Bones evolved to act like batteries, 400-million-year-old fish suggest

The first bones containing living cells provided key minerals that allowed the fish to undertake longer journeys–changing the trajectory of vertebrate evolution.

By studying the fossilized remains of ancient fish, scientists have uncovered a turning point in the development of one of the most important parts of humans and other animals: bone. While bones primarily provide a structure to support the body, these hard tissues are always changing and provide other benefits to vertebrate bodies. Bone maintains itself, repairs injuries, and provides key nutrients to the bloodstream.

The earliest bones, however, were very different from human skeletons today. In the prehistoric past, bone was more like concrete, growing on the exterior of fish to provide a protective shell. But according to a new study in the journal Science Advances, the first bones with living cells—like those found in humans—evolved about 400 million years ago and acted as skeletal batteries: They supplied prehistoric fish with minerals needed to travel over greater distances.

The fossilized creatures in the analysis are known as osteostracans. “I affectionately call them beetle mermaids,” says Yara Haridy, a doctoral candidate at the Berlin Museum of Nature and lead author of the study. These fish had a hard, armor-encased front end and a flexible tail growing out the back. They had no jaws, and their bone tissue encased their bodies. These kinds of fish are critical to understanding the origins of the hard parts that shaped vertebrate evolution.

Haridy’s research focuses on osteocytes, the cells that become walled-in by the hard, mineral part of bone as part of skeletal growth. The earliest animals with bone didn’t have osteocytes, however, and some modern fish also don’t have these cells, leading paleontologists to wonder when and why these bone cells first developed.

“I basically started to become obsessed with the question: Why osteocytes?” Haridy says.

A new 3D technique

Solving the mystery of bone cells has proven challenging for paleontologists. Traditionally, Haridy explains, researchers study the microscopic structures of bone by slicing off thin sections and examining them on two-dimensional slides. But this method doesn’t provide a full, three-dimensional picture of what bone cells really look like.

A method developed for materials science and other engineering applications allowed Haridy and colleagues to reveal bone structures that scientists have not previously been able to study. “I saw one of my colleagues’ posters in the hallway with amazing images of pores in batteries, and they looked like cells,” Hariday recalls. The method used to make those images was called focused ion beam milling and scanning electron microscopy (FIB-SEM), which creates detailed, three-dimensional scans. Haridy asked what objects the technique could be applied to, and when she learned that a stable, dry object is best, she says she “basically screamed, What’s more stable than rock?”

The resulting scans of osteostracan fish fossils were beyond Haridy’s expectations. “My amazing co-author Markus Osenberg nonchalantly sent me an email of early images,” she recalls, “and I called him to make sure it wasn’t a model but our actual data, that’s how unbelievable it was.”

The scans did not show the actual bone cells—which decayed long ago—but [ … ]

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