How did kangaroos evolve to hop?

For a long time, biomechanics experts believed that giant, extinct kangaroos were simply too heavy to hop. While the largest modern kangaroos weigh around 90 kg, their prehistoric relatives were much larger and weighed more than 250 kg. And scientists previously calculated that if you took the anatomy of a modern kangaroo and scaled it up to that weight, the forces generated by hopping would snap their ankle tendons and fracture their bones.

However, a January 22 study has challenged this limit: by looking at the fossils rather than relying only on mathematical scaling from modern kangaroos, researchers have found that kangaroo ancestors didn’t just grow larger: they also evolved a different skeletal structure to handle the immense weight.

The study identified two major adaptations that made hopping mechanically possible for these giants. First, their weight-bearing foot bones, the fourth metatarsals, were significantly shorter and thicker than those of modern kangaroos. This compact shape made the bones more resistant to bending forces, preventing fractures upon landing. Second, their heel bones, called calcanea, were much wider, providing a large area  for the gastrocnemius tendon to attach. This suggested the animals possessed thick tendons capable of withstanding the tensions required to lift their bodies.

There was one trade-off, however. While the adaptations allowed them to hop without injury, the giants likely sacrificed speed and energy efficiency. Unlike modern kangaroos that bounce efficiently over long distances, the extinct giants probably hopped only for short, high-power bursts, perhaps to escape predators.