How a single zebra tooth builds an archaeological picture of Africa’s ancient climate
A tiny shaving of enamel from the tooth of a long-dead zebra or a recently dead lion can be the puzzle pieces that help archaeologists piece together a picture of what Africa’s climate might have looked like during the time when our early ancestors were evolving on the African savannah.
Written by: Leonie Joubert
Photographs by: Sam Reinders
Archaeologist Dr Julie Luyt is busy with a finicky lab process that ultimately will allow her to better understand of how Africa’s climate has changed in the past 2.6 million years. She’s not poring over weather station data, such as wind speeds, changing precipitation, or thermometer readings. She’s not sampling ancient air bubbles trapped in ice cores that would give a glimpse into a past atmosphere, from which someone might extrapolate the likely climatic conditions that would have existed then.
She’s working with the powdered enamel taken from the scraping of a zebra tooth — a molar, to be precise — that was collected from a skull at the De Hoop Nature Reserve about three hours drive east of her lab at the University of Cape Town (UCT). The sample is so small the dusty material could fit into a pill capsule. Even though the powder might look unremarkable to an untrained eye, the specific makeup of its chemical isotopes will tell her what the animal ate as it roamed through the veld as a youngster, at the time when its molars were forming.
‘Enamel forms gradually from the crown to the base of a tooth,’ Luyt explains, a research officer at the Department of Archaeology who runs her own samples and also assists colleagues with their processing. ‘In the case of zebras, a molar can take around two years to form. As the tooth grows, the enamel progressively mineralises.’
This mineralising preserves the material that was used by the body to build the tooth during that window in the animal’s life, preserving a permanent record of what the animal ate and drank during that time. The carbon isotopes show what the animal’s diet was, and the oxygen isotopes show the nature of the water that occurred in the environment at that time and which the animal would have drunk.
‘Sampling at regular intervals along the tooth can give an isotopic record of seasonal changes over the period during which the tooth formed,’ she goes on.
The chemical makeup of the enamel is extremely stable and does not leak or change after it formed, so it is a good proxy for the ancient climate and environment.
‘Among herbivores, oxygen is related to the body water of an animal, both drinking water and water found in food. Since the oxygen isotopic composition of water is linked to local temperature, by sampling serially along the molar, from the top to the bottom of the tooth, you get a seasonal temperature signal.’
Since carbon isotopes relate to the types of plants consumed by herbivores, these isotopes show the seasonal change in carbon if an animal had a slightly different diet from one season to the next.
Peering this deep into the isotopic makeup of a tooth is technically complicated, though. She explains some of the finicky details of putting the sample, in a small vial, through the stable light isotope mass spectrometer.
‘Once the enamel is at the bottom of the vial, we're left with atmosphere in the vial, too,’ says Luyt.
‘We don't want that, because there's carbon dioxide in the atmosphere, and we only want the carbon that’s in the sample. So, we flush the vial with helium. This is an inert gas, so once all the atmospheric gas is gone, all we’re left with is the enamel sample.’
It gets even more complicated from there, involving a drop of phosphoric acid to the tiny vial so that it can cook some carbon dioxide out of the powder. The resulting gas is then piped through a mass spectrometer where she’ll measure the carbon and oxygen isotope ratios.
On its own, this sample may not mean much. But it is one piece of a much bigger puzzle that spans time and space. Placed alongside the isotope analysis of hundreds of teeth of other wild animals — modern-day wildlife whose teeth reflect today’s vegetation distribution, as well as long-dead or even fossilised animals whose remains are preserved in museums and tell of environmental conditions in a more ancient Africa — archaeologists like Luyt can show how the vegetation across a region has changed over time. This can allow them to understand what climatic factors might have been at play, to shape the vegetation at the time, and therefore know more about the environment in which early humans evolved.
Elements such as carbon, oxygen, strontium and hydrogen are all present in water and soil, and sometimes have unique isotopic ratios that are particular to the geographic or environmental conditions of a place. As plants take up soil nutrients and water, and animals eat those plants, the elements move up the food chain, stamping their isotopic fingerprint into the tissues of the plants and animals that absorb them. By studying the isotopic ratios in certain organic tissues from plants or animal remains, researchers can understand the environmental conditions at the time when that plant or animal lived, whether it was one week ago, or 2 million years ago.
Luyt’s work has advanced the discipline’s understanding of past climates, particularly in the winter rainfall region of Southern Africa by analysing the stable isotopes in animal remains.
Her work focuses on stable carbon (C-12 and C-13), nitrogen (N-14 and N-15), and oxygen (O-16 and O-18) isotopic ratios in bones and tooth enamel of contemporary and fossil animals, since these signatures are a good proxy for past environmental conditions, offering insights into historical climates.
Luyt’s work in the field of stable isotope analysis has allowed her to reconstruct climatic conditions during a time in the past 2.6 million years in this part of Africa, including various meteorological factors, such as annual rainfall and temperature.
Enamel scrapings from an animal that died hundreds of thousands of years ago can tell us what conditions our early ancestors survived and evolved in.
The south-western part of South Africa has had a winter rainfall climate for about 5 million years. This part of the continent also has important archaeological evidence of early human presence during this time.
People living in the winter-rainfall are of southern Africa — in today’s Western Cape province of South Africa — would have adapted their resource use and behaviour as climate factors shaped what foods were available for them to glean from the environment.
Archaeological work combined with isotopic analysis at the Boomplaas cave near the Cango Caves, just inland of Plettenberg Bay, have shown, for instance, how a suddenly warming climate at around 17,000 years ago forced hunter-gatherer communities into diplomatic negotiations. As Earth slipped into a warming phase, glaciers at the poles melted, and global sea levels rose. Here on the Agulhas Plain, an area the size of Ireland disappeared under an advancing tideline. Until then, hunter-gatherer clans had been able to share the abundant plains where they hunted game and collected plants. Now their pantry had shrunk dramatically, and groups needed to meet to negotiate how to share limited resources.
This is the kind of societal behaviour archaeologists like Luyt are able to make sense of, simply be knowing how to read the mysteries hidden inside the teeth of long-dead grazing animals.
Layering together different scientific and archaeological methods of enquiry, archaeologists can piece together an evidence-based story about how climate and environmental changes might have shaped ancient human societies, influencing migration, agricultural and settlement patterns, the pursuit of and negotiation around resource use, and related social and political dynamics.
The De Hoop Nature Reserve where Luyt sourced the zebra molar is in the same window of winter rainfall that these diplomatic gatherings at Boomplaas took place. Once Luyt is done with her analysis of this single puzzle piece, she will return the tooth to its rightful place — she will put it back into the jawbone of the animal at the reserve from where she borrowed it.