One thing we like to look for is whether there are any marks on the surfaces of the fossil bones we find. The photographs below show three bones we have found in the past few days, all with interesting marks on them. Fossil bones can be marked in many ways. They can be scratched by the teeth of carnivores or rodents who eat the animals or the bone. They can be scratched or smashed by the stone tools of hominins. They can be rubbed or scratched by sediment or gravel that is trampled or pushed into the surface of the bone. Or they can be accidentally scratched or nicked by the excavator's tools as they're being excavated. Can you see the marks on the bones? What do you think made these marks?

The small square piece in this first photograph is a small bit of cranium - in fact, it's a piece of brain case. The marks on this bone are narrow, V-shaped, grooved striations or lines. This pattern of marks is usually made by a stone tool. With the naked eye, however, it is very difficult to be sure, since we can't see the very small variations within the mark.

The photograph in the second photograph is of the part of the smooth end of a limb bone, where it meets another bone at a joint. When I first looked at this bone surface in the full sun, it looked like the entire area was badly damaged. I knew that carnivores, just like any pet dog, like to chew on the ends of bones, so I thought these marks were from a carnivore. When I looked at the bone again under a low-angle light, however, I saw that the end of one of the scrapes on the bone is V-shaped, which is more typical of a mark made by a stone tool. Is it possible that a hominin, rather than a carnivore, was cutting this joint? In order to answer this question, we will need to look at the bone in a lab under a powerful microscope.

Finally, the bone below is a real mystery. The two marks on the surface could be caused by a carnivore's tooth, when it grabbed the bone. They could also be caused by a stone tool that was used to smash the bone. They are not distinctly V-shaped like marks from a stone tool used for cutting, nor are they U-shaped like marks from a tooth. The marks are filled with sediment, so they're not new ones made by accident with an excavation tool. The marks themselves are clear, but the interpretation is not. What do the marks look like to you?