Gail Ashley likes to peer into the past and reassemble moments that shaped humankind’s trajectory from hominin to human. Of course, it’s a puzzle that can never be fully resolved. Like everything in science, the best solutions represent statistically significant estimations. While this openness may sound daunting, it also allows researchers like Ashley to exercise a fair amount of creativity in their approach to gathering and interpreting data. Whether it is using geological layers of sediment to date artifacts or smashing atoms together to indirectly confirm the existence of Higgs bosons, coloring between the lines can come from any number of sources. In her case, it was water – specifically ground springs.

A recent collaboration between Ashley and Tom Gleeson (University of Victoria) attempted to simulate the movement of early humans out of Africa using access to water as the key variable in their model. Their results were as surprising as they were enlightening. 

Professors Ashley set aside time from her hectic schedule to discuss the paper and a life in science with SINQ.

SCIENTIFIC INQUIRER: What is the general consensus regarding the conditions early humans faced when they left Africa?

GAIL ASHLEY: The general consensus is that humans originated in Africa. They gradually moved northward and into the Levant area and then into Europe. We know the timing to be about two million years ago. However, the exact path and detail of how they survived are unknown. There’s not a lot of evidence about their lifestyle during that period nor about the route by which they exited Africa. We can, however, surmise the route.

SCINQ: Were the conditions dry at the time?

GA: We know the conditions in Africa between two and three million years ago. The climate fluctuated in twenty thousand year cycles. There were time periods when it was wet and ten thousand years later it was dry. Then there were transition periods between wet and dry. These were naturally occurring changes in climate.

A lot of people made the assumption that humans only moved when it was wet and that they used lakes and rivers as a source of water. They ate whatever food that might be found in the vicinity.

SCINQ: Can you discuss hydro-refugia and how they fit into the picture you just painted?

GA: The term itself is new and introduced in the paper. One of our coauthors, Tom Gleeson, claims the term. It captures the two very important elements of the model of how humans left Africa which is that water was absolutely key. As you know there is no substitute for water. There isn’t now and there wasn’t in the past. Food was key as well. Hydro means water and Refugia means a site where people or plants take refuge. So hydro-refugia are places where people can survive even though surrounding areas might not have been conducive to living. It is a refuge connected to a water source. We feel hydro-refugia are key to how humans dispersed and migrated out of Africa.

SCINQ: Why is it key to how humans migrated out of Africa?

GA: During dry periods in Africa, like now, most people rely on groundwater for drinking water. It is likely that this was true in the past. Most lakes in east Africa are salty because of high evaporation rates. They are not drinkable. Rivers tend to be seasonal – they disappear in the dry seasons. East Africa relies on groundwater.

It became obvious to us as scientists looking into how things were in the past that conditions were similar. During droughts animals and people had to rely on groundwater as a water source for drinking.

SCINQ: Would there be animals around these areas, the hydro-refuges?

GA: Yes. There’s a double edged sword with groundwater. Groundwater can be a continuous source of water day in and day out, year in and year out, and springs continue to flow whether the ambient weather is dry or wet. Springs are extremely important for life. We’re focusing on humans but they would be attractive to animals as well. That leads us to a number of connections. Hunting might be easier. These water sources are an attraction for animals. Not only would they be an attraction for vertebrates but also for carnivores that eat the vertebrates. While springs might be a predictable water source, they were also a place of danger.

SCINQ: Can you explain the significance of water that your group’s paper proposes. How does this alter our understanding of the migration?

GA: The paper involved collecting an enormous amount of data on the modern environment — What the landscape is like today –from northern Tanzania all the way north to Ethiopia. That’s about two thousand kilometers or more. We have mapped out the location of springs in this area using current maps and Google Earth. We made a regional map of where springs were located. The modeling that was done is called agent-based modeling. It’s a computer program that predicts what a person might do or could do given some limited physical constraints, namely that a person might be able to walk as much as a hundred and eighty kilometers in three days. This is based on a military calculation. If a person could make it from one spring to the next, in three days, that person lived. If that person could not, they died. The program would run on the model under wet and dry conditions for the equivalent of a few thousand years to see how successful any single person could be walking from spring to spring.

Our model showed that people could migrate out of East Africa even during dry periods if they went from spring to spring. Migration did not have to take place only when the climate was wet but could take place when it was dry. They were completely reliant on springs that were present on the landscape. People were tied to the springs;. these hydro-refugia. When conditions got really dry and people couldn’t make it from one spring to the next, they were isolated and had to stay at that area for a long time. These periods of isolation could set the scene for natural selection to take place. It’s possible with these kind of constraints – being connected to a spring with abundant water. But when walking to the next one wasn’t possible – people would be isolated for a long period of time. Under these circumstances, you can get gene transfer and/or isolation of genes. When people finally moved, genes could transfer. This might have been a factor in the evolutionary process.

SCINQ: Were your results in line with what you suspected at the start, or were there things that surprised you?

GA: There were a couple of things we did not expect. First of all, we didn’t suspect that there were so many springs. People haven’t focused on springs so there hasn’t been a lot of research on their locations. We were surprised at how many springs there really are in what is considered arid landscape. Secondly, they are pretty small. They aren’t huge gushing things. You can see them from the satellite but on the ground they are measured in meters. They are smaller than we anticipated but the water that was coming out of them was more than enough to keep flowing year in and year out. Our definition of a perennial spring was something like a thousand cubic meters a year of water. That’s a lot for a small hominin group and associated animals. That was a surprise.

The other surprise was that, during the wet period, it appears that most of the migrations were across the East African rift as opposed to along it . It’s a very large depression that extends from Israel down through the Red Sea and into Ethiopia. It then continues south into Kenya and Tanzania. It was caused by plate tectonics and runs north to south. One of our surprises was that during wet periods, the agent-based modeling suggests humans migrated across the rift from east to west preferentially. I guess that is because the water sources are closer together. Whereas during the dry periods they could only move north and south. The actual direction of the migration and dispersal varied with the climate picture.

SCINQ: Can you place this study into the context of your past work?

GA: I started working in east Africa in 1994 and discovered ancient springs in my first year there and have been working on them ever since. For me this is a culmination of a long period – two decades of studying groundwater resources in East Africa, and its connection to early hominins.

I’ve been working for a number of years with paleoanthropolgists on extremely small scales, looking at archaeological sites, and seeing what water sources were connected to those very small archaeological sites. They were about size of a room. I’ve been looking at these very small sites and studying the associated springs.

With this new paper in collaboration with new colleagues, I was able to take what I’ve been thinking about for a long time, and test if springs were important – not just on a day to day basis but long term for humans. I was able to see that there is likely a connection to the evolution of hominins in this area.

SCINQ: Can you explain where the springs and groundwater originate?

GA: The East African rift is a depression. It’s a low area, a valley, but adjacent to the valley are high areas. They’re blocks of rocks and volcanoes. The East African rift has lots of volcanoes associated with it caused by plate tectonic processes. The really high areas stick out. Some are two, three, four thousand meters high. They act as traps for rainwater from moist clouds originating from the Arabian Sea. This area of East Africa, where we are did our study is where winds, known as easterlies, blow from east to west. The winds come off the ocean, blow across the continent, and then come up against big mountains. [Volcanoes are high topographic areas.] The moisture and clouds cool and the rain falls on this area. The precipitation soaks into the ground and moves by gravity down hill and comes out at the bottom and into the Rift Valley. So the groundwater originally comes from rainfall on the peaks associated with the rift valley.

SCINQ: What brought you to a life in science?

GA: What brought me to a life of science? When I was younger, I was an outdoors kid. I loved being outside. I actually didn’t understand that science was a subject until eighth grade. Then I loved that class. I gradually became aware of my interest and attraction to the outdoors. At that time, I lived next door to a geologist, a professor, and I spent a lot of time over at his house talking to him and finding out what geologists did. He was always doing interesting things. I became interested in geology at an early age. When I entered college, I took science courses and became a scientist following my natural interest.

As a scientist, when I graduated with my PhD, most of my work had been involved in flowing water of different types. My doctorate was at the University of British Columbia in Canada. I worked on tidal flow channels and rivers. My whole training in college pertained to water, flowing water, lakes, that sort of thing. It was my area of expertise. My early career at Rutgers involved studies of different types of water in modern environments. I had natural background, interest, and expertise in water.

When I went to Africa, I was pretty much “a fish out of water” because it was arid. Extremely dry. But when I examined the geological record, the rocks left from the last one to two million years, I noticed that there was evidence of water bodies of different types like rivers, swamps, and lakes. I mapped those and found out that springs, in particular, were very close to and associated with archaeological sites, places where they found hominin skeletons and hand tools. It was guilt by association.

Here, I found paleo springs in the same age rocks as archaeological sites. It seemed logical to me that people would need water. Where we were working, it was extremely hot and dry with no water on the surface at all except for lakes that were precipitating salt. I realized there was a smoking gun association between the springwater and the evidence of water of being connected tonear where hominins were living and carrying out their lives. It just became obvious that hominins were using springs as a water source., realizing there’s no substitute for water. I also discovered that there was nobody working on this topic in Africa. There was a big scientific void there, a niche to fill, so I stepped into it. I’ve been working on it ever since.

SCINQ: How much do you think doing your PhD prepared you for an actual life in research?

GA: I wasn’t a typical candidate going through school. I married and had children early. I went to graduate school over a long period of time as a maturing adult. I wasn’t a kid. By the time I entered graduate school, I was thirty-two and by the time I finished, I was thirty-six years old. On one hand, I was lucky to go into school with a mature head. I had experience approaching problems that I think, I would have lacked had I gone to school when I was twenty-two. I wasn’t a typical case. I had a lot of mentors helping me along the way. By the time I finished my doctorate, I had already started to publish papers and was on the professional track. I knew how to conduct myself.

SCINQ: What do you think is the single most important trait for a scientist and why?

GA: That’s easy. Curiosity. You really have to be curious. If I narrow it down to one thing, I’d say curiosity. If you don’t have it, how you can develop ideas and formulate questions? And when things don’t work out and you get discouraged you think about another track. So that is one trait. (Persistence is another.) Curiosity is something I look at when working with my students. I think without that we are just following the things that other people have done. I don’t think you can open doors to new discoveries without it.

For more information about Gail Ashley and her research, visit her Rutgers University lab page. 


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