Early Humans: Climbing/Walking Tradeoff Pushed Back

"Early Humans Were Poor Climbers"
LiveScience (April 13, 2009)

"Our ancient human ancestors traded in the ability to climb trees for the power to walk on two legs, but it is unclear when this happened in evolutionary time.

"A new study could help pin down the timing of this exchange, revealing that human ancestors as far back as 4 million years ago didn't have quite the climbing skills of modern chimpanzees, so climbing was phasing out by this time.

"The evidence: Early humans lacked the ankle structure that assists chimps in climbing, according to anthropologist Jeremy DeSilva of Worcester State College in Massachusetts. ..."

It's not that the early humans couldn't climb: but it looks like they were better at walking upright than the were at climbing.

One piece of evidence is the arrangement of their feet. Walking on two legs - bipedalism - takes a lot of energy for lifting the foot and ankle. That puts a premium on making those parts as light as possible. There's an arrangement of bones that would make our feet better for climbing trees - but it'd be an add-on to what we have. Which would mean more weight.

Our feet are pretty good for walking - but not so much for climbing. And, it looks like the tradeoff between good climbing and good bipedal walking started three or 4 million years back.

The article gives a brief overview of the benefits and risks of walking around the savanna without being good at climbing, and includes something I haven't run into before:

"...Other research suggests that early humans at this time had limb proportions similar to other mammal species that are particularly aggressive. Perhaps early humans used aggression to discourage predators from targeting them...."

I wish they'd given some hint of what the limb proportions were - but it does make sense. I'd think it would be easier to prey on a creature that tries to run away, rather than one that's crazy enough to attack back.