Hi everybody, it's Hank. On your friendly, local Eons social media, of some kind.
Do not forget: Our 2023 calendar featuring some of our favorite “main characters of
paleontology” from classic Eons episodes - available at ComplexlyCalendars.com!
One day in eastern Africa, about 3 million years ago, a tree branch cracked and broke,
sending a small human ancestor plummeting to the ground.
This wasn’t any old tree, but one that was nearly 14 meters high,
almost as tall as a four-story building.
And the poor little hominin didn’t survive this, breaking most of the bones in her body upon impact
And maybe you know this hominin. Or at least you might have heard
of her. Scientists call her Lucy and her species is Australopithecus afarensis.
And a paper published in 2016 argued that this might’ve been
how our famous fossil ancestor met her end, by falling out of a tall tree.
But, in order to fall from a tree, Lucy would’ve had to spend time in trees–right?
And scientists are split on whether or not her species was still doing a lot of climbing.
So, what actually happened to Lucy? Did she really take an ancient nosedive?
The answer is part of a decades-long debate over how exactly our ancestors
transitioned from life in the trees to life on the ground.
And it turns out that Lucy’s fall might be a question that’s as much
about how our ancestors lived as it is about how she died.
In 1974, when Lucy was first discovered in what’s now Ethiopia,
it was immediately clear that she was a big deal.
From the arid soil, scientists recovered a lot of her fossils,
including fragments of her jaw, spine, ribs, pelvis, arms, and legs.
Her skeleton was about 40% complete, which meant that she contained tons of information.
In contrast, previous discoveries from Africa were often limited to skulls, jaws, and teeth.
Also, she was really old. By dating the volcanic layers surrounding her fossils,
scientists found that Lucy had been deposited just after 3.18 million years ago.
This made her the oldest fossil human ancestor found at the time. And this meant that she
could answer questions about the evolution of some of our most unique adaptations.
For example, in what order did our ancestors acquire each of the
traits that make up our unique suite of features today? Like,
did we evolve a big brain first or the ability to walk upright on two legs?
Probably the biggest thing that Lucy revealed is that bipedalism
had already evolved by the time her species appeared – a topic we
covered in another Eons episode, called “When We First Walked.”
The shape of her pelvis, as well the inward angle of her femur and some other features
suggested that she was already an obligate biped – she had to get around on two legs, not four.
So the combination of Lucy’s completeness, her old age, and the insight she provided
into our evolutionary history were the major factors that made her famous.
Of course, it didn’t hurt that she had a catchy nickname. Borrowed from
the Beatles' song ‘Lucy in the Sky with Diamonds,’ the name was given to her the
night of her discovery, when the team was celebrating and that song came on.
But since Lucy was uncovered in the 1970s, her story has only gotten more interesting.
Scientists have continued studying her for decades and they’ve found more than
400 additional specimens belonging to her species, Australopithecus afarensis.
These come from across Ethiopia, Kenya, and Tanzania, and date from
3.7 to 3 million years ago. This makes Australopithecus afarensis one of the
longest-existing species of early human ancestors.
But, even after decades of research, we still haven’t learned all there is to know about Lucy.
In 2015, a group of scientists became interested in the amount of damage they
could see on her fossilized bones. Lucy appeared to have a lot of fractures.
Based on the pattern, they concluded that a fall from a tall tree had caused the breaks.
Their interpretation of the fractures was that Lucy had stretched her hands out in front of her,
to try and stop her fall, breaking her hands, wrists, arms, shoulders, and jaw in the process.
Basically, she was a wreck.
It was an astonishing idea,
one that allowed us to vividly imagine what might’ve been her final moments.
But while the idea that Lucy fell made headlines all over the world,
it was received a bit more skeptically across the scientific community.
Because it fueled a debate that – like Lucy herself – had been around for decades:
Did life in the trees remain important to Australopithecus
afarensis even after they found their footing on the ground?
Now, we know that before our lineage split off from the one that led to chimpanzees,
our shared ancestors were up in the trees.
After that split, the members of our branch of the family tree transitioned to life on
the ground, potentially starting around 6 or 7 million years ago.
But the move down to the African forest floor didn’t happen overnight.
There were likely many different stages in the transition. And because of where
Lucy sits in the family tree, her species potentially is a key piece of the puzzle.
And since we can’t go back in time and observe whether Lucy
was hanging out in trees, we have to go on the skeletal evidence.
We already know from her lower body, specifically her femur, that there’s no
question that Lucy and other members of her species were adapted to be skilled walkers.
But when we look at their upper bodies, the picture starts to get interesting.
A shoulder blade preserved from a juvenile of her species,
nicknamed “Lucy’s baby,” looked more like a tree climber than a permanent ground dweller.
And the slightly curved finger bones seen in her species are well-adapted for grasping branches.
So Australopithecus afarensis had some adaptations for moving through
the trees --but did they actually use them?
Some scientists think that these traits
in Australopithecus are just a useless evolutionary holdover.
See, natural selection really only modifies what’s important at that time. Other stuff
sometimes just gets left as-is, sticking around long after losing its evolutionary purpose.
And this brings up an important distinction: does the skeletal evidence point to things that
a hominin’s body could physically do? Or, is it pointing to a thing that hominins actually did?
There might have been some advantages to being a good tree-climber,
even after becoming a good walker.
For example, some other ground-dwelling apes,
like chimpanzees, often build nests in tree canopies to sleep.
This strategy would have worked well for hominins like Lucy. She was small,
only about a meter tall, and not particularly fast– a prime
candidate to be preyed upon by big cats and other fierce predators.
But behaviors like nest building and canopy sleeping don’t really fossilize.
And this is where Lucy’s fall comes in. It offered a potential new type
of evidence that theoretically showed what Lucy actually did.
By arguing that she fell from a tree, the researchers were
suggesting that she was essentially caught in the act of being in trees.
And while broken bones that might’ve been caused by falls have been proposed for other hominins,
there was one major problem in this particular case:
the fracture patterns weren’t unique to Lucy.
When the 2016 study that put forward the fall idea first appeared,
researchers working at her discovery site, a place called Hadar, quickly objected.
They said that similar fractures were prevalent on the bones of other animals
like elephants, giraffes, and rhinos found throughout the site–not just on Lucy.
Lucy’s discoverers pointed out that it was more likely that the damage was produced
by fossilization processes, resulting from millions of years in the ground.
During that time, the bones were probably exposed to water, crushing events,
pressure from the sedimentary layers above, erosion–and even plate tectonics.
This type of damage better explains why these fractures
appeared across species at this single location.
Because, as one researcher pointed out,
rhinos weren’t exactly falling out of four-story-high trees.
While Lucy’s fall was possible the scientists hypothesizing it didn’t
consider all the other explanations for these kinds of fractures, which is a must in science.
And the question of whether her species still spent time in the trees,
or how much time they spent there, is still open.
So, while the fall hypothesis kind of… lost its footing,
what the study really shows is that Lucy’s bones still have things to teach us.
And even after decades of research, we still love Lucy.
We’re falling for this month’s Eontologists! Jacksy Weiss, Colton, Melanie Lam Carnevale, Annie
& Eric Higgins, John Davison Ng, and Jake Hart. Become an Eonite at patreon.com/eons and you can
get fun perks like submitting a joke for us to read. Like this one from Kassy W.
Where did the female dromaeosaurid live? I don't know, but Alaska.
That didn't make sense until I heard myself say it out loud,
and then it just became so much worse. And as always thanks for joining
me in the Adam Lowe studio. Subscribe at youtube.com/eons for more fabulous fossils.
This line just reminds me of Cher:
"And since we can't go back in time and observe whether Lucy was hanging out on trees..."
If I could turn back time...
And since...
And since we can't go...
Now it's in my head!
Picturing Cher on the battleship...
Yes!