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SCIENCE T H E LO OM

Meet
Meet Your
Your Inner
Inner Mole
Mole
Rat
Rat
By Carl Zimmer
August 15, 2005 • 6 min read

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.)

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