UC Berkeley

Evolutionary changes in locomotion modify the way an organism interacts with its environment. Changes in locomotion coincide with changes in various other features, including procurement of both food and mates. Beginning roughly 7 million years ago, a major shift in hominid locomotion began, resulting in the obligate bipedality of Homo sapiens. Humans today exhibit a unique shift in locomotion throughout ontogeny, from a locomotor form that requires use of an individual’s forelimbs and hindlimbs to full bipedality that appears around 1 year of age. To date, it is unclear whether this ontogenetic shift in how infants move originated with the beginnings of hominid bipedality, or began later in evolutionary history. This dissertation offers insight into how growth in human limb bones correlates with ontogenetic shifts in locomotion and provides a framework for understanding the skeletal morphology preserved in the fossil record of human evolution.This dissertation uses a newly published dataset of medical CT scans of recently deceased humans from the New Mexico Decedent Imaging Database (NMDID) to study long bone growth qualitatively and quantitatively in human infants and children. Results are correlated with relative onset of locomotor development such as crawling, bipedal standing and walking, and development of hand preference. Each chapter focuses on a different aspect of skeletal growth. Chapter 3 examines intra-element growth and variation by comparing growth rates and patterns of growth at multiple different positions along each long bone diaphysis. Chapter 4 explores inter-element growth by comparing growth at homologous positions on different skeletal elements (e.g., the proximal humerus and the proximal femur). Chapter 5 investigates the development and variation in bilateral asymmetry between paired right and left elements. Each chapter focuses on one broad hypothesis with two or three subsequent questions outlined in each chapter. Results of Chapter 3 show that there are differences in growth patterns and rates of growth at different positions along the same skeletal elements, but only in the humerus. Results from Chapter 4 show different rates of growth between the humerus and femur at different ontogenetic stages. These changes in growth rates and bone strength broadly correlate with the onset of locomotor developments such as crawling and walking. Results from Chapter 5 indicate that asymmetry in long bone length is present at birth, but does not follow the same pattern of asymmetry as is seen in adults or expected from hand-preference data. As individuals get older, they are more likely to exhibit asymmetries that align with an overall populational right-hand 2 preference. Asymmetries in the femur are also present at birth, but patters of asymmetry do not change with age to the same extent as the humerus. This dissertation expands current knowledge of variation in human ontogenetic long bone growth and acts a preliminary study on the growth, development, and variation in growth of human long bones. Future research will focus on increasing sample sizes of human individuals, as well as investigating long bone ontogenetic growth in non-human primates, with the eventual goal of understanding the evolution of crawling throughout human evolution.