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Science and Technology

Exploradio - Our cousin in the trees
A fossil foot discovered in Ethiopia shows that nature experimented with more than one manner of upright walking on the path to humankind.
This story is part of a special series.

Reporter / Host
Jeff St. Clair
'Ardi' was a survivor. Ardipithicus ramadis is 4.4 million years old, and its descendants were still around when "Lucy" walked the earth a million years later.
Courtesy of Jay Matternes, Science News
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The human family tree just got a little bushier. Cleveland researchers say fossil foot bones recently discovered in Ethiopia belong to a distant cousin of humankind.  But, while the foot’s original owner lived in trees, they say our direct ancestors walked a different evolutionary pathway.

In this week’s Exploradio:  A fossil foot and the origin of the human family.

Exploradio - Our cousin in the trees

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Owen Lovejoy on the origin of upright walking

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Trees by a lake
Yohannes Haile-Selassie celebrated his discovery at a press conference late last month in Ethiopia.  

On his way back to Ohio, the Cleveland Museum of Natural History curator described where in the Ethiopian desert he found the eight fossilized foot bones.

“The local name where we found this partial foot is Burtele. It’s located in the central Afar region of Ethiopia.”

The area is bone dry now. But Haile-Selassie says it once at the edge of a large lake.

“And also trees [were] associated with this water body. So we think that it was sort of a wooded environment where this species to which this partial foot belongs, was living in.”

Trees were the home of this animal 3.4 million years ago, according to Case Western Reserve University professor Bruce Latimer, who worked with Haile-Selassie in publishing the find. Its mode of locomotion surprised Latimer. He says the creature had a grasping big toe, ideal for tree climbing. But it also stood tall like a human.

“This is an animal that was still climbing trees. However when it came to the ground, it walked on the side of its foot as a biped, an erect biped. Which is peculiar.  Who would have thought?”

Our cousin in the trees
It’s the ability to walk upright that defines the human lineage. And the human foot is uniquely adapted to handle the stress of a vertical lifestyle. But Latimer says, THIS foot, with its thumb-like big toe, was not.

“When it came down, we can say that, but it was doing it with an awkward kind of gait, and it certainly didn’t go distances. It wasn’t going long distances at all.”

What’s significant about the find is that there already was a primate living 3.4 million years ago in that part of Ethiopia. But it walked just like us. Her name was “Lucy.”

Latimer says the foot discovered recently in Ethiopia belonged to a much older animal that persisted in the trees while our ancestor Lucy strode across the ground.  

“You can imagine that you’d have had Lucy looking up and wondering, ‘What is that thing?’”

Kent State’s Owen Lovejoy, who was not involved in the foot find, says it complicates our picture of the past.

 “The evolution into Australopithecus afarensis, or Lucy’s species was a bit messier than we’d like to have it if we were going to draw it on a blackboard.”

But he says the foot provides an important clue as to what may have led to our existence.  

Lucy takes the low road 
Lovejoy says the foot probably belonged to an older species, similar to Ardipithecus, which lived a million years earlier than Lucy. 

Lovejoy says with Ardipithecus still hanging around in the trees a million years later, Lucy and her kin sought opportunities for survival on the ground.

“Probably the existence of these descendants of Ardipithecus accentuated and accelerated the evolution of upright walking in our ancestors.”

But what was the advantage of upright walking in the first place? Lovejoy has a theory for that too.

“Upright walking provides the capacity to carry things.”

The first family 
Lovejoy says the ability to carry two things in particular, babies and food, was the genesis of human kind.

“The selection for upright walking was in fact a shift in reproductive strategy where males would pair bond with a specific female and exchange food for that pair bond.”

In essence, according to Lovejoy, the invention of the family in the first primates to walk upright, is the defining moment in human evolution.

“Humans are the only example of where you have monogamy but within a social setting, and that was probably the big breakthrough in our evolution.”

Back in Ethiopia, Yohannes Haile-Selassie and his team are trying to figure out just who the newly discovered fossil foot belongs to. He says fossil teeth discovered nearby don’t belong to any known species of human ancestor.  Another few years of fossil hunting in the Ethiopian desert may provide more clues to the puzzle of how we came to be human.

Related Links & Resources
"Lucy" Lived Among Close Cousins -

Burtele Foot Indicates Lucy Not Alone -

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