Our hips don’t lie: They evolved for walking

Posted On 28th May 2019 By Adam Benton

	Thanks to Dominik, the latest supporter to help make these posts possible

It turns out, at least when it comes to locomotion, our hips don’t lie. They feature numerous crucial adaptations for walking upright. As well as being handy for getting around today, studying these traits allows us to track their evolution; revealing how we became a weird, upright ape.

Spot the odd one out

Let’s twist again

One of the big changes to our hip is that it’s become very, very short, but very, very wide. It’s the exact opposite of perverse. This shape is often described as “3D”, as it creates a nice bowl all of our bowels can fit into. Meanwhile, a chimp pelvis is designed to provide a rigid structure for their horizontal body¹.

Where the hips are (and what they look like), in case you didn’t already know.

These changes are a big help in our different ways of walking. The rigidity of the chimp pelvis reduces the effort required to walk on all fours. Ours makes us more flexible¹.

Notably, a chimp pelvis is too tall to rotate independently of their upper half as it would bump into their ribcage. However, our shorter hip means that we can twist their top and bottom halves individually (like we did last summer)¹.

This ability to twist our hips is important for walking upright. As we step forwards we rotate our forwards with the leading leg. This is known as “pelvic tilt” and increases the length of our strides, making walking more efficient. It also decreases the distance that leg has to travel to reach the ground, reducing the force of impact².

The hips from above, showing how a bit of rotation leads to longer strides

Read more here: The hips did lie: our pelvis helps us walk in more ways than we thought

Origin of the hips

Clearly, understanding how our hips evolved is vital for figuring out human evolution. So where did they come from?

The classic march of progress image would have us ascending from a knuckle-walking ancestor. As such, we might expect the earliest hominins to have the tall, rigid hips seen in chimps.

The original march of progress image

However, the pelvis is challenging this simple “ascent of man” imagery. The oldest hominin we have is from Ardipithecus ramidus which lived just a few million years after the human family was born. So you might expect them to either still be knuckle-walking, or at least preserve some vestigial knuckle-walking traits³.

In a dramatic twist, they have neither. Their pelvis, in particular, already looks similar to ours; being that classic 3D shape we know and love³.

From left to right, Chimp, Ardipithecus, Human³.

This trend is repeated in other body parts, both in Ardipithecus and even earlier hominins. All of which provides strong evidence that our ancestors were never knuckle-walkers. Instead, they were upright from an early age; most likely living in the trees.

In fact, the evidence for that upright bodyplan stretches so far back it looks like the common ancestor of all modern apes was vertical. In other words, knuckle-walking evolved from an upright body; not the other way around.

Redrawing the old “march of progress” family history with this new data produces something like this:

Read more here: Hominins were walking upright before it was cool

Creationists do lie (about hips)

All of these unique features our hips have accumulated play another key role in understanding our evolution: identifying members of our family.

Since our way of walking is unique (and thus so are the hip adaptations for it) it effectively serves as a fingerprint for our lineage. Thus, finding it in a fossil is compelling evidence it belongs in the human family.

Australopithjecus shares our hips, so gets to join the family

All of which makes bipedalism and the pelvis anathema to creationists, as they often try to claim key hominin fossils don’t actually belong to the human family. That way, they can just ignore the evidence they contain.

Perhaps the most notable example of this is with AL 288-1, better known as “Lucy”. This female Australopithecus is arguably the most famous hominin fossil. Crucially, she is clearly a hominin as she features many of those bipedal fingerprints, including a hip quite like ours.

All of which creationists have to lie about in order to try and exclude her from our family. This drove them to create this hilariously awful reconstruction of her for a creationist museum.

The reconstruction of Lucy from AIG’s creationist museum

This led to a great little back and forth between me and the museum, where they dedicated several thousand words and a couple of PhDs to trying to refute little old me. Their “arguments” of why her hip wasn’t human-like were especially amusing.

Read more here: Lucy the knuckle walker? Answers in Genesis v EvoAnth

References

1. Lovejoy, C.O., 2005. The natural history of human gait and posture: Part 1. Spine and pelvis. Gait & posture, 21(1), pp.95-112.
2. Gruss, L.T., Gruss, R. and Schmitt, D., 2017. Pelvic breadth and locomotor kinematics in human evolution. The Anatomical Record, 300(4), pp.739-751.
3. Lovejoy, C.O., Suwa, G., Spurlock, L., Asfaw, B. and White, T.D., 2009. The pelvis and femur of Ardipithecus ramidus: the emergence of upright walking. Science, 326(5949), pp.71-71e6.
4. Hogervorst, T. and Vereecke, E.E., 2014. Evolution of the human hip. Part 1: the osseous framework. Journal of hip preservation surgery, 1(2), pp.39-45.