Panel showing the timeline and sample images of hominin feet that were analyzed by the study.
Timeline of species related to humans since the split from chimpanzees. The photographs show the fossils that were used in the analysis.

Species studied

The lineage leading to humans (Homo sapiens) and chimpanzees (genus Pan) diverged around 7 million years ago. There are some uncertainties and much debate about the precise ancestry of some of these species and the type of locomotion mode they may use.

We examined the fossilized foot bones from key species in the time period between that divergence and today. Their scientific names and common names are given in the table below.

The names of extinct species link to information hosted by the Smithsonian National Museum of Natural History. In an active research area such as this, there is unlikely to be immutable facts or single authoritative sources, but some sources are updated more regularly using the latest advances than others.

Living species

  • Homo sapiens (human)
  • Pan troglodytes (chimpanzee)
  • Gorilla gorilla (gorilla)
  • Macaca nemestrina (pig-tailed macaque)
  • Chlorocebus aethiops (vervet monkey)

How to work with partial skeletons?

The metatarsals have a twisted shape because of the transverse tarsal arch, which is used to measure the shape of the transverse tarsal arch.

The metatarsals are arranged in an arc towards the middle of the foot because of the transverse tarsal arch (TTA). Towards the front of the foot, the metatarsals lie flat on the ground. This transition, from flat at the front to arched in the middle, creates a twisted geometry for the metatarsals. This twist, called metatarsal torsion, increases from the first to the fifth metatarsal. The fourth metatarsal was found for all the extinct species that we studied, thus allowing us to measure the shape of the TTA using incomplete skeletons of the foot.

Comparing feet of different sizes

Despite overall similarities between species that are related to humans (hominins), there are considerable differences in the overall size and shape of the feet. The understanding of the biomechanics and physics of how the transverse tarsal arch works allowed us to derive a parameter that adjusts for differences in length, width, and thickness between the different feet.

We call this parameter \hat{c}, pronounced ‘see-hat’. The curvature parameter showed how the TTA evolved over time and how it differs amongst primates. Most striking was that the TTA of the nearly 3.5 million-year-old foot from Burtele in Ethiopia was quite similar to humans. The foot belonging to the same species as the famous fossil Lucy did not have as well-developed a TTA as humans, but far more than other primates.

A human-like transverse arch may have evolved almost 1.5 million years before the genus Homo emerged.

The plot shows the value of \hat{c} for different feet. The dot shows the median value and the bars show the effect of anatomical variation (middle 50th percentile). Read the original paper to know how \hat{c} was estimated for the different species.