Read more: “Instant Expert: Fossils“
Look a little closer at fossils and you can discover how ancient organisms lived and interacted. To tease out these remarkable stories palaeontologists turn to taphonomy, the study of how fossils form. Decay, weathering and transport by water all leave telltale signs, which can be pieced together by analysing marks on bones, for example, or the microscopic or chemical make-up of fossils.
See diagram: World firsts
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The cutting edge
The analysis of cut marks on fossilised bone has come on in leaps and bounds. Marks left by stone or metal tools can now be distinguished from the tooth marks of carnivores. Percussion – hitting a bone with a rock, perhaps to extract nutritious marrow – also leaves characteristic marks and types of fractures. As a result, we can find out whether an animal was prey for a carnivore or processed and eaten by our forebears. This is helping pin down the origins of tool use.
The oldest stone tools yet found come from a site in the Afar region of Ethiopia and date to 2.6 million years ago. Although there are no hominin remains associated with them, and nothing resembling a campsite or living floor was found there, cut marks and percussion marks on the bones of antelopes and other animals found nearby prove that our ancestors used these tools to butcher meat. The search for older stone tools has proved difficult. In 2010, however, a team led by Zeresenay Alemseged of the California Academy of Sciences found what they interpreted as cut marks on fossil bones from Dikika, Ethiopia. The remains were almost 3.4 million years old, which would push the invention of tools back by around 800,000 years. Without the tools themselves, this interpretation is controversial. To explain the missing evidence, the researchers suggest that the very first tools were expediency tools – rocks with natural sharp edges that were used opportunistically. To resolve the debate we need to find more marks on bones older than 2.6 million years, carry out studies into the sorts of marks that naturally sharp stones can produce, and find out what else might make marks similar to those found by Alemseged’s team.
What’s bred in the bone
Clues in fossilised bones can give a good idea of an extinct species’ diet. Animals incorporate various trace elements from their food into their bones, which can be detected by chemical analysis, giving evidence of their place in the food chain. Strontium and calcium are useful elements for this type of analysis. They are derived from the soil, so plants incorporate the local ratio into their tissues. Animals preferentially incorporate calcium into their bones, so herbivores have a lower strontium/calcium ratio than the plants they eat. A carnivore feeding on the herbivores will have an even lower ratio, with apex predators, which feed on carnivores, having the lowest.
It is even possible to find out the type of plant a herbivore ate by measuring isotopes of carbon in its fossilised remains. Tropical grasses and sedges use a photosynthetic pathway that results in carbon-12 being preferentially incorporated into their tissues. Temperate grasses, trees and many other plants produce lots of carbon-13 in their tissues. The ratio of these isotopes in a fossilised bone reveals the proportion of the two plant groups in the animal’s diet.
Fossilised teeth provide yet more insight into diet. Even fairly soft foods such as fruit produce scratches, pits or polishing on enamel and dentin, and these marks are much more distinct with tougher or grittier foods. Building up a reference collection of the microwear on the teeth of animals with known diets creates a key to which the teeth of extinct species can be compared. Until recently, this was done by counting and measuring the marks on teeth. Peter Ungar at the University of Arkansas has pioneered a more accurate approach. This uses high-resolution scanning microscopy to create a 3D model of the tooth’s surface, allowing statistical comparisons with other teeth.
This approach has revealed that two early hominids, which became extinct around 1.2 million years ago, had surprisingly different diets. Paranthropus boisei‘s consisted mainly of grasses and sedges, whereas P. robustus favoured soft foods ().
If dead, eat, human or not…
In 1986, analysis of 6000-year-old Neolithic remains found in a cave at Fontbrégoua, France, caused a stir. Of the 13 disposal pits excavated there, 10 contained the bones of butchered animals – domestic sheep, deer, a wolf, a few badgers and martens, and a fox. The remaining three held human bones. Paola Villa at the University of Colorado and her team concluded that they had found evidence of cannibalism.
The team’s criteria for identifying cannibalism remain the gold standard to this day. First, the animal and human remains showed similar patterns of butchery in terms of the frequency and location of cutting and chopping marks. Second, the patterns of long-bone breakage to give access to nutritious marrow were similar. Third, none of the pits had a special design or location and all seemed to be used in the same way. Finally, no remains, animal or human, had convincing evidence of cooking or burning. If the animals had been eaten – an uncontentious interpretation – then the humans must have been eaten too ().
The similarity of the animal and human remains suggests the cannibalism was not prompted by starvation or religious beliefs, concluded the team. Instead, the Fontbrégoua people simply viewed dead people as being no different to dead animals.
