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What a waste are two thumbs on the
space bar. There they sit, nearly flaccid, punctuating the end of
each word, awaiting the call to crack stone or to use sharp flakes
to incise wood.
It is easy to think of other traits as making
us human. We talk, use metaphors, empathize, follow fashions, laugh,
play politics and Angry Birds. But many of us still work with our
hands, crafting fine objects or simple tools, digging and harvesting.
Or texting. As they have always done, thumbs and fingers connect us
to our social lives.
No living ape has such digits, each long
thumb with its own wide, flat fingertip bone. Ape thumbs are short
afterthoughts, jutting awkwardly below the long, strong bones of the
hand. No ape supplies each thumb with its own flexor pollicis longus,
deep in the forearm. This muscle pulls powerfully on the thumb bones,
clamping them forcefully onto the stone. The pinky bones, too, get
into the act, flexing inward to cup the palm around a rock. No ape
has a pinky as mobile as ours, or short fingers with broad
fingertips. Those wide fingertips, wider than any ape or monkey’s,
spread the pressure over a broader surface, making the grip fast
against the percussive rebound as the stone wallops into another at
the abrupt end of its arm-driven ballistic path.
Toolmaking is
not ours alone. Primatologists have known for more than 50 years that
apes make and use tools in their natural habitats. Even in Charles
Darwin’s time, the manipulatory abilities of monkeys and apes were
widely known. Today we can watch as a chimpanzee makes delicate
termite probes, using hands that are built like meat hooks. All
living apes have long and powerful fingers, each finger phalanx
wrapped with a strong sheath enclosing the cable-like tendon that
stretches from arm to fingertip. Their anatomy gives them a forceful
curling grip that no human can match, but little control at the
fingertips.
Only after long training could Kanzi, the famous bonobo, chip flakes comparable to the earliest artifacts made my
human ancestors. The problem was not his brain: He seemed perfectly
to understand the utility of the flake, how and why to cut things
with the sharp edge. The hard part was mechanical: controlling the
arms and hands without a powerful fingertip hold.
The
proportions of our hand, including our long thumb and relatively
short fingers, emerged early in the human lineage, over 3 million
years ago. The species of the famous “Lucy” skeleton,
Australopithecus afarensis, had digits proportioned much like ours,
very different from chimpanzees and other apes. The thumbs of some of
these early ancestors were equipped with the flexor pollicis longus
muscle, though they were generally weakly developed. The muscle
within the fleshy mound at the base of the thumb was likewise weak.
These were not hands for making stone tools, not yet. The broad
fingertips and manipulatory abilities of later humans came later. The
hands of Au. afarensis kept strong flexor tendons and curved finger
bones, likely because the species still used its hands to climb and
cling to branches. Like Kanzi’s hands, these were useful both for
using tools and climbing, but not for inventing tools from stone.
As
anthropologists probe deeper into the origins of humans, they
naturally emphasize the brain. They point to the brain’s rapid
enlargement during the past two million years, the expansion of the
neocortex relative to the whole brain and the apparent reorganization
of cortical areas of the frontal lobes. The brain’s evolution touches
upon all those behaviors that seem to separate us so clearly from
other creatures: language, social complexity, invention.
Individually, these features overlap with other primates, but in each
case evolution has tweaked existing abilities to enable new cultural
abilities in the human lineage. Even when they consider early
toolkits, archaeologists often write of the cognitive abilities and
behaviors driving tool use—selecting raw materials, transporting
carcasses, organizing food sharing. Functional MRI scans have even
shown the recruitment of brain areas as expert stone-tool-makers
visualize their craft. One assumption underlies much of this
thinking: technology may make us human, but the brain is in the
driver’s seat.
So isn’t it interesting how natural it seems to
describe this evolution—“point,” “probe,” “touch,”
“tweak”—with terms that apply to our hands?
Among the first cave paintings are outlines of human hands—living stencils outlined by blown-on pigments.
Did hands
make us human? Did they allow our ancestors to finish the climb out
of the trees and enable them to become the uniquely intelligent,
social, adaptable primate that colonized the planet? The discovery of
a boy’s skeleton in Olduvai Gorge in 1960 made it seem that stone
tool manufacture, larger brains, and human hand anatomy evolved
together. The boy was around 10 years old when he died, leaving us a
fragmentary braincase, a jaw, and bones from his right hand. Broad
fingertips and a very mobile joint between the wrist and metacarpal
for the thumb gave the right anatomical signs for stone toolmaking.
Olduvai Gorge was also where anthropologists were finding the
earliest known tools. Believing they had found the toolmaker,
discoverers Louis and Mary Leakey and the describer Phillip Tobias
called the fossil Homo habilis: “handy man.”
But from this
simple picture, our understanding of human hand evolution has
dramatically shifted during the last two years. Two million years
ago, a quarter-million years before the Homo habilis boy, the species
Australopithecus sediba lived on the high plains of South Africa.
This species had small bodies and brains, but hands (pictured above)
that were more human in some ways than Homo habilis, with stronger
flexor pollicis longus muscles; flatter, straighter finger bones; and
some details of the wrist made to absorb the compression coming from
powerful blows. Au. sediba had a tiny hand compared to living people,
but it shares many of the key features useful for stone tool
manufacture. Today, the earliest known stone tools are 2.6 million
years old, and by the time of Au. sediba, somebody—maybe even a
broad array of species—left toolkits for archaeologists intensively
analyze. We know, judging by which stone outcrops the materials came,
that they appreciated the fracture mechanics of the rock. We know how
the tool-users took apart animal carcasses, transported the parts,
and smashed into marrow cavities. But what archaeologists cannot do
is place the tools into the hands of any particular species. Until we
know how tools really fit into the lives of these later
australopithecines, the steps leading up to the more complex brains
of our genus may remain a mystery.
Hand evolution did not end
with the appearance of Homo, as later people continued to increase
the muscular power and bony strength of their hands. Along with our
cognitive talent for invention, we require a more mechanical
facility: hands capable of building and precisely controlling the
tools we imagine.
Since the beginning, we’ve also been using
those hands to communicate. Among the first cave paintings are
outlines of human hands—living stencils outlined by blown-on
pigments. These traces come only from the last stage of our
evolutionary journey, but even here we see how pointing and touching
are so fundamental to human communication. Humans talk, but even
simpler, we point. Psychologists studying learning in human children
and chimpanzee juveniles have found that a key difference between our
two species is the ability to share attention jointly with another,
more experienced individual. When a mother holds a toy in her hands
and names it, the child can learn that word. When a mother points at
the toy later on, the child will understand. By directing attention
with our hands, early humans may have laid the groundwork for spoken
language.
And now, we type and text.
When it comes to
the information content of human imagination, statisticians and
information theorists sometimes muse about one question: How long
would it take a million monkeys to type the works of William
Shakespeare?
As an anthropologist, I have to believe this is
going to take even longer than they expect: Without the individual
finger control for touch typing, the million monkeys will have to
hunt and peck.
John Hawks is a paleoanthropologist at the University of Wisconsin-Madison, investigating the Neandertals and recent human evolution. His blog follows research and the development of his upcoming massive open online course in human evolution.
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