About Development
A Big-Picture View of Development
About Development
A Big-Picture View of Development
About Development is a free collection of readings about what development is and how it works. It brings together ideas from psychology, neuroscience, biology, and the study of evolution. It is written mainly for students, but also for anyone interested in how bodies, brains, and abilities develop. The aim is to connect ideas that are often taught separately and to make the larger questions easier to see.
One of those questions is why development is so reliable. Human beings develop similar kinds of bodies, brains, and abilities, but these are not simply present in miniature at the beginning. Nor are they built by following a complete set of instructions. The readings look at how organized forms develop so dependably while still differing from one person to another.
Some of the readings trace this problem through the history of ideas about development. Others look at what DNA does, how cells work together, and how later development grows out of what has already happened. Development does not start from nothing, but what happens next is not fully settled in advance.
The resource also looks closely at bodies and brains. A brain does not develop separately from the body and then begin controlling it. Brain development takes place in an active, growing body. Movement and neural activity help shape developing brain systems, while earlier changes affect what becomes possible later. This helps explain why brains can be highly adaptable without being able to develop in just any direction.
Finally, the readings connect development with evolution. Natural selection helps explain why some forms become more common across generations, but explaining how those forms are produced in individual organisms requires an account of development. Changes in developmental timing and organization can also generate new variation and contribute to evolutionary change.
The readings are connected, but each can be read on its own. You can follow them in order or begin with a topic that interests you and use the surrounding readings to explore further.
This resource originates in work by Peter J. Marshall at Temple University in Philadelphia (USA). For background on this open educational resource (OER) as well as reading tips and information on licensing and attribution, see this page.
Part 1 starts by widening our view of development. It looks at change across different timescales, considers how development can have direction without following a fixed plan, and ends with the origins of multicellular development.
We often use development as another word for childhood, growth, or change with age. Looking beyond that everyday notion, this reading describes development as a history of changing organization in which earlier changes alter what becomes possible later.
1.2 The Big Picture of Development
Development can be examined at many timescales, from changes unfolding over moments to the history of an individual life and the much longer histories of generations and species. This reading introduces explanatory frames as a way of asking how these different processes and histories fit together.
1.3 Life Cycles and Developmental Direction
Development follows recognizable paths, but that does not make adulthood the goal of childhood or mean that the path was fixed at the start. Ideas about telos and the life cycle help us understand how development can be ordered without being planned in advance.
1.4 The Origins of Multicellular Development
Multicellular bodies have not always existed. This reading looks at how cells came to remain together, take on different roles, and maintain a larger individual, turning the body itself into something that must be built again during every life cycle.
The second set of readings asks where organized living form comes from if it is not already present or specified at the beginning. It starts with the old debate between preformationism and epigenesis, then moves through questions about form and matter, DNA, developmental systems, and evolution.
Is organized form already given at the beginning, or does it arise as the organism develops? From the miniature humans of preformationism to Wolff’s embryos and Driesch’s sea urchins, this reading shows why rejecting a fixed plan is necessary but not enough. Developmental reliability still needs to be explained.
2.2 The Riddle of Form and Matter
A living thing cannot be explained simply by listing the materials from which it is made. Drawing on Aristotle and later debates about form and stability, this reading treats form as changing organization, made and maintained through activity rather than stored as a hidden plan.
2.3 DNA as a Developmental Resource
DNA does something far more specific than “build an organism.” Looking closely at the genetic code, proteins, gene regulation, and the work of cells shows why DNA can be an essential developmental resource without being a blueprint, program, or master controller.
2.4 Developmental Systems Theory
Behaviors that look “innate” may depend on developmental experiences that are easy to miss. Research in developmental psychobiology challenged the nature–nurture split and helped lay the groundwork for Developmental Systems Theory, which treats heredity as the reliable reconstruction of a whole developmental system.
Evolutionary differences can arise through changes in the timing, rate, and organization of development. Examples from axolotls, giraffes, mouse paws, and bat wings introduce heterochrony and evolutionary developmental biology, leading into the debate over whether development should have a more central place in evolutionary theory.
The final section turns to brains, but it begins with bodies. It follows the evolution of nervous systems within active animals, then examines how the cortex becomes organized through relations among cells, neural activity, bodily movement, sensory systems, and surrounding conditions.
3.1 The Evolution of Bodies and Brains
Brains arrived late in the history of life. Following a path from older forms of cellular signaling to neurons, bilateral bodies, heads, and vertebrate brains shows that nervous systems evolved within animals already organized to move, sense, and act. Jellyfish remind us that brains were never evolution’s destination.
The mammalian cortex provides a close look at how reliable neural organization takes shape during individual development. From the neural tube and embryonic organizers to cortical areas, retinal waves, and body maps, each developing structure helps create the conditions for what happens next.
The developing cortex is plastic, but it cannot become anything at any time. Work on altered sensory input, rerouted pathways, different bodies, and deprivation reveals a changing but structured set of possibilities. Plasticity means movement within that space, not freedom from every constraint.
The Coda returns to the central problem of explaining both reliability and variation. It considers what it means to see a life cycle as reconstructed and how that changes our understanding of brains and human lives. It ends by discussing what questions remain open.