We Do We Walk on Two?
Introduction
If you're one of the curious ones, you may have asked yourself this question a couple of times already. Most animals are quadrupedal, so why aren't we, and what's the science behind bipedalism?
What makes hominins special in the Hominini tribe is that, at one point, they stood up and figured they could walk upright. From that point we're talking about early humans.
It's hard to tell which early hominin was the first to stand up, and, what's more, there are 12 distinct
hypotheses that try to find an answer! It seems that some questions, including ours, will hardly ever be answered accurately.
But we're curious about what is known so far!
So dive in - you may find out about the real power of your butt.
The Benefit of Walking Upright
One feature that helps us walk on two so efficiently is that our big toes (hallux) are not opposable or grasping. A 2019 study about Ardipithecus ramidus shows what the early steps toward bipedalism could've looked like. It seems that the species was the first facultative biped. Ardi, as the first specimen has been named, lived between 4.4 and 3.9 million years ago, and though she had grasping big toes, they were abductable, making it possible for Ardi to be bipedal on the ground, and quadrupedal in tree branches. The problem is, the skeletal remains are just fractures, so it's hard to get a full image.
Another feature is the angled part of the upper part of the thigh bone. You can see that when we stand up, it's natural that our knees are rather close to each other as they support the body's weight. This feature is first well documented in the 6 million-year-old Orrorin tugenensis, which therefore pre-dates A. ramidus. A strong knee is key to making it possible for you to briefly stand on one leg every time you take a step. So, perhaps O. tugenensis was the first biped?
Finally, a later species, Australopithecus anamensis, had a strong, wide bone just under the knee joint, providing evidence for habitual bipedalism in Hominins 4.1 million-year-old ago.
Looking at our closest living relatives may shed light on why bipedalism could be beneficial. Check out the chimpanzee (Pan troglodytes) in the video, who collected precious coula nuts, but had no stone lying around to crack them open. Scared that others would steal the nuts while getting the stone, the chimpanzee found a solution which tells us about one of the hypotheses: that bipedalism freed the hands of early humans to valuable items, such as food.
The Thermoregulatory Model
While there are several hypotheses and theories, some better than others, explaning the origins of bipedalism, there is one view showing just why standing upright is good for our body when it's hot... in the savannah.
The thermoregulatory model proposed by Dr. Peter Wheeler states that walking upright raises most of the body high above the ground, so as the sun shone down on our ancestors in the savannah, and as it still shines down on us today, there's less heat gain and better heat dissipation. As humans stand upright, the direct exposure to the sun is minimized and the body loses more heat as it gets access to more wind.
This way the proportions of our body changed as well: we grew taller.
The Efficient Walker Theory
When prof. McHenry and prof. Rodman studied chimps that were taught to walk upright, they wanted to see whether it's energetically more efficient than quadrupedalism. They formulated the Efficient Walker theory.
This theory suggests that bipedalism is much more energy efficient than walking on four. If we don't run, that is. Yeah, humans suck at saving energy when they run, in comparison with most quadrupedal mammals, but simply walking is just as energy efficient as quadrupedalism in most mammals, and much more efficient than bipedalism or quadrupedalism in chimps.
Well, as for those chimps that were trained to walk upright, most of them showed no fundamental improvement in saving energy when they walked on two, and in some individuals, bipedalism was just a waste of energy.
You might just think: Duh! But what makes the difference between us and them?
Of Chimpanzees and Men
After our ancestors started walking upright, their body proportions changed dramatically. They grew taller and thinner, their limbs grew longer, the knee joint became stronger, and this goes hand in hand with changing behaviour.
Longer legs allowed our ancestors to waste less energy as they took longer steps. But that's not the only thing: our vertebrae became organized in a way that distributed the weight down to the pelvis. And if you look at our pelvis and compare it to that of a chimp or other great apes, it's very, very different. It's wider and shorter.
The image shows differences not only in the bones. Just look at the robusticity of the three semicircular canals of the inner ear, which are important in maintaining balance (upper left corner). They're there to control the position of your head.
Illustration by: Tom Moore (source)
It also shows that chimps have more muscle connections between their shoulders and their head - they need these muscles to hold their head up when they walk on four, while the human head can just sit on the neck peacefully.
However, we gained muscle connections somewhere else - in the glutes. So, next time you see a nice bum, think to yourself: dayum, this person's gluteus maximus must be great at keeping that thighbone and the pelvis together (because that's one of its functions).
We also have springs in our feet! The Achilles tendon and the tendon in the arch of the foot work kind of like springs when you run: energy in and energy out between each step. The Achilles tendon in chimpanzees is really short, so no wonder they don't jog.
Although I stated that we're not really good at saving energy when we run, there's something that we, humans, have that almost all quadrupedal animals, especially those who are destined to run a lot, wish they had...
Why Are You Running?
Walking upright, as we have already found out, helped our ancestors to cope with heat caused by the solar radiation and thus allowed them to do more in the daytime. This way they gained access to more food and could expand their home range easily.
As their body proportions changed, hominids were able to walk greater distances and dump heat. But running was, and still is, really expensive in terms of energy consumption.
There is, however, something that made our ancestors overcome this expenditure and become so successful in hunting. Sweating. We're pretty damn good at it.
Walking upright and sweating is a fantastic combo when it comes to cooling the body and getting rid of excessive heat, but it seems that a significant hair loss was also needed for extended daytime activity. Most quadrupedic mammals don't sweat (horses being an exception), so endurance running is out of question in their case. But humans, as you may know, are really good at endurance running.
Professor David R. Carrier concludes in his 'The Energetic Paradox of Human Running and Hominid Evolution (1984)' that hominids overcame the energetic disadvantage thanks to their ability to alter their breathing pattern while running, and also thanks to sweating. What helped them even more is their ability to adjust diet to improve their physical performance.
Most mammals are unable to change their breathing pattern since their ability to dissipate heat is strongly connected to respiration. If they ran for too long, they would overheat because they wouldn't be able to get enough air to cool down their body. Humans lack this connection, which earned them an advantageous position.
Endurance running turned humans into successful persistence hunters, and if you're a fan of jogging, you may be familiar with one of its effects..
Anandamide is a fatty acid neurotransmitter, a cannabinoid, whose name comes from the Sanskrit 'ananda', meaning "delight and joy" and, of course, amide. It is one of those euphoriants that are responsible for the exercise-related euphoria, also called the "runner's high".
Seems like evolution made sure you get a reward for spending so much energy!
Photo credit: Tiia Monto
| Common name: | Ardi |
| Scientific name: | Ardipithecus ramidus |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Primates |
| Family: | Hominidae |
| Status: | Extinct |
| Scientific reading: |
Photo credit: By Lucius
| Common name: | "Original man" in the local language of Tugen |
| Scientific name: | Orrorin tugenensis |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Primates |
| Family: | Hominidae |
| Status: | Extinct |
| Scientific reading: |
Photo credit: Ghedoghedo
| Common name: | Australopithecus anamensis |
| Scientific name: | Australopithecus anamensis |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Primates |
| Family: | Hominidae |
| Status: | Extinct |
| Scientific reading: |
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| Common name: | Chimpanzee |
| Scientific name: | Pan troglodytes |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Primates |
| Family: | Hominidae |
| IUCN Status: | Endangered |
| Population trend: | Decreasing |
| Scientific reading: |