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The first humans,
from knuckle-walking
to bipedalism
Flooded forest
Modern  chimpanzee Homo sapiens
Our skeleton went through considerable straightening during our evolution.


The inundated forest theory

Flooded forest
© Luiz Bento
How would chimpanzees survive to this?

Humans haven't invented bipedalism

Our ancestors began walking on two feet about 7 million years ago, while some archosaurs started doing so about 240 million years before them.
Later, some bipedal theropod dinosaurs became the first birds, about 150 million years ago.
Birds are mainly known for their flying abilities, but they spend most of their time on the ground, walking on two feet.
Other theropod dinosaurs, like the tyrannosaurs, were also bipedal.
Bipedalism has evolved several times in archosaurs and four times in mammals, notably in kangaroos.

The Savannah theory

Since 1925, it was thought that becoming «bipeds» was the result of leaving the trees for the ground.
Called the «Savannah theory», this hypothesis has been seriously challenged, but it is still present in scientific literature and popular culture.
Bipedalism was supposed to have evolved slowly and was needed to free the hands to carry things around and hunt on the ground.
But the most recent findings show that bipedalism occurred very early in our history, before the savannahs appeared.
In fact, bipedalism is now considered the first and most important trait that distinguishes humans from the other apes.

Why give up knuckle-walking?

It appears that changing from knuckle-walking to bipedalism was the event that triggered our speciation.
Humans are the only bipedal primates.
This shift in behavior also seems to have taken place rapidly, at the very start of our history since even the oldest human fossils exhibit a significant modification in posture.
What could have caused such a change?
Chimpanzees and gorillas had been using this type of locomotion for several million years.
It seems to me that only external pressures, coming from their environment, would be sufficient to alter their conduct so drastically.
I don't think that bipedalism was much of a choice; it was the result of a series of events.

Apes can't swim

A few monkey species can swim quite well, but apes generally have a fear for water; some will drown in knee-deep ponds.
Others may feel confident enough to wade in shallow water, but none are proficient swimmers, except humans.
This inability to cross water formations was crucial in their evolution, since it isolated some populations who went on to become different species.
For example, the formation of the Congo river (about1.5 million years ago) could have led to the speciation of the bonobos, now separated from their chimpanzee ancestors by water.

Chimpanzees stranded in a
flooded forest

The best logical reason to explain our sudden abandon of knuckle-walking is the absence of dry land for an extended period of time.
This could easily happen if a population found itself stranded in a newly inundated forest with no dry land to be seen for miles.

Flooded forest
© Col Ford and Natasha De Vere
No knuckle-walking for chimpanzees in this environment. They could only wade on two feet or swim to get around.

They would have to adapt very rapidly, but if this situation lasted for several generations, important physical and behavioral changes would occur.
They would have to wade bipedally to get from one tree to the other and finally learn how to swim.
This could explain why humans have become water aficionados and have lost their fear of it.
Many people have suggested that water has played a role in our history; the most important proposal is called the «Aquatic ape hypothesis». It covers a whole range of aspects and I don't necessarily agree with many of them.
Nevertheless, the idea that our ancestors went through a semi-aquatic period, but returned to terrestrial living before they adapted to it completely, may explain a few of our evolutive traits.
My inundated forest theory is much simpler and could have led to bipedalism in just a few hundred years.

Back to knuckle-walking? No way!

Flooded forest
© Col Ford and Natasha De Vere
What did our chimpanzee-like ancestors do after the extended flood?

They stood on two feet!
Our ancestors, now experts at wading on two feet, found it easier to walk erect on dry land.
Abandoning knuckle-walking also freed their hands for other uses.
When the water receded, our ancestors wouldn't return to walking on their knuckles again.
Putting their weight on them had become much too painful.

Flat of the knuckles
The skin on your knuckles is delicate
and not conceived to support your weight

Young apes learn how to knuckle-walk when they are young and light.
Returning to it as an adult would be very painful and hard on the skin.
One can wonder how and why knuckle-walking could have evolved in the first place.
The skin is much thicker and harder on the palm side.

Humans still living in the trees

In the beginning, our chimpanzee-like ancestors probably couldn't walk very well on two feet, but they spent most of their time up in the trees.
At night, they slept there and, during the day, they would head back to the trees anytime conditions became dangerous on the ground.
Bipedalism involved major transformations of the body;
•The whole body had to straighten up from the bent and curved posture of the chimpanzees,
•The legs lengthened while the arms shortened,
•The rib cage became smaller while the digestive tract got shorter,
•There were structural changes in our hands and feet; the big toe, for example, had to evolve from a grasping position to become aligned with the foot about 2.5 million years ago.
Some say this was the moment when humans stopped climbing up trees, but I disagree.
I think they still needed the safety of the trees to sleep for 2 million years afterwards.

Only one race of humans left

Since bipedalism is an instantly recognizable trait, making it possible to easily identify another human.
Did all those early humans congregate into a big family?
Judging by our history, it would rather seem that our ancestors promptly killed anything walking on two feet they would meet.


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