Researchers have discovered the first fossilized evidence for bone marrow in the fin of a fish that lived 370 million years ago. The research was conducted by scientists from Sweden and France and the results were published in the Proceedings of the Royal Society B.

Bone marrow is a soft tissue found in the middle of long bones. There are two types of bone marrow: red and yellow. Red bone marrow is responsible for the creation of all red blood cells and most of the white blood cells, while the yellow marrow is mostly fat storage and can produce some white blood cells. At birth, all marrow is red but about half will convert to yellow over time. If there is extreme trauma to the body and a great deal of blood has been lost, the yellow is able to revert back to red within a matter of hours to ramp up red blood cell production.

In order to determine where bone marrow originated, the team would have to take a careful approach. They would have to detect any traces of bone marrow without causing damage to the bone itself and therefore had to use a technique called synchrotron microtomography, which allowed them to see the inside structure of the bones in great detail. Previous research has shown that the humerus of a fish that lived 370 million years ago in the Devonian already showed hallmark signs of having bone marrow—including long channels running down the bone and enlarged canals for the blood vessels—so the Eusthenopteron became the obvious choice when taking a closer look. The fossils originated from Miguasha National Park in Quebec. 

The researchers brought the Eusthenopteron fossils to the European Synchrotron Radiation Facility in Grenoble, France. They were able to scan the fish’s long bones and  view the internal structure of the bone on the submicron level and create a 3D representation of how the bone marrow would have been arranged within the bone. Scanning the bone without having to cut through it was the most preferable way to model the inner structure in detail, as the cut marks would have impaired the quality of the 3D image.

Investigating the evolutionary changes that occurred within the cancellous bone tissue and bone marrow is essential to understanding how the bones in fish fins eventually elongated and allowed them to begin to walk. Other studies have already shown this relationship on the molecular level, but it is important to see how it actually played out during our evolutionary history. The earliest tetrapods required bones that were strong enough to support their body weight on land and the evolution of bone marrow was essential to that transition.