Gail Ashley is a geologist and the director of the Quaternary Studies Program at Rutgers. She’s been working in paleoanthropology since 1994 and has conducted extensive field research in the East African Rift Valley, especially on the hydrology and ancient environment of the region. We discussed her work, including the role of water in human evolution and how geologists try to understand landscapes of the past.
Why did you decide to focus on water to study human evolution in the East African Rift Valley?
My whole approach right from the beginning was looking at resources, especially water. If you spend any time in Africa, one day is a problem without water. So I was thinking of smaller spatial scales, shorter time scales, and looking at things from a much higher resolution, from the point of view of what I call “bare feet on the ground,” people having to survive on a daily basis.
The record that’s preserved after 2 million years—there is not always a lot. Sometimes you have to use a bit of creativity to connect the dots, and then figure out how to prove that scientifically. It has been getting more arid through time, but that’s operating on such a long time scale that it couldn’t possibly be a major factor to me. To me, it’s a day-to-day basis that affects whether people live or die, and it’s that long-term accumulation of day-to-day activities that leads to natural selection and evolution.
I find myself sometimes a voice in the wilderness in trying to talk about things at a scale like that. I think water is key. Water for drinking, obviously, but also water was important in terms of resources, the plants and even protein sources that would be associated with springs and wetlands.
Does the geology of freshwater resources play any role in the size of the populations for early hominins?
There is a logical connection to that. Everybody envisions the populations to be relatively small. They’re being modeled after things like chimpanzee troops and other types of primates that tend to occur in relatively small groups. People could also look at the size of groups of the few hunter-gatherer humans that we have now, like the Hadza. But I I’m not an anthropologist, I’m not a biologist, and so I have to go by what I’m being told.
An area can only support so many people, particularly if the springs are small. That’s what I’m trying to figure out, why the springs tend to be relatively small. It’s all connected with the geology: how fast the water can flow, what the recharge is from rainfall, the rate of flow out of the spring and how fast it evaporates. A spring that’s small and slow-leaking can actually provide quite a bit of water, whereas one that’s flowing faster might evaporate more quickly.
How did volcanoes and earthquakes impact the populations?
The tectonics associated with the Rift Valley are an ongoing thing that certainly affect the topography and where groundwater exits, so it’s really important in connection to the plumbing, so to speak. In terms of faulting or earthquakes, those do not operate on the timescale that’s going to affect a person that lives 20 years. They might feel an earthquake or see the effect of a fault somewhere, but the only thing that might impact them would be volcanism. Volcanism has occurred and when it does it can be very dramatic and have a high death toll, but that’s local. The land and people tend to recover from it. Overall, tectonics is something that happens as an ongoing part of the system, but I don’t think it really has a major impact on the individual hominin or hominin family, on a day-to-day basis.
How do you use geology to answer questions about food sources and water?
I am a sedimentologist, so I spend most of my time looking at rocks that start off as loose sediment like sand, silt, or clay, and then get cemented with time and turn into rocks. The record of the homimin environment is encased, or archived, as I often say, in these sediments.
Some of the things that are archived are organic materials, and that organic material is a record of what was on the land’s surface: what microbes were there, what plants were there, and in some cases, what animals were there. It made a lot of sense to me, when I was trying to reconstruct what the land’s surface would’ve looked like, to find out if there were trees there. Were they in the middle of a forest or a small woodland? Was it a grassland with scattered acacia trees like everybody seems to talk about? When a hominin was looking around, what would he see? The way to determine that is by looking at the organic matter.
What’s the most useful tool for your research, the thing that people might not understand about using geology to study the landscape?
The most fundamental thing that is important to the research that I’m doing, is that you need be able to correlate beds over great distances. Which means the stratigraphy—your understanding of the layers and how they vary—has to be well documented. If you don’t know that you’re within a time horizon and you slip off into something that’s 10,000 years older or younger than you think, then you’re not reconstructing the landscape at all. You’ve got a hodgepodge.
For the work I’m doing, precise stratigraphy is really important. If you don’t have good geology, or good precise locations of where your samples are coming from, then your samples aren’t worth anything. And if you’re samples aren’t worth anything, the data that comes out of them aren’t either. So it all boils down to good field work.
What are the big unsolved questions for you? Is there anything that drives your research?
One of them is the quandary that many of these hominins, or at least Paranthropus, had teeth that appear to have a geochemical record of them living on a savannah or eating grass. Which doesn’t make a lot of sense. For one thing, there’s not a lot of nutrition in grass. This is because scientists have been very narrowly associating the isotope signature that appear in the teeth to be only grass. What we’re finding is that there are a lot of other plants that have a similar signature but can occur in different environments. There are plants that occur within springs that have a signature which is similar to the grass.
Knowing if it’s grasses or wetland plants makes a big difference, because one of the pervading ideas about what drove evolution is the aridity forced hominins out of trees to go walk on the grassland and eventually hunt. And it might very well have been that they lived in an entirely different environment. I’m hypothesizing that they were tied to springs, and that they were eating vegetation that provided a signal that was similar or the same to what would’ve been derived from eating grass. This would change the whole idea of how people evolved, what was the driving mechanism for their evolution. But it’s not very well worked out yet.
What you would like to see if you traveled back in time?
If I could go back in time, I would like to see what kind of movements hominins did on a daily basis and on a seasonal basis. I’m pretty sure that hominins did move around on the landscape and that a lot of the water that we’re talking about, the water I’ve been studying, is in low areas. I think hominins might’ve moved down in low areas to acquire meat and then moved back up on the highlands. It could’ve been done a daily basis, but definitely on a seasonal basis. I suspect they might’ve moved up higher on the slopes where it might’ve been a little cooler, but then they would come down low, into the valleys, because there would be more grazing animals and that’s where the springs often disgorge, at the bottom of slopes. So there would be animals and opportunities for scavenging or hunting.
What would you be afraid of running into, if you traveled back in time?
For the most part carnivores are not so worrisome to people, they much prefer animals to eat. Of course, if you meet one at the wrong time it’s not good, but for the most part they prefer getting a good chunk of buffalo. But snakes—I could do without snakes, thanks.