did the discovery happen?

The finding grew out of a collaboration between two researchers studying development from different angles.

Before joining Rowan, Shylo studied veiled chameleon development as a postdoctoral researcher at the Stowers Institute for Medical Research in Kansas City, Missouri. There she captured images of developing chameleon embryos. Co-author Antonia Weberling, a developmental biologist at the University of Oxford who studies early embryonic development and stem cells across many species, reached out to collaborate.

When Weberling reviewed Shylo's images, the resemblance caught her completely off guard.

"Are you sure you sent me chameleon embryos?" Shylo recalled Weberling asking. "This looks exactly like a human embryo."

That startled reaction prompted a closer look at how the embryos develop — and ultimately to the published study.

Do chameleon and human embryos develop the same way?

Not quite. Both form similar internal cavities, but they take different biological routes to get there.

The difference in process:

• In human embryos, cells organize around a central space as the cavity develops.
• In chameleon embryos, the organism begins as a flat sheet of cells that folds inward and gradually closes — much like a drawstring bag being pulled shut.

Despite those different mechanisms, both embryos end up with remarkably similar structures.

That convergence carries an evolutionary lesson: evolution can sometimes produce similar outcomes through different biological processes. The finding also raises new questions about how early development evolved across species, questions the researchers have not yet answered.

What are the caveats?

The study examined one species of chameleon, so the results do not automatically extend to other reptiles or to all non-mammalian vertebrates. And while the embryos look strikingly alike at certain stages, the researchers found similarities in structure, not identical underlying biology — the route to the cavity differs between the two lineages.

Whether chameleon embryos can serve as a practical, widely used laboratory model for human development also remains to be demonstrated. The findings are a first step, and Shylo plans to continue investigating some of the earliest and most mysterious stages of human development.

For her, the work is also a reminder that important discoveries can emerge from unexpected places — in this case, from an egg laid by a lizard with a prehensile tail and a projectile tongue.

Publication details: Antonia Weberling et al., "Epiblast lumenogenesis is not a mammalian-specific trait," Nature Communications (2026). DOI: 10.1038/s41467-026-73768-9.