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The origins of walking
Walking is much more ancient than scientists realised
– possibly as old as 20 million years.
Coming
down from the trees
Hard evidence of walking
Until very recently, the debate over walking seemed to
concentrate on some 
was
simple. When they came down from the
trees, they had to become thoroughly at home on the ground very quickly – or
nothing more would have been heard of them.
It now turns out that I was being too timid!
A comprehensive article in the top journal Science (1.Jun.07) claims that tree
living apes had long been into “hand assisted bipedalism”. They walked along the slender outer branches
of trees, holding on to other branches for security, to get at the best
fruits. They also used these branches as
a bridge to the next tree.
This picture gives the game away. It comes from a more readable but less
comprehensive article in New Scientist
(9.Jun.07). The work involved much study
of living orang-utans in
You need long arms and legs to walk in the trees. And you need elbows and knees that are able
to straighten fully. So it should be
fairly easy to detect this ability in fossils.
The theory is that all the great apes were once bipedal in this way,
including of course the common ancestor of us and chimpanzees, Ardipithecus – but that many of them
lost it for reasons that we are about to come on to.
Coming down from the trees
What brought us down from the trees? In a word, climate change. As we’ve said, the climate dried, and the
trees became further apart. The apes
appear to have had two choices. Some
moved over to the large trees, so that they didn’t have to come down to ground
much. They sacrificed their walking
ability in order to become much better climbers. They developed permanently-bent hips and
knees and elbows for example, and no doubt other features as well. This way they could get quickly from bottom
to top of a tree and back again. For
when they did need to walk on the ground, each species developed it own form of
knuckle-walking. I’ve always thought
that knuckle walking looked terribly half-baked. Now we know why. It hasn’t been around long.
Others stuck with the old ways, and stayed with the
smaller trees. This meant that they had
to come down to the ground a good deal more, to get from tree to tree. They were already well adapted to this so it
was an easy option to take.
At
some point, one small group may have found itself without any trees at all –
perhaps just bushes and scrub. They had
to adapt fully to ground living, and we will have been descended from them.
Hard evidence of walking
Enough bones have been found to suggest that the
Australopithecines were fully ‘bipedal’, walking erect like us. We’d expect that now, but I think it was a
surprise to the scientists that we started walking properly “so early”. In fact I’ve read that they are having one of
their ‘acrimonious debates’ about it.
The Australopithecines
were certainly walking properly by about 3.6 million years ago, because
footprints have been found. This
remarkable picture comes from “Earth Story” by Lamb & Sington. It seems that a volcano erupted, spreading a
layer of ash over the ground. A couple
of adults walked over it, leaving clear footprints. (The prints on the right are those of an
extinct 3-toed horse.) It rained, which
turned their footprints into concrete.
And then another layer of ash covered the tracks.
The scientists took careful plaster casts of the
footprints, and they proved to be very similar to the kind of footprints that
we make. They are quite different from
those of chimpanzees.
However the Australopithecines
were still not true ‘people of the plains’.
They could not walk and run long distances like we can. That capability had to await the arrival of
‘the mighty hunter’, Homo erectus.
[Click for the next chapter,
our first serious ancestors]