Mole rats are a pretty ugly, obscure bunch of creatures. They live underground in Africa, where they use their giant teeth to gnaw at roots. Those of you who know anything about mole rats most likely know about naked mole rats, which have evolved a remarkable society that is more insect than mammalian, complete with a queen mole rat ruling over her colony. But according to a paper in press at the Journal of Human Evolution, mole rats are important for another reason. Their evolution and our own show some striking parallels that may shed light on how our ancestors diverged from other apes.
The authors of the paper, Greg Laden of the University of Minnesota and Harvard’s Richard Wrangham, believe that the rise of hominids was marked by a shift in food. Reviewing the evidence from fossils and living apes, they argue that common ancestor of humans and our three closest relatives (chimpanzees, bonobos, and gorillas) dwelled in a rain forest. If this ancient ape was anything like living chimps and gorillas, it depended mainly on fruits. When it couldn’t find fruits, it turned to other so-called “fallback foods” such as soft leaves and pith.
Judging from the fossils of plants and animals found alongside early hominid bones, it seems that hominids shifted from dense rain forests to woodlands, and much later to open, arid savannas. It would have been harder to survive on the diet of a gorilla or a chimpanzee in such places. Laden and Wrangham point out that in Gabon, gorillas that live in rainforests don’t venture into the surrounding savannas, despite the fact that the savannas get a lot of rain. The problem is that outside of rainforests, there just aren’t enough of their fallback foods to sustain them.
So how did hominids survive? Laden and Wrangham argue that they began to rely on a new fallback food: roots, tubers, and other “underground storage units.”(To me this term sounds too much as if it came from a subterranean Ikea catalog, so I’ll just use the word tubers.) The idea was first proposed in 1980 by other scientists who observed that one important difference between hominids and other apes is their teeth. Chimpanzees and gorillas have shearing edges on their teeth that help them slice up leaves. Hominids had teeth that resembled those of pigs and bears, which can chew tough, fiber-rich food. Pigs dig up tubers with their snouts, bears with their claws. Fossil discoveries suggest that hominids might have used sticks or horns. But they all chewed the tubers in much the same way.
In the new paper (posted by Laden here), Laden and Wrangham explore this idea in much more detail. They point to evidence that tubers are more diverse in savannas than in rain forests, and grow at densities that can be hundreds of times higher. This makes intuitive sense when you consider that tubers are probably adaptations to dry, unpredictable climates where plants need to store away energy underground. In the stable dampness of a rain forest, there isn’t much use for a tuber. Laden and Wrangham also point out that human foragers who live where lots of tubers grow take advantage of them. They prefer other food, like ripe fruits, but in tough times they dig up their meals.
Laden and Wrangham then turn from the present to the past. If their hypothesis is right, hominids must have lived in places where they might have eaten tubers. That’s a tricky question to answer directly for most sites where hominid fossils have been found, because scientists haven’t found enough plant fossils associated with them.
Enter the mole rats.
Mole rats love tubers, and where you find mole rats, you generally find a lot of tubers for them to gnaw on. What’s more, mole rats and humans have a taste for many of the same species that produce underground storage units. Mole rats have left a long fossil record in Africa since they first appeared some 20 million years ago–not coincidentally when tuber-rich habitats may have begun to spread through Africa.
Laden and Wrangham predicted that hominids and mole rats should tend to have left fossils in the same habitats. They looked at fossil sites from six million years ago to half a million years ago in eastern and southern Africa, where hominids lived. They then picked out sites where either hominids or mole rats had been found, or both. Of the 21 sites that had mole rats, 17 also had hominids. Less than a fifth of the sites without mole-rats had hominid fossils. The pattern suggests that mole-rats and hominids both evolved to take advantage of the rich supply of tubers in African savannas. They came at the tubers from below, we from above.
Dribs and drabs of this hypothesis have trickled out over the past six years. In a 1999 paper in the journal Current Anthropology, Laden and Wrangham and their colleagues suggested that tubers were important to hominids and then became really important about 1.9 million years ago. At that time, hominids began emerging who were much taller and bigger-brained than their ancestors, and who also had smaller teeth. Laden and Wrangham argued that hominids at this time must have discovered fire, which would have allowed them to cook down tubers, liberating much of the nutrition in them. In this 2002 article Natalie Angier offers a nice summary of their thinking at the time—along with the skeptical reaction it drew from some experts. One big problem is that the oldest good evidence for fire is only a few hundred thousand years old, not almost two million.
The new paper doesn’t address the skepticism about this later part of their scenario. Instead, it looks back at the first four million years of our life with tubers. Laden and Wrangham propose testing their hypothesis by looking at the trace elements and isotopes in tubers to see if the patterns are reflected in the composition of hominid fossils. I also wonder about how they got hold of the tubers. Were the earliest hominids able to fashion digging sticks, or were they merely using their hands, the way savanna baboons do today? How exactly, I wonder, did we get to be the upright mole rats?
(Update: 8/15 10 am: Thanks to Hoopman for pointing out some new findings that may show evidence of fire 1.5 million years ago. Here’s a BBC article with some details. As far as I can tell, though, the results have only been presented at a conference. They haven’t been published in a journal.)
Originally published August 14, 2005. Copyright 2005 Carl Zimmer.