Evolution and development

My interests in evolution and development (Evo-Devo) are driven by two classic questions in human evolution:

1) How did our ancestors stop relying on tree climbing when this is an essential aspect of the niche of all other apes?

2) Why do we grow up so slowly compared to other apes?

I address these old questions in a unique way, which is via the growth and development of locomotion in modern humans, our living primate cousins, and fossil hominin relatives.

Current research projects

The role of climbing in hominin evolution

All (non-human) apes climb trees, and for very good reasons. Trees provide access to food and safety from danger. The ability to climb enables infants to cling on to their mother for safety and transport. Fully abandoning this ubiquitous aspect of the primate niche would have come with significant costs. Surprisingly, we know little about the role that climbing has played throughout hominin evolution.

Addressing this question requires a uniquely interdisciplinary approach, integrating biomechanical analysis of climbing in living apes and humans, high-resolution 3D imaging of extant primates and fossil hominins, and phylogenetic techniques to disentangle the developmental, evolutionary and behavioural signals hidden behind morphological traits: which traits contain evidence of within-lifetime behaviour throughout development, and which traits reveal information about longer-term evolutionary selection?

The first results are promising!

Here I have plotted how the ratio bone volume fraction (a measure of bone stiffness) between the upper limb and lower limb neatly predicts a reduction in the frequency of arboreal locomotion relative to total locomotion in chimpanzees and gorillas. Younger, more arboreal chimps and gorillas have relatively more robust upper limbs compared to lower limbs. Additionally, more arboreal chimpanzees have relatively more robust upper limbs compared to more terrestrial gorillas.

Below you see a 3D heatmap of bone volume fraction in the humerus of a chimpanzee, created with my R package trabmap. I've identified several areas of high bone density (in red) that correspond to areas that are highly stressed during climbing activities. These areas correspond either to muscle attachment sites or specific regions of the elbow joint.

Link: Saers (2023). Frontiers in Ecology and Evolution

CRAHL - Comparative Reconstruction of Ape and Hominin Locomotor ontogeny

Marie Skłodowska–Curie Postdoctoral Fellowship at Naturalis Biodiversity Center

The development of locomotion is a fundamental, yet often neglected aspect of an organisms life history. For any trait to appear in adults it first has to make it through the gauntlet of growing up. Immature animals are more susceptible to predation than adults, resulting in significant selective pressure on locomotor ability.

Modern humans are very different from other apes in how our locomotion develops. All other apes are highly arboreal when they are young, either hanging on their mothers or in trees. This ability to climb by themselves enables infant apes to avoid predators by seeking cover in trees and by clinging on to adults. Non-human apes can do this because their brains and nervous systems are more mature at birth compared to humans, they are more precocial. This illustrates the fundamental links between the development of the brain (the neuromotor system), the development of locomotion, and life history.

Here is where bone functional adaptation comes in. I've shown in the papers illustrated below, that trabecular bone is a faithful indicator of several important aspects of locomotor development:

Locomotor onset - when infants start moving by themselves

Locomotor maturation - how quickly their locomotor ability develops to adult like ability

Locomotor repertoire - climbing using their upper limbs or walking on the ground

In this project I investigate how hominin locomotor development evolved and which factors may have driven or limited the evolution of the unique aspects of modern human locomotion and life history.

The brain-bone connection

In a recent paper published in PNAS we show that brain development in humans and other primates is causally linked with skeletal development, a finding that creates new opportunities for studying the evolution and development of locomotion, the brain, and life history.

This connection between brains and bones seems arbitrary until one realises that the brain controls the movement of the rest of the body. These movements subject bones to different forces to which they adapt by adding more bone. This provides a causal mechanism linking the development of the brain to the development of locomotion, and, in turn, to the development of the skeleton.

The paper reminds us of the interconnected nature of biology. By studying variation in one part of the body, the musculoskeletal system in this case, we can infer much about associated changes in the nervous system. Examining evolutionary trends in bone structure allows us to also learn new things about the evolution of aspects of the nervous system that, unlike bones, do not fossilize.

No mind-body dualism here... move over Descartes!

Saers et al. (2022) Proceedings of the National Academy of Sciences

Phys.org

Identifying the onset of locomotion from bone morphology

The onset of locomotion is an important milestone in the life history of any organism and is associated with the timing of several important aspects of brain development. Learning how to walk is not just a developmental precursor to numerous psychological changes but rather plays a causal role in their formation. Travel, therefore, quite literally 'broadens the mind'.

If we can detect at which age our hominin relatives were able to move around by themself we can learn alot about the rates at which their brains developed and how self-reliant they were as infants. So how do we go about this?

In this paper we demonstrate exactly how we can identify the age at which an animal starts walking independently from its parents by looking at the configuration of the internal trabecular architecture of its bones.

Link: Saers et al. (2022) Journal of Anatomy

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