Understanding the relationships between different groups of plants and animals can sometimes be an entirely unintuitive exercise. For example, if you were asked to name the four main groups of reptiles, chances are good that your answer would be wrong. Most non-biologists be able to get at least two of the groups: alligators/crocodiles (order Crocodylia) and turtles/tortoises (order Testudines). However, identifying the remaining two is a bit trickier.

One group, tuataras (order Rhynchocephalia), has been around since before the rise of the dinosaurs, yet have only one or two remaining species. Tuataras live only in New Zealand and, although they resemble iguanas at first glance, are different from lizards in very fundamental and significant ways. For example, tuataras’ skulls are shaped very differently than lizards’ skulls, and they have a light-sensing organ on top of their heads called a “pineal eye.” In fact, the structure of their skeletons has changed relatively little when compared with other groups, which is why they are sometimes referred to as a “living fossil.”

The fourth group of reptiles combines snakes and lizards in the order Squamata. This group also includes a rare group of under-studied, worm-like reptiles called Amphisbaena. Squamates all have shared characteristics like tough, scaly skin, specialized reproductive anatomy and the ability to move their upper jaw independent of the rest of their skull. This grouping reflects a fascinating truth about the way this group of reptiles evolved: Snakes are essentially legless lizards.

And now the “Journal of Vertebrate of Paleontology” has published a new analysis of a fossil that sheds some light on how exactly snakes lost their legs. Researchers used a powerful new tool called synchrotron radiation computed laminography (SRCL), which uses X-rays to examine portions of fossils still embedded in rock. SRCL allowed them to look at vestigial hind leg bones in a fossilized snake from around 95 million years ago.

Their analysis revealed that this snake had a simple pelvis and two hind limbs, each with the familiar femur-fibula-tibia pattern found in all vertebrate limbs. Interestingly, it had four tiny ankle-associated bones but no forefoot or toe bones.

SRCL also allowed them to observe the fine detail of the fossilized skeleton in ways that were previously unthinkable. By comparing the micro-scale architecture of these bones to those of modern lizards, researchers were able to show that their bones likely developed in the same way, but were reduced in size because they either grew more slowly or for a shorter period of time.

Of course, reptiles alone won’t tell you the whole story. From an evolutionary perspective, you’d be missing one large piece of the puzzle: birds.

Birds and reptiles both belong to the group Sauropsida because they shared a common ancestor with alligators/crocodiles around 220 million years ago. Multiple lines of evidence, like the genetic similarity between birds and reptiles, and the famous transitional fossil Archeopteryx, strongly suggest that birds are simply dinosaurs with feathers.

I think the reason so many Americans have trouble understanding and accepting evolution is because it can sometimes be so unintuitive. Our limited experiences with snakes and lizards, or birds and reptiles, tell us that these groups are very different. But upon careful inspection, a wealth of fundamental similarities becomes apparent. This profound unity of life was Darwin’s greatest lesson; one I still struggle to wrap my mind around.