Our bones did not begin deep inside the body. They started in the skin, not long after the first complex animals took shape - because why do things the easy way when you can do them the weird way?
Ever since, skin bones have remained a recurring motif in evolution, popping up in turtles, crocodiles, lizards, snakes, and even dinosaurs. Yet we still know surprisingly little about them. Why do they keep reappearing? Was there a single ancestor with skin bones that gave rise to them all?
In a new study published in the Biological Journal of the Linnean Society, researchers combined fossil evidence with modern computational tools to reconstruct 320 million years of reptile skin bone evolution. What they found concludes a centuries-long debate: skin bones have indeed independently evolved across multiple lizard lineages. In the process, they also traced a unique evolutionary comeback in one of their most iconic groups - goannas.
The oldest skin bones in the fossil record may date back 475 million years, when some of the earliest vertebrates evolved an elaborate bony exoskeleton. This seems counterintuitive, since vertebrates are literally defined by having backbones, but their bony internal skeleton didn’t evolve until 50 million years later. Priorities, apparently.
Throughout evolutionary history, the skin’s ability to form bony tissue has resurfaced again and again - fish scales, for example, or osteoderms, the skin bones of land-dwelling animals. After leaving the water, osteoderms may have helped animals adapt to terrestrial life. Beyond that, the picture becomes less clear. Osteoderms vanished in most lineages, yet they kept reappearing, especially in reptiles.
To understand how this happened, researchers pieced together a complex evolutionary puzzle, like arriving at a bank robbery long after it happened. Their witnesses were 643 living and extinct species, each offering a unique perspective. They kept investigating until the stories began to converge.
They found that most lizards first evolved osteoderms during the Late Jurassic and Early Cretaceous, more than 100 million years ago, when dinosaurs like Brachiosaurus, Allosaurus, and Stegosaurus roamed. Armour may have helped lizards survive predators, cope with harsh environments, or move into new habitats. After those early bursts, the pace slowed, and most groups held onto their skin bones ever since.
The ancestors of monitor lizards - known in Australia as goannas - lost osteoderms entirely, likely because their active lifestyle and efficient bodies functioned better without the extra weight. But when their descendants reached Australia about 20 million years ago, something remarkable happened: they grew them back. Researchers pinpoint this re-evolution to the Miocene period, when Australia’s climate was becoming drier. Skin bones may have helped reduce water loss and offered protection in open, arid landscapes.
Strikingly, goannas are the only known lizard lineage to reacquire osteoderms after losing them. This challenges Dollo’s law, which holds that once a complex trait disappears, it cannot re-evolve. Early in the 20th century, researchers assumed lizards inherited osteoderms from a common ancestor. Later, that view gave way to the idea that these bone plates evolved independently between select groups. Debates about evolutionary mechanisms followed, but they raced ahead without anchoring the origin of osteoderms in a clear timeline.
This study provides that foundation - published in the same journal where Charles Darwin first shared his groundbreaking ideas. Fossil evidence helped resolve a longstanding question, but modern computing made it possible to narrow thousands of evolutionary scenarios into a single, coherent story. The evidence is clear: osteoderms evolved multiple times, independently, across different lizard lineages over hundreds of millions of years.
Among lizards, goannas stand out as the only lineage known to have lost this armour, only to regain it in a remarkable evolutionary twist. This pattern fits seamlessly among other evolutionary oddities found in Australia, where marsupials reign and mammals lay eggs. It also shows that evolution rarely follows a straight path, instead meandering through the ever-changing conditions on our planet.