Walking on two feet

Walking on two feet

Lucy’s shattered bones prove our ancestors lived a dangerous life in the trees

A mock grave for Lucy at the University of California Museum of Paleontology in Berkeley. Wikimedia Commons, CC BY-SA

A ground breaking new study of the bones of a 3.2 million year old human ancestor (‘Lucy’) has revealed that she died from the crushing impact of a fall from high in the trees.

This exciting research published this week in the journal Nature adds great weight to the idea that Lucy and her Australopithecine kin spent much of their life in the trees, in addition to walking on the ground, an idea which has been controversial up till now.

But before we take a look at the new study about Lucy’s life ending fall, let’s briefly consider the main elements of what we know about the evolution of our uniquely human form of locomotion.

Humans stand out among all the mammals as being the only species to totter about on two feet. We are the bipedal apes.

Why did evolution transform a four-limbed, tree-dwelling ape, into a ground dwelling biped? How did it happen?

To try and answer these key questions, we need to take a look at the incredible environmental transformations that occurred during the Miocene Epoch, and the way they shaped the evolutionary trajectory of our ancestors.

This is a time when there were more than a hundred species of apes living across Africa, Europe and Asia.

In stark contrast, today we live with just a fraction of this past ape diversity, with only around dozen living species, from chimpanzees to gibbons, more or less clinging to the tropical zones of Africa and East Asia.

Among the early African apes was the one that would be destined to be the first biped.

Now, we need to hone in on a few dramatic events that set the scene for the emergence of bipedal apes; staggering global shifts that would shape the planet for millions of years to come.

The first was a series of dramatic falls in the concentration of carbon dioxide in the Earth’s atmosphere. Each time, dropping by roughly a third, and sending the planet’s climate swinging from warm and wet to cold and dry, and back again.

Three such fluctuations happened between 26 million and 11 million years ago, but it’s the final one that’s of most interest to us here. For it was the most dramatic plummet in carbon dioxide at the time, and ushered in a new and long cold phase, and we’re still living through it today.

Large, permanent, ice caps were formed, like those we have today, including the ice sheets of Antarctica.

Continental drift brought the Indo-Australian plate crashing into the Eurasian plate, lifting up the Himalayas, with its enormous consequences for global and regional climates like the Monsoons.

Massive forces tearing apart the African continent formed the Great Rift Valley of East Africa, bringing with it a new era of volcanic activity, mountain building, and the great rift lakes.

The final dramatic event was the spread of C4 grasses and the forming of new kinds of open, savannah, ecosystems. Likes those we’re all familiar with today from TV documentaries about the wondrous herds of wildebeest and antelope on the savannah plains of East Africa.

C4 plants fix carbon from the atmosphere in a more efficient manner than the geologically much older C3 plants, and are well suited to arid environments, like those that prevailed from 11 millions years ago.

C4 plants include arid adapted grasses, and most of the cereals such as millet, sorghum and maize, that we depend on today. C3 plants, in contrast, include trees, herbs, shrubs, tubers, fruits, nuts and most vegetables, as well as rainforest species.

The cooling and drying of the Earth’s climate saw a massive decline in forest cover, and by about 8 million years ago, C4 plants had spread in an explosive manner across the planet, replacing the older C3 plants in many places.

The spread of C4 plants also saw the rise of new kinds of mammals, ones adapted for feeding on grass. These are the grazing animals and they include the African antelopes, hippos, camels, deer, giraffe, sheep, goats and cattle.

Almost every domesticated mammal we humans eat regularly today, and those that provide us with milk, evolved during this time.

It was this tumultuous episode in Earth’s history – a period in which the apes flourished and then died out again – that saw the beginnings of the human evolutionary line itself.

It ushered in the rise of the bipedal apes.

Historically, the origins of bipedalism have posed a major conundrum for anthropology. Last time I checked, more than a 100 theories had been published aiming to explain its development.

Theories like: upright posture evolving for the purposes of sexual display, or to free the hands to carry weapons or tools; to allow our ancestors to see over tall grass and spot predators or prey; to feed from bushes; shuffle on their bums on the ground to feed off grass seeds; or even to run and hunt.

The theory that receives strongest support to this day is the ‘savannah hypothesis’, which I have just outlined above, keyed into the tumultuous events from 11 million years ago.

Surprisingly perhaps, bipedalism didn’t just arrive as a complete package from day one. It evolved in a series of step like changes, from 7 million until about 2 million years ago.

The earliest bipeds are found in the fossil record of Africa and date between about 7 million years and 4.5 million years ago.

These are the species Sahelanthropus tchadensis, Orrorin tugensis and Ardpithecus kadaba and ramidus.

They were probably all bipeds, but from the most complete skeleton we have – the 1.2 metre tall Ardi – they were firmly tree climbing apes as well, with grasping hands and feet, and very ape-like hip bones.

In fact, some anthropologists simply aren’t convinced that Ardi walked on two feet at all, while others see her lifestyle as mostly in the trees, combined with some ground dwelling.

The next evolutionary step appeared with the species of Australopithecus, the most famous example being Lucy.

Lucy is a roughly 40 percent complete skeleton of a female Australopithecus afarensis who lived around 3.2 million years ago in the Ethiopian rift.

There are no doubts that Lucy was a biped. But her skeleton and lifestyle differed greatly from our own, and this has led to an almighty controversy spanning the past four decades.

She was also a midget. We humans have long legs for walking and short arms, chimpanzees short legs and long tree climbing arms, and Lucy was proportioned somewhere in between.

At the same time, Lucy’s hands were curved like an ape’s and show that she had a powerful grip for climbing in the trees.

Were the ape-like features of here skeleton just evolutionary ‘hangovers’? Vestigial features even? Or was Lucy an active tree climber?

Enter the new research from a team led by University of Texas at Austin anthropologist John Kappelman. It’s a praiseworthy example of great lateral thinking in science, and one that amazes for both its innovation and implications for these long controversial questions.

Kappelman’s team of detectives found that many bones in Lucy’s skeleton bear the fractures of a substantial fall from a tree. The fall that ended her life.

Lucy fell from high in a tree, hitting branches on her way as she plummeted at 60 km/hour or more to the ground.

Landing on her feet first, she fell forward, and in a very human and futile quest, tried to break her fall with her hands.

Her leg bones were fractured and knee joints destroyed. Lucy’s pelvis was seemingly shattered into pieces by the unrelenting force of her hip joints being pushed deep and decisively up into her spine.

She was still conscious when see landed, and as she tried to break her fall, her should joints were shattered, ribs broken, and her spine twisted and its bones displaced.

As her head hit the ground knocking her out cold, and into the slumber of her death, Lucy’s skull fractured, cracking her jaw and the bones enveloping her brain.

The new study shows us dramatically how Lucy lived and died: in the trees.

She must have at the least slept in the trees, as all primates on the African savannah do today. But she very likely fed in them, probably lived in them, as well, at least some of the time.

Chimpanzees, with their similar body size, and grasping hands and feet like Lucy’s, routinely climb 100 metres or more searching for fruit.

They also build sleeping nests in the relative safety of trees, sometimes as high as 23 metres, the equivalent of an 8 story building.

Just occasionally, like Lucy, they fall to their death as well.

Soon after Lucy’s kind disappeared from the African savannah, the final step in the evolution of our bipedal walking arrived, in the form of the human genus, Homo.

Our kind seems to be the first fully committed humans to a life on the ground, with our skeletons having lost all of the features associated with climbing; like those seen in Lucy’s and Ardi’s skeletons.

Homo ergaster, at around 2 million years ago, more or less marks the completion of the evolution of our modern style of upright posture and locomotion.

We’ve still a great deal to learn about how we evolved to be the only two-footed walking mammals.

But Ardi and Lucy show us, in sometimes surprising ways, that our evolution wasn’t a linear path, with the slow and gradual perfection of bipedalism along the way.

As we learn more about our evolutionary journey, we’re forced to think of our ancestors as creatures in their own right. Not just as more primitive versions our ourselves.

Lucy had a unique lifestyle, and form of locomotion that was in some ways quite familiar, and in other ways, very distinct, from our own.

But her death, in all it’s drama, is something we can relate to, even though it happened millions of years ago. Her last desperate attempts to break her fall; to survive; seem very familiar, very human, to us today.

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