The T-Shirt Was A Lie!!

This is really just a stream-of-consciousness piece more than anything. It's come about from me going over old uni notes when clearing out some stuff at my mum's; I hope you'll indulge me!

Let's get straight to the titular t-shirt, shall we? I'm sure you know the one - it shows the evolution of man from knuckle-dragging primitive, slowly straightening up in stages until we reach a modern, upright human. Or javelin thrower. Or a fisherman. Or Darth Vader. Or whatever depending on which design you opt for. There are a lot of options!

And why wouldn't there be? It's a cool design. More than that, it nicely and simply encapsulates the progress of human evolution right up to the pinnacle, the most advanced state: us. But what if I told you.... the t-shirt was lying!! Shock horror for all those who look to their wardrobe for scientific understanding.

Where it may be wrong, and the notion I'm going to challenge, is this thought that our upright stride is some ultimate evolutionary advancement. I'm going to rock the very foundations of that tee's design by suggesting to you that rather than being a nice visual shorthand for how amazingly far on we are from our chimpanzee relatives, our walking on two legs is actually a demonstration of humans holding on to an ancient, primitive behaviour.

To illustrate this school of thought, let's first have a crash course in the primate family tree. We - humans, Homo sapiens - are part of what we call the Great Apes. The other Great Apes still alive today are gorillas, orangutans, chimpanzees and bonobos. You could think of us all as cousins. We're related species who are all branches of a family tree; branches that are pretty close together. The branch closest to our human branch, and therefore our closest living relative, is the chimpanzee and bonobo branch. The point where their branch forks away from our own is our last common ancestor. If you follow t-shirt logic, this ancestor knuckle-walked around on the ground on all fours. But from what we know about its habitat and what we can see in the fossil record, this perhaps isn't the case.

You see, evidence suggests that a common ancestor of ours lived in abundant, lush rainforest. They didn't really walk around on the ground at all, but rather lived in the trees, very much like a modern orangutan. It's believed they developed a way of moving that allowed them to reach the very end of tree branches in order to get at food sources that would be otherwise inaccessible. These creatures probably moved around much as a modern orangutan does. If you've ever seen them, orangutans walk around the very ends of tree branches using their arms to assist them. But there's one very important detail to how they move - they're upright. They use a hand-assisted, but two-legged, method of locomotion. And the skeletons of various ancestral species for both human and chimpanzees suggests they moved in the same way. So that may be the root for both modern humans and our chimpanzee cousins: an arboreal biped.

We're already well off-piste from the t-shirt! The far left image for the design, the ancient starting point for modern humans, is upright, not on all fours. If that's the case, what happened next? Where do the knuckle-walkers come from?

As I said, our orangutan-like ancestors lived in dense rainforests. What probably happened next is a common theme in the history of the planet - the climate changed. The once expansive forest coverage started to contract. Where there was previously never-ending foliage, you were now seeing open, treeless expanses developing. This change in landscape created challenges for the species living there. And opportunities. When you have challenges and opportunities present themselves after a change of some kind, you have the right selection pressures for evolution.

The basic thinking behind the theory I'm very much taking my time in explaining to you is that there were two different approaches to handling this change in landscape. These approaches guided the evolution of the Great Apes through to the modern day. Just for reference, the evolutionary branching of the orangutan line happened well before the human/chimp split. These animals continued their life in the trees, employing their arboreal bipedalism.

The first possible route for dealing with the changing habitat conditions was to leave the trees and embrace the newly opened-up ground space. That's the route we believe our ancestors took - committing to life on the ground using a starting point of arboreal bipedalism. Over time, evolutionary pressures refined terrestrial bipedalism to what you see in modern humans. Body proportions shifted a little bit from our arboreal ancestors, who perhaps had a bit more of a resemblance to orangutans in that regard. Our arms became shorter compared to our legs and other aspects of our body morphology changed throughout our evolution to better fit our new lifestyle.

So that's life on the ground covered. The other route was to try and have the best of both worlds. The evolution of chimpanzees and gorillas can be thought to have been driven by adaptations to exploit both the ground and the trees. If you keep that in mind, knuckle-walking makes sense as the best way to get around. Picture a chimp walking the open plains on all fours. They reach a tree. Now, this next part is a little bit of an oversimplification, but it demonstrates the thinking of this locomotive theory quite nicely. Take the chimp's ground-based knuckle-walking... and simply rotate it 90 degrees. Hey presto, they're climbing that tree! The chimp-like way of moving around on all fours is a way to create a movement system that is equally as efficient when walking on the ground as when climbing into the trees. It's believed that this same adaptation for life both on the ground and in the trees evolved independently in gorillas, rather than it being an ancestral, shared trait.

It's often said rather erroneously that humans evolved from chimps, which may be where a lot of common misconceptions about our evolution come from. As you've seen, this isn't the case; rather, humans and chimps share a common ancestor that we both branched off from around 7 million years ago. The two resulting evolutionary lineages eventually led to modern humans and modern chimpanzees, respectively. If you cast your mind back to the very start of this piece, I said that rather than being a status symbol of our advancement and sophistication over other species, the fact we walk on two legs is actually evidence of our species holding on to something ancient and primitive. From what we see in the fossil records, various species from around the time of the human/chimp split were muddling around on two legs. Fossils of Sahelanthropus tchadensis, Orrorin tugenensis, and Ardipithecus kadabba - all ancestral species from around this time of our divergence from chimpanzees - show adaptations for bipedality.

So if you look at the skeleton of an early human or an early chimpanzee ancestor, you don't see a knuckle-dragger. Instead, you see the skeleton... of an upright walker! That was in fact the ancient starting point, and as chimps evolved over time, they actually went further down towards the ground - the exact inversion of the famous image of humans rising up from all fours.

If there's one thing to take away from all this, I suppose it's that I have a perfect design to start my new line of t-shirts for chimpanzees...

Or, perhaps, you could take away a new appreciation of what the word 'advanced' means within the context of our evolution.

I've been pretty reductive in how I've talked about bipedalism as I've tried to illustrate this theory. I've talked about it being ancient and primitive purely as a way to help think about it as an ancestral starting point. The truth is, the modern bipedalism of Homo sapiens is very different from that employed by our ancient ancestors. Over the millions of years of human evolution, our locomotion has developed, changed, refined and 'advanced', for lack of a better word.

If there is one thing I'm trying to highlight it's that alongside our evolution, chimps have had exactly the same amount of time to develop. Their modern knuckle-walking is also the result of millions of years of parallel evolutionary refinement, beginning at the same starting point we did. From this point of view, it is just as 'advanced' as the way we walk around. There may even be an argument that it is more so - our evolution refined an ancestral upright state, while theirs developed an entirely new locomotor system. But that's maybe too hard an idea to make a snazzy clothing design out of...

So there you have it - the theory of the arboreal origins of hominin bipedalism, as my lecture notes so eloquently put it. I really just wanted to write this because I was feeling a bit nostalgic going through my old uni materials. I wanted to share with whoever may read this one of the most interesting theories I came across during my studies - a fascinating piece of our human family history that always stuck with me.

But I would be remiss if I didn't end this with a disclaimer that I think should be present with any piece of scientific musing: this is a theory. While it's probably the best theory out there and is pretty much now taken as the explanation for how our upright walking evolved, it is still just one school of thought for what was going on in our species' evolution. I'm sure there are those within the scientific community who don't subscribe to this thinking and, instead, hold to the old notion that we started on all fours and ended upright and modern.

..... you can recognise these scientists by their t-shirts.....