Hominin bipedalism in two? or three? steps

Senevirathne et al 2025 reported,
“Bipedalism is a human-defining trait.”

This, of course, ignores Hylobates, the extant gibbon (Fig 1), and all extinct bipedal gibbon-line hominids and convergent chimp-line australopithicines (Fig 1).

These are the first words of the abstract and they are false. Don’t make this mistake. Announce your conclusions at the end, not the beginning.

BTW, we all know many lepidosaurs and archosauriformes are also bipedal.

“It is made possible by the familiar, bowlshaped pelvis, whose short, wide iliac blades curve along the sides of the body to stabilize walking and support internal organs and a large-brained, broad-shouldered baby.”

Another false statement. Gibbons (Fig 1) don’t have a bowl-shaped pelvis.

“The ilium changes compared with living primates are an evolutionary novelty. However, how this evolution came about remains unknown.”

Another false statement. How this evolution came about is known (Fig 1).
Apparently bipedalism came first, then the pelvis changed shape.

The authors go on to describe
“heterochronic and heterotopic shifts in ossification.”

The authors looked at mammal pelves
at several ranges including histology, x-ray and µCT scanning.

By contrast, the large reptile tree (LRT, 2337 taxa) looks at 238 aspects of every tested mammal, both extinct and extant.

Speaking of extinct taxa, the authors reported,
“Although fossil pelves are not available for the earliest period of hominin evolution (8 to 5 million years ago), it is noteworthy that some of the earliest bipedal traits present in Ardipithecus (4.4 Ma, Fig 1) and Australopithecus (3.85 Ma, Fig 1) include a short and wide ilium. But how did this reconfiguration happen during evolution?”

‘How’ questions are difficult, perhaps as difficult as ‘why’ questions. Both involve speculation constrained by bracketing and analogy.

“We envision a three-step model.

That’s incompatible with the headline of this paper, “The evolution of hominin bipedalism in two steps. 

Apparently none of the 19 authors, none of the editors and none of the referees noticed this typo = discrepancy.

“First, early on (8–5 Ma), there was an abrupt or gradual shift away from vertical growth to transverse growth by way of a chondrocyte reorientation. This occurred as locomotion went from ape-like to facultative bipedalism with muscle function shifting for the need for increased lateral stabilization.

Let’s remind ourselves that only orangutans, chimps and gorillas practice ‘ape-like’ locomotion = knuckle-walking. Gibbons and their lineage leading to humans never knuckle-walked. The term ‘knuckle’ is not found in the authors’ text.

Gibbons practice ‘facultative bipedalism’ when terrestrial. The authors don’t seem to know this.

Nor do they seem to know that Australopithecus was bipedal by convergence with gibbon-human line primates.

“Second, later (5–2 Ma), as hominins shifted from facultative to obligatory bipedalism, this growth plate orientation became fixed or locked in by additional or new molecular changes, leaving a remnant signal of vertical differentiation. During this stage, we predict a concomitant ossification shift to the posterior ilium to permit anterior growth in response to muscle function.

As you can see the authors’ are focusing on the minutia, which is fine, but they are ignoring the wide-gamut big picture that the LRT presents.

Third, more recently (2 Ma onwards), with the increasing demands of bipedalism for running, and as fetal heads and shoulders became larger, the timing of ossification became delayed to enable enhanced growth and the retainment of a complex iliac shape. According to this model, we argue that the ilium is part of highly important adaptive musculoskeletal pelvic complex that is shaped during evolution under the constraints of complex polygenic underpinnings.”

One could make these statements without x-rays, µCT scans and histology studies (Fig 1). First: get the big picture with a wide-gamut phylogenetic analysis based on traits. Then focus on the microscopic. Just a suggestion.

References
Senevirathne G, Fernandopulle SC, Richard D et al. 2025. The evolution of hominin bipedalism in two steps. Nature  https://doi.org/10.1038/s41586-025-09399-9

Source: https://pterosaurheresies.wordpress.com/2025/08/28/hominin-bipedalism-in-two-or-three-steps/