The arches of hominin footprints reflect motion, not just anatomy – once again ‘footprints are not feet’

In this post, I want to tell you about a paper that I co-authored with Kevin Hatala and Stephen Gatesy, which was published in Nature Ecology & Evolution. It’s called “Arched footprints preserve the motions of fossil hominin feet” and it’s about how we can use fossil footprints to learn about the way our ancient relatives walked and ran on two legs.

You may be familiar with the famous footprints from Laetoli, Tanzania, and Ileret, Kenya, which are some of the oldest hominin footprints. Some have argued that the Laetoli footprints show that early hominins had arched feet like us. This is important, because the longitudinal arches in humans are said to be a major part of our locomotion – absorbing shock, storing and releasing elastic energy, and stiffening the foot during push-off.

But as we’ve previously published about dinosaur tracks, ‘footprints are not feet.’ They are in fact, a complex mix of anatomy, motion, and substrate response.

For this paper, we used a combination of techniques to study how footprints are formed and what they reveal about foot kinematics (these methods were published in Hatala et al 2021). We used biplanar X-ray, (XROMM), to record how four human subjects walked across different substrates, from solid to wet mud. We also used 3D animation and particle simulation to visualize how the foot deformed the substrate and created the footprint shape.

Image from Hatala et al 2021 showing how we used XROMM and the Discrete Element Method (DEM) to capture, then simulate the foot and sediment interaction. Note that getting the foot to smoothly deform in LIGGGHTS was a big break through!

We found that arched footprints are not faithful copies of arched feet, but rather result from a specific pattern of foot motion that is characteristic of human walking.

We then applied our findings to reinterpret fossil hominin tracks from Laetoli (3.66 million years ago) and Ileret (1.5 million years ago), as well as more recent tracks from Walvis Bay, Namibia (~400-500 years ago). We compared them with experimental tracks made by habitually barefoot humans and chimpanzees walking bipedally. We measured the relative arch volumes (RAV – we have an upcoming paper about this measure) and pitches of the tracks, which are related to how deep and arched they are.

We discovered that the Laetoli tracks, which were probably made by Australopithecus afarensis, show only partial evidence of human-like walking style, with a similar heel strike but a different pattern of propulsion. The Laetoli tracks are also positively pitched, meaning that the heel is deeper than the forefoot, unlike modern human tracks which tend to be negatively pitched or flat. We think this most likely has to do with the way they used their forefoot to push off against the ground.

The simulations let us visualize how the sediment is moved under the foot, and you can see that the arch that is left behind is far taller and more prominant than was ever present on the foot that made it – *footprints are not feet*, and features can be more determined by motion, than by anatomy.

The Ileret tracks, which were presumably made by Homo erectus, show the earliest evidence for fully human-like bipedal kinematics, with high RAVs and negative pitches similar to modern human tracks. We argue that this indicates important changes in foot anatomy and function at or before the emergence of the genus Homo, possibly related to adaptations for long-distance walking or endurance running.

This paper shows how fossil footprints can provide unique insights into hominin locomotion, but also how they can be misleading if we don’t understand how they are formed. By using a combination of experimental and computational methods, we were able to reveal the dynamic nature of footprint formation and interpret fossil tracks in a new light. This paper also highlights the diversity of locomotor strategies within the hominin clade and the evolutionary significance of foot kinematics for bipedalism.

If you want to learn more about this paper, you can read it here: Arched footprints preserve the motions of fossil hominin feet

Hatala KG, Gatesy SM, Falkingham PL (2023) Arched footprints preserve the motions of fossil hominin feet. Nature Ecology & Evolution 7:32–41.

You should also chekout Kevin Hatala’s blog post for Evology and Evolution.

(Full disclosure: I was feeling lazy, so had bing chat (ChatGPT) help me write this post!)